Tag: National Geodetic Survey

NGS releases beta version of NCAT 2.0

My last column highlighted the next phase of the National Geodetic Survey’s (NGS) GPS on Bench Marks program; that is, the development of the 2022 transformation model. It provided web links to material explaining the new GPS on Bench Marks program. NGS continues to update this site so I would encourage users to periodically check the site for updates. At the time of this column, the site was updated on Sept. 13. See the box titled “GPS on Bench Mark Web Page.”

GPS on Bench Mark Web Page

This column is going to focus on the newly released beta version of NCAT 2.0, which includes the new beta version of VERTCON 3.0. See the box titled “NGS Product Updates.”

NGS Product Updates

On Sept. 24, NGS sent an NGS News Update through its data delivery system. See the box titled “NGS News Announcement of VERTCON 3.0.”

NGS News Announcement of VERTCON 3.0

As a side note, anyone can sign up for NGS News announcements by clicking on the button titled “Subscribe for email notifications” on the left side of NGS Home Page. See the box titled “Subscribe for NGS Email Notifications “

Subscribe for NGS Email Notifications

At this time, there are four NGS subscription services available:

NGS News
NGS Webinar Series
NGS Training
NGS GPS on Bench Marks.

For more information on how to sign up for each of the subscription series click here. (See the box titled “NGS Subscription Web Page.”)

NGS Subscription Web Page

Why is NGS Updating VERTCON now?

First, NGS published a technical document that provides a brief history of previous versions of the VERTCON model and the technical details of the new beta VERTCON 3.0 model. The report is titled “NOAA Technical Report NOS NGS 68,The VERTCON 3.0 Project” and can be downloaded here.

NGS decided to update the existing VERTCON tool with two primary purposes in mind:

to support as many of the vertical datums of the NSRS as possible, and
to prepare users for the new North American-Pacific Geopotential Datum of 2022 (NAPGD2022).

NGS plans include incorporating the new VERTCON 3.0 model into its integrated products and services. See the box titled “Excerpt from NOAA Technical Report NOS NGS 68, The VERTCON 3.0 Project: Motivation for VERTCON 3.0.”

As a matter of fact, the beta version of VERTCON 3.0 is included in an updated beta version of the NGS Coordinate Conversion and Datum Transformation Tool (NCAT). This column will provide examples converting NGVD 29 heights to NAVD 88 heights using the new beta versions of NCAT and VERTCON.

Excerpt from NOAA Technical Report NOS NGS 68, The VERTCON 3.0 Project: Motivation for VERTCON 3.0

The greatest driver for VERTCON 3.0 was the pending release of NAPGD2022, expected in late 2022. As part of that release, NGS intends to release grids to transform between existing vertical datums and NAPGD2022. As the build software used to create all previous versions of VERTCON was no longer available, it was decided (like NADCON; see Smith and Bilich, 2017) to completely recreate the entire suite of VERTCON build software.

However, unlike horizontal datums, the history of vertical datums at NGS is, as mentioned earlier, quite limited. As a transformation can only exist if two datums are released in a region, this limits what expansion to VERTCON 2.1 might be possible. Nonetheless, most regions at least had “Local Tidal” heights published by NGS as well as some other official vertical datum of the NSRS, so a decision to support transformation in these regions was made.

Knowing that such a re-build would replace VERTCON 2.1, the new project and its build software were designated from the beginning as “VERTCON 3.0”.

Other expected advantages with this project were the chance to update documentation and the delivery of the transformations, through incorporation into newly integrated products and services like the NGS Coordinate Conversion and Datum Transformation Tool (NCAT, available at https://www.ngs.noaa.gov/NCAT/) and VDatum (available at https://vdatum.noaa.gov/).

Users can access the VERTCON 3.0 model by clicking on the VERTCON 3.0 link on NGS Home Page. It will direct the user to this website.
See box titled “VERTCON 3.0 Web Site.” The user can also download the Technical Report from this site.

VERTCON 3.0 Web Site

Clicking on the “Access Tool” button” connects the user to the Beta NCAT website. See box titled “Beta NCAT Website.” Two links have been highlighted in the box: “About Conversion Tool” and “Horizontal+height.”
The default values for the beta NCAT are “Horizontal” and “Geodetic lat-lon.” If the user wants to use the VERTCON 3.0 option, he or she must click on the button “Horizontal+height.”

Beta NCAT Website

Clicking on the “About Conversion Tool” provides a brief description of the tool. I’ve highlighted a section in the description that should be pointed out to users. See the box titled “NCAT Brief Description” and the statement below.

“Please note that, although either orthometric or ellipsoidal heights can be used as inputs to NCAT, at this time NCAT does not convert between orthometric and ellipsoidal heights. Only orthometric-to-orthometric and ellipsoidal-to-ellipsoidal height transformations are currently possible in NCAT.”

NCAT Brief Description

What this means is that you can convert, at this time, stations located in the Conterminous United States and Alaska from NGVD 29 to NAVD 88, and from NAVD 88 to NGVD 29. In order to convert from one orthometric height system to another, you have to click on another button. I’ve highlighted the button in the box titled “Single Point Conversion – Horizontal+height.” Clicking on the button “Horizontal+height” initiates another set of buttons under the section titled “select a height.” There are two options ellipsoid or orthometric. The ellipsoid button is the default option. If you want to convert a height from the NGVD 29 datum to the NAVD 88 datum the user needs to select the button titled “orthometric.”

Single Point Conversion – Horizontal+height

The box titled “Select a Height Option” is a screenshot of the site after the user clicks the “Orthometric height option. The user can now select the input and output vertical datums. The input and output datum options are highlighted in the box.

Select a Height Option

Once you select the input and output vertical datums, you need to input the latitude and longitude of the station, select the reference frame, and input an orthometric height value to be converted. You must enter an orthometric height that you want to be converted. The box titled “Converting from NGVD 29 to NAVD 88 – Input Parameters” provides an example for station RU 36 (PID FA1337) located in Rutherford County, North Carolina.

Converting from NGVD 29 to NAVD 88 – Input Parameters

After you enter your input parameters, click on the button titled “Convert.” The box titled “Converting from NGVD 29 to NAVD 88 – Output Solution” provides the output from Beta NCAT tool. The input height and output heights are highlighted in the box. The solution also provides an estimate of the accuracy of the value (SigOrthoht).

Converting from NGVD 29 to NAVD 88 – Output Solution

The published information for RU 36 (PID FA1337) is listed in the box titled “Published Information for Station RU 36.” The NGVD 29 height converted to a NAVD 88 orthometric height from the Beta NCAT tool agrees with the superceded NAVD 88 height to within a couple of millimeters (281.753 m minus 281.755 m = -0.002 m). Saying that, the station was superseded with a GNSS-derived orthometric height and the difference is a little larger, 281.79 m minus 281.753 m = 0.037 meters. I’m not saying that there’s anything wrong with the conversion model, I’m only highlighting that, in this case, it agrees with the NAVD 88 leveling-derived heights even though that station has been superceded by a GNSS-derived orthometric height. Users should be aware of this.

Published Information for Station RU 36

Also, it should be noted that the current version of VERTCON is based on published NAVD 88 heights as of a certain date. If a station has been readjusted since VERTCON 3.0 was generated, then the difference between the modeled value and the published value may be different. The actual difference will depend on how much the newly published orthometric height differs from the previously published orthometric height. If a single station’s height changed due to being disturbed by a local phenomenon such as road construction equipment, then the VERTCON value should still be valid.

However, if the heights of several stations in a region changed due to a regional phenomenon such as crustal movement and/or a large adjustment distribution correction due to a regional vertical control network adjustment, then the VERTCON values may not provide the best estimate of the difference between the two datums.

An example of this is provided in the boxes titled “Published Information for Station R 1036” and “Converting from NGVD 29 to NAVD 88 – Output Solution for Station R 1036.” Station R 1036’s NAVD 88 height was updated in September 2019 ,which was after the creation of the VERTCON model. This means that the latest published NAVD 88 height (6.269 m) would not have been used in the model. The newly adjusted NAVD 88 height and the superseded height differ by –23.7 cm (6.269 m – 6.506 m). In this case, this is not an isolated change of a single station’s published height. The adjusted heights of the stations in the region have all changed due to apparent crustal movement and/or a large distribution correction due to a vertical network adjustment.

Published Information for Station R 1036

The box titled “Converting from NGVD 29 to NAVD 88 – Output Solution for Station R 1036” provides the converted NAVD 88 height using the NCAT tool. The converted NGVD 29 to NAVD 88 value differs by 23.5 cm (6.504 m minus 6.269 m). Which is expected because the newly published height and superseded height differ by 23.7 cm. It agrees to within 2 mm of the previously published NAVD 88 height (6.506 m minus 6.504 m = 0.002 m). Once again, this is not implying that there is something wrong with the VERTCON model. It’s only to note the limitations of the model. Users need to remember that it is a model and it does not produce geodetic quality coordinate values.

Converting from NGVD 29 to NAVD 88 – Output Solution for Station R 1036

Users can also convert from other vertical datums published by NGS. These datums include:

Puerto Rico Vertical Datum of 2002 (PRVD 02)
American Samoa Vertical Datum of 2002 (ASVD 02)
Northern Mariana Vertical Datum of 2003 (NMVD 03)
Guam Vertical Datum of 2004 (GUVD 04)

An example of converting a station with a PRVD 02 orthometric height to a Local Tide (LT) value is provided is the boxes titled “Converting from PRVD 02 to Local Tide (LT) – Input Parameters“ and “Converting from PRVD 02 to LT – Output Solution for Station 11 R RESET.”

Converting from PRVD 02 to Local Tide (LT) – Input Parameters

Converting from PRVD 02 to LT – Output Solution for Station 11 R RESET

Notice that the difference between the PRVD 02 and converted LT value differ by 0.062 m but the accuracy estimate is +/- 0.102 m.

In my opinion, the VERTCON model and the NCAT tool are extremely helpful tools to the surveying and mapping community. NGS is developing these models and tools to support the implementation of the North American-Pacific Geopotential Datum of 2022 (NAPGD2022). I would encourage all users to download the technical report and perform a couple of conversions in your area of interest.

NGS would like individuals to use the beta products and services and provide feedback. What do you like about the tool and its features? What would you like changed or added to the service? I hope everyone will try the beta version and contact NGS with their comments.

NGS is in a listening mode and wants to develop models and tools to assist users in their transition to the new reference frames in 2022. This is your opportunity to let NGS know what you need (desire) to implement the new reference frames.

October 2, 2019

Datums, feet and GNSS vectors: The 2022 NGS upgrade
In what seems like just yesterday, GPS World published my article in the May 2017 issue of Survey Scene on the upcoming datum change by the National Geodetic Survey (NGS) in 2022.

With the calendar pages turning rapidly and as we get closer to the witching hour of geospatial voodoo, more items have surfaced to discuss and educate ourselves on in relation to “the change.”

Let’s delve into these topics and break each down into what the common surveying and geospatial practitioner will need to know with the advancements in coordinates, geodesy and our everyday uses.

NATRF2022: The continental U.S. replacement for NAD83 and NAVD88

It is no secret that with the advancing use of GNSS technology, flaws in both existing horizontal and vertical datums establishing our National Spatial Reference System (NSRS) have been identified and exposed.

NGS estimates that NAD83 is non-geocentric by over two meters, while the model establishing NAVD88 contains a tilt of approximately one meter across our continent.

For most geospatial practitioners, these flaws are minimal to the integrity of their data. It does, however, give us a glimpse of how assumptions of geodetic information can produce incorrect modeling of surveying and mapping data and could lead to more flawed earth models without significant changes to their structure.

With a great number of surveying and mapping practitioners using GNSS technology with little or no knowledge of the origins of our NSRS, it is a good time to provide the primers below to explain the history of our geodetic datums.

Besides my previous article, follow these links for much more thorough technical information:

• GPS World Contributing Author David Zilkoski

• NGS 2022 Informational Videos

• NGS Publications and Webinars

• NGS Video Library

• NGS / COMET Program YouTube Channel

With changes in both horizontal and vertical datums, slight variations in the data we are used to seeing will seem insignificant, but will require the user to pay close attention to potential data traps when converting between the old and new systems. The NGS graphics below depict the severity of datum change in the horizontal and vertical component across the U.S.

Image: NGS

Depending on where you are working, new state plane coordinates will vary from –2 meters to +4.5 meters from previously published values, with elevations fluctuating up to one meter from previous norms. All these changes are due to the increased knowledge of our world using various forms of emerging technology not thought possible several decades ago.

These new measuring methods and studies, including GNSS and gravity monitoring, have allowed scientists and geodesy experts to establish more accurate geographic location systems than past terrestrial ways and procedures.

We have geodetic monuments and marks everywhere; will they still be usable?

The short answer to this question is an unequivocal yes, but with some caveats. Use of GNSS monitoring has proven we reside on tectonic plates that move slowly over time; thus, the geographic values (latitude and longitude) used to calculate any number of coordinate value systems are changing as well.

Image: NGS

Relational data between established points are not likely to change, but studies have shown significant shifts in areas that result in movement of our previously considered “unmovable” monuments.

With additional parameters and characteristics being introduced with the 2022 datum, time and tectonic plate shift are main factors in establishment of a point.

The concept of a “permanent” point no longer exists in relation to a published and unchangeable coordinate value of horizontal and vertical data. The surveying and geospatial data collector must recognize that the user is establishing a particular X/Y/Z or N/E/Z value for that exact moment in time and it, theoretically, will change from the moment one steps away from the point.

This may be too “splitting of hairs” for most users, but the new system simply recognizes the reality of the moving data-collection stage, no matter how minute.

This datum re-establishment has been a monumental undertaking (no pun intended), and NGS deserves many kudos for coming up with a realistic solution for a complex problem.

However, most of its users still have a problem, and it lies within the standard unit of measurement: the U.S. survey foot. NGS (and its predecessor, U.S. Coastal and Geodetic Survey) have always used the meter for the basis of all units of measurement (as does the rest of the world.) The new 2022 datum is bringing us, the surveyors and mappers, to a new reality — nationwide adoption of the international foot. Let the grumbling and arguments begin!

The meter vs. international foot vs. US survey foot

The unit of measurement aptly named the “foot” has existed since early times, with most sources crediting King Henry I of England making a decree that his foot shall become the standard for measurement.

No matter where the definition of the foot came from, it has varied slightly throughout history. The origin of the meter (or metre, as it’s known worldwide) also has a variety of beginnings. The most established story starts from John Wilkins, an English philosopher, who published in 1668 what he described as a new standard of measurement based upon the length of a pendulum that swings approximately 38 inches across in one second. This length was eventually named the meter by an Italian scientist.

Another century later, King Louis XVI of France issued a integration law establishing the modern metric system with weights and measures having a base-ten system of units and sub-units. Within this system was the meter with a new length definition of being one ten millionth (1/10,000,000) of the distance from the North Pole to the Equator.

Upon completion of the calculations, a rectangular bar made of platinum and iridium was created to establish the “standard” meter from which all future measurements would be based.

The United States first recognized in 1866 the metric system and the meter (set forth as one meter equaling 39.37 inches). During this time, the International Commission of the Meter officially adopted the physical meter bar as the standard.

Over the next 100+ years, many studies were undertaken to re-establish the length of the meter. Using wavelengths of various elements, including cadmium, mercury, neon, zinc, helium, thallium and krypton, new definitions were created. In 1983, the current definition of the length of the meter was finalized.

The meter is now based upon the speed of light in a vacuum (299,792,458 m/s) with the meter being the length traveled in 1/299,793,458 of a second. While the length is very close to the original measurements set forth over the centuries, it is better defined for reproduction worldwide without having to possess a standard bar or other device.

To further muddy the standardization of units, in 1959 an international agreement was made by Australia, Canada, New Zealand, South Africa and the United Kingdom so one yard would equal 0.9144 meters. Meanwhile, the U.S. National Bureau of Standards published a notice that all survey-related measurements will remain based so one one yard equals 3600/3937 meters or 0.91441083 meters.

Image: NGS

We have two different measurements for the foot. What’s the big deal?

The difference between the two standards is two parts in one million; while that doesn’t affect everyday physical measurement, it does cause havoc on coordinate systems with values beyond the millions. (See NGS video “Two Right Feet?” for details).

What makes it even more confusing is that states across our country vary on which “foot” is standard within their legislation and daily practice. Currently (at the date of publication), six states recognize the International Foot as their standard unit of measurement, with four states not defining it. The remaining states have officially adopted the U.S. survey foot as their standard unit of measurement.

NGS has suggested that starting with the 2022 datum change, the U.S. survey foot will not be supported in applications and software produced by them for geodetic computations. It will be limited to meters and the international foot, so they are recommending that states update their existing definitions to change to the international foot along with recognizing the 2022 datum as the official coordinate-system base.

How to train our profession, the construction industry and John Q. Public on the new datum

I would be lying to you if I said I’m not concerned with the rollout of the new datum and with converting all surveying and mapping work to the international foot. My biggest concern is not with those direct relationships I have with my staff and fellow professionals within my company.

My main concern starts with these two areas: the tens of thousands of surveying practitioners working within projects containing state-plane coordinate systems in addition to contractors and other mapmaking providers using survey-grade equipment for construction and other mapping applications.

Both groups have little to no technical knowledge of the intricacies of state-plane coordinate systems and the geodesy network “behind the curtain.” To paraphrase a well-known mortgage company with an app-based home loan system, “push button, get data” is the limit of most users’ knowledge when it comes to state-plane coordinates.

Add to this the double-edged sword of real-time networks, where the user does not have to be concerned with setting up a base station, and the potential problems could get worse.

While there will be a few early and timely embracers of the new datum, the majority will dig their heels in and refuse to switch. When the conversion to the 2022 datum is upon us, many users will drag their feet on learning about the new system as existing projects continue under the old datums.

Until there is a mandate by government agencies and others, many newer projects beginning around the adopting time will remain on NAD83 and NAVD88 until directed otherwise.

Most practitioners I have spoken with on this issue agree that it will be a tricky period for surveying and mapping. Rather than get bogged down with negativity and fight change, the surveying, mapping and geospatial community should do the following:
- Rally our professions around these significant changes to educate our technicians and future professionals.
- Coach contractors and other trades who rely on the technology to understand the new system.
- Work with governmental agencies at all levels to educate them about what these changes entail and why to make the appropriate revisions to codes and statutes now.
- Capitalize on this opportunity to teach the public about who we are and how spatial data is part of everyone’s life.
All these points are paramount to the success of the datum upgrade and need to be followed through to the end. Ultimately, the faster we adopt and adapt, the better our geospatial world will be. There is lots of work ahead of us, but as the staff at NGS has shown us, the hard work necessary to make significant change is well worth the effort.

CALLING ALL SURVEYORS AND GEOSPATIAL PROVIDERS!

NGS announces GVX data format for GNSS vector processing

The National Geodetic Survey (NGS) is requesting input and feedback on a new data format for sharing real-time kinematic (RTK) GNSS vector information.

The new format will be like the static GNSS standard, Receiver Independent Exchange (RINEX), and is utilized by most software packages and the Online Positioning User System (OPUS).

The new GNSS Vector Exchange format (GVX), will introduce a new industry standard for sharing of RTK vectors across differing platforms and software packages.

Earlier users of GPS-based data collection remember the number of proprietary files created by each manufacturer, and having their own unique format for data and attribute interpretation. In response, the NGS created RINEX to help standardize data collection as a universal file format that would easily be adopted by receiver and software producers.

That same goal is being set with the introduction of the GVX format as the next step in data-collection standardization for GNSS RTK vectors. GVX elements include (but are not limited to) the following:
- Mark-to-mark Earth-Centered, Earth-Fixed (ECEF) vector components
- Variances and covariances of vector components’
- Reference frame information
- Start and stop time of the observation
- A-priori coordinates for the end points of each vector
- Receiver and antenna types
- RTK and real-time network (RTN) settings, if applicable
- Quality control metadata (e.g., PDOP, number of satellites used, orbit type, etc.)
The introduction to the new format along with technical specifications and examples are on the NGS website.

The National Society of Professional Surveyors (NSPS) works directly with NGS to provide input on maintaining and updating the National Spatial Reference System and will include significant assistance with educating geospatial data providers with the upcoming 2022 datum change and implementation of the North American Terrestrial Reference Frame of 2022 (NATRF2022).

Image: NGS

You can send your feedback here.

For more information, visit the NGS website.

Takeaways from this geospatial refresher…

The surveying, mapping and geospatial professions have exciting times ahead with these cool upgrades from NGS, so we need to take advantage of the calm before the storm to educate ourselves to make the most of the opportunity.

Geospatial data surrounds all of us, and we are the profession specifically educated for correctly and efficiently keeping a handle on it all. It all starts with growing your knowledge a little bit each day. Please join me in growing the profession as well.
September 4, 2019

Using the new interactive ‘GPS on Bench Marks’ map

The National Geodetic Survey (NGS) is now developing the 2022 transformation model. Once again, NGS requests the assistance of the surveying and mapping community. This column provides examples to explain the symbology and use of the new version of the GPS on Bench Marks program for developing the 2022 transformation tool.

My last column discussed the results of the Beta hybrid Geoid18 model, and the differences between the Beta model and the official hybrid geoid model, Geoid12B. It provided examples to explain the symbology of the Beta Geoid18 Web Map. It was noted that NGS analysts rejected stations based on pre- and post-modeled residuals but many times there wasn’t enough redundant information available to ensure the station should be rejected or used in the creation of the hybrid geoid model. As I have mentioned before, users should be commended for their participation in the GPS on Bench Marks program. The Geoid18 model is still in “Beta” so, hopefully, users will continue their support by evaluating the Beta hybrid geoid model and reporting their issues to NGS. Saying that, NGS’ GPS on Bench Marks program is now in a different phase.

NGS held a webinar in July on the latest GPS on Bench Marks program for developing the 2022 Transformation tool. The webinar was recorded and users can find the presentation here. This was an excellent webinar and explained the functions of the web map. I would encourage readers to watch the webinar. It is an hour long but is worth while watching. See Figure 1 for information on the webinar.

Figure 1: GPS on Bench Marks:2022 Transformation Tool Campaign Webinar. (Photo: National Geodetic Survey) — Figure 1: GPS on Bench Marks:2022 Transformation Tool Campaign Webinar (Photo: National Geodetic Survey)

As in the past, the NGS on Bench Marks program can be accessed from NGS’ web page (see Figure 2). The user clicks on the “GPS on Bench Marks” button to access the program’s web page.

Figure 2: NGS Home Web Page (Photo: National Geodetic Survey)

Figure 3 depicts the home web page of the GPS on Bench Marks Program.

Figure 3: GPS on Bench Marks Home Web Page (Photo: National Geodetic Survey)

The web page provides several reasons why users should continue to participate in the GPS on Bench Marks program. Figure 4 lists three reasons for helping NGS develop the 2022 Transformation Tool.

Figure 4: Excerpt from GPS on Bench Marks Home Web Page

GPS on Bench Marks

Help improve the National Spatial Reference System (NSRS) and prepare for the NSRS modernization in 2022 by participating in the GPS on Bench Marks (GPS on BM) for the Transformation Tool campaign. Your efforts will support the following objectives:

• Improve the 2022 Transformation Tool, >which will enable conversions from current vertical datums to the North American-Pacific Geopotential Datum of 2022 (NAPGD2022) and will be integrated into the NGS Coordinate Conversion and Transformation Tool (NCAT).

• Update Passive Control Status: mark recoveries and shared solutions provide NGS and other users of the NSRS with insight into the health of the passive control network and updated information for project planning.

• Automatic Reprocessing in 2022: Shared data will be automatically reprocessed and given new coordinates after the NSRS modernization occurs in 2022.

I’d like to highlight a few of the benefits for participating in the GPS on Bench Marks program.

(1) Improve the 2022 Transformation Tool, which will enable conversions from current vertical datums to the North American-Pacific Geopotential Datum of 2022 (NAPGD2022) and will be integrated into the NGS Coordinate Conversion and Transformation Tool (NCAT).

> A goal of the transformation tool is to provide a model that will allow users to convert from the current North American Vertical Datum of 1988 (NAVD 88) to the new North American – Pacific Geopotential Datum of 2022 (NAPGD2022). The more bench marks that are occupied by GNSS and included in OPUS Shared solutions will enable NGS to generate a more detailed relationship between NAVD 88 and NAPGD2022. This will provide an accurate transformation tool in local areas which will facilitate the implementation of NAPGD2022 in surveying and mapping products and services.

(2) Update Passive Control Status: mark recoveries and shared solutions provide NGS and other users of the NSRS with insight into the health of the passive control network and updated information for project planning.

> An important part of the GPS on Bench Marks program is that it provides an indication of the status of the station. The last time a bench mark was leveled to varies greatly across the Nation. Many of stations in NGS’ Integrated Dataset haven’t been visited in over 50 years. The GPS on Bench Mark program can be useful to identify stations that have moved since the last time it was part of a leveling project. The mark recoveries will provide the latest status of a station which will help others in future project planning. More important, in my opinion, is that the OPUS shared solutions will identify stations that no longer have valid NAVD 88 published heights, and should be used with caution and flagged with a warning

(3) Automatic Reprocessing in 2022: Shared data will be automatically reprocessed and given new coordinates after the NSRS modernization occurs in 2022.

>> Any station that is part of the GPS on Bench Marks program and included in the OPUS Shared solution database will be given 2022 coordinates. This means that users will not have to resubmit their data to obtain the new coordinates in the new 2022 reference frames. This information will be useful during the implementation phase of the 2022 reference frames.

As in the past, NGS is developing web-based products and services to facilitate users incorporating their data into the National Spatial Reference System (NSRS). They have developed a GPS on Bench Marks Web Map Application to inform users which stations they would like occupied by GNSS equipment. They realize that everyone is busy so they are trying to provide information, in near real time, on stations that have been occupied to reduce users occupying a station that already has two occupations. Figure 5 depicts the buttons that will connect the user to an interactive web map application. There are several ways the user can access the application: (1) click on the link titled “Web Map Application” – the red rectangle and arrow in the box titled “GPS on Bench Marks Web Map Application Site,” (2) click on the figure of the web based application – see the blue ellipse and blue arrow in the box, and (3) download the prioritized marks in XLS or Shape file format – see the green pentagon and green arrow in the box.

Figure 5: GPS on Bench Marks Web Map Application Site (Photo: National Geodetic Survey)

Clicking on the Web Map Application button or picture will direct the user to a new website. It informs the user that they are leaving a U.S. Government Web Site for another site. See Figure 6. The user can either click on the statement or just wait until they are redirected the website. (See Figure 7.)

Figure 6: Clicking on the Web Map Application Button or Picture (Photo: National Geodetic Survey)

Figure 7: GPS on Bench Marks For the Transformation Tool Interactive Web Map (Photo: National Geodetic Survey)

Just click on the “OK” button to remove the splash screen. You can click the button “Do not show this splash screen again” so it doesn’t show up every time you access the web page. At the bottom of the web map is a legend that provides information about the map and allows the user to select various options. Figure 8 provides an example of legend buttons. The information box appears by clicking on a particular icon in the legend bar (the arrows indicate the icon and information box for that icon).

Figure 8: Legend on GPS on Bench Marks Web Map Application Site (Photo: National Geodetic Survey)

There’s a lot of information provided in the information box. There’s a scroll bar on the right side of the box that provides the entire write up. Figure 9 provides several sections of the write up. I’ve highlighted sections in the write up to emphasis what NGS is trying to accomplish. NGS’ goal is to minimize the amount of work performed by users and maximize the amount of GNSS data provided to the development of the 2022 transformation tool.

First, NGS has prioritized marks at two spatial resolutions: 10 km and 2 km. They want to reach a 10 km density to provide good national accuracy and a 2 km level to improve local accuracy. The Interactive Web Map allows users to zoom down to a level to identify individual stations selected by NGS. A 10-kilometer hexagonal lattice was developed to define the desired data density on the ground. For each hexagon, the goal was to identify a primary mark and a list of up to 4 secondary marks. The primary mark for each hexagon was added to the priority mark list. Secondary marks are listed and should be observed in cases where the primary mark cannot be found or is unobservable.

To reduce duplication, when a single mark within a 10 km hexagon has two GPS observations that meet NGS requirements, that hexagon is marked as done and the station is removed from the prioritized list. This will help to reduce the number of surveyors occupying the same station over and over again, and increase the number of prioritized stations occupied with GNSS. After a 10-kilometer hexagon is marked as done, a group of up to thirteen 2 km hexagons is generated to define the opportunities to densify the model with additional marks.

To assist in the selection of stations to be part of the GPS on Bench Marks program, NGS has prioritized stations as Priority A and B. Priority A being more important than priority B for the development of the 2022 transformation tool.

Figure 9: Excerpts from GPS on Bench Marks for the Transformation Tool Technical Details

For questions or comments on this tool please email NGS at [email protected].

NGS has developed a prioritized list of bench marks on which new GPS observations will be most helpful to develop the best transformations between the current vertical datums and the modernized NSRS in 2022.

• NGS has labeled marks as Priority A or B based on the quality of previous geodetic measurements, the stability of the mark, and other criteria. GPS observations on Priority A marks will be the most helpful.

• NGS has also prioritized marks based on two spatial resolutions: 10 km and 2 km. 10 km spacing will provide good accuracy at the national scale. Users can improve local accuracy even more by collecting data at the 2 km level.

• NGS will build the transformation tool with data submitted by December 31, 2021. The tool will interpolate over areas without GPSonBM data, meaning that the transformations will be less accurate in those areas.

Priorities A and B

Priority A
Priority A marks meet the following specific criteria from their datasheets and are most likely to be used to create the transformation tools:
• Vertical Order: FIRST, SECOND
• Stability: A, B, C
• Satellite: USEABLE
• Last Recovery Condition: excluding “MARK NOT FOUND”

Priority B
Priority B marks are lower quality marks that will only be considered for use in the transformation tool to fill data gaps if no other data exists in the region.

Spatial Resolution
NGS has also prioritized marks at two spatial resolutions: 10 km and 2 km. NGS wants to reach a 10 km density to provide good national accuracy. Additionally, users can help improve local accuracy by collecting data at the 2 km level.

To prioritize marks based on the two spatial resolutions, NGS created the following system:
• A 10-kilometer hexagonal lattice was developed to define the desired data density on the ground and help select appropriate marks within those areas throughout the U.S. and territories.
• Hexagons with appropriate bench marks were identified.
• For each hexagon, a primary mark was selected and a list of up to 4 secondary marks — if available — were identified. The primary mark for each hexagon was added to the priority mark list. Secondary marks are listed and should be observed in cases where the primary mark cannot be found or is unobservable.
To communicate when observations in a hexagon have been completed, the following process was developed:
• Once a single mark within a 10 km hexagon has two GPS observations that meet the requirements, that hexagon is marked as done and the observed mark is removed from the prioritized list.

• Once a 10 km hexagon is marked as done, a group of up to thirteen 2 km hexagons is generated to define the opportunities to densify the model with additional marks.
• In each of the 2 km hexagons, a primary mark is identified and a list of secondary marks is provided in case the primary mark cannot be found or is not observable. The new primary marks are added to the priority mark list. The number of 2 km hexagons will vary since not all areas have bench marks inside the 2 km lattice. See graphic below:

Clicking on the Web Map Applications “Instructions” button will provide a summary of all of the tools available on the Web Map. See the arrow in Figure 10. The instruction page provides a lot of information and explains the function of each tool.

Figure 10: GPS on Bench Marks Web Map Instructions Site (Photo: National Geodetic Survey)

Figure 11 provides an excerpt from that web page. All of the icons on the Web Map are explained on a mock up of a sample map in the beginning of the Instruction web page.

Figure 11: GPS on Bench Marks Web Map Instructions List of Tools (Photo: National Geodetic Survey)

The list of detailed descriptions of the tool is fairly long so I’ve provided some of the descriptions in Figure 12. The reader is referred to this page for the descriptions of all of the tools.

Figure 12: Partial List of Descriptions of GPS on Bench Marks Web Map Instructions Tools

Legend
Clicking this button will display the legend for all of the active layers displayed on the map.

Layer List
Clicking this button will bring up the list of available layers to display on the map. By default, only the Priority List of marks at 10 km spacing appears. Users can select other layers to display on the map by clicking on the box to the left of the layer name. Once clicked, the box will show a check mark, and all layers with check marks are displayed on the map.

Layer Descriptions:
• Priority List 10 km – Marks requested for national coverage
• Priority List 2 km – Marks requested to densify local areas
• Priority List Done – All marks with enough observations to be considered for use in the Transformation Tool.
• Hexagons 10 km – Areas where GPSonBM data is still requested to complete broad national coverage
• Hexagons 10 km – Done – Areas where sufficient data exists
• Hexagons 2 km -Areas where GPSonBM data may still be submitted to increase local accuracy of the transformation tool.
• Hexagons 2 km – Done – Areas where sufficient data exists.

Working with the Layer List
Clicking on the ellipsis to the right of each layer opens a window with actions for that layer. Set visibility range allows the user to set the zoom level at which each layer appears. By default the visibility ranges are set to prevent too much data from being plotted at once which would slow down the application. Users with fast internet connections can change the visibility range to allow data to be displayed when zoomed out far enough to see the extents of larger states.

Mark Selection Tool:

This tool provides several options for selecting marks. First, change the layer to select from, click in the box to the left of the layer name. Click the green Select box and choose a selection method, then use the mouse to left-click on the map to draw the selection region. Selected marks’ icons will turn blue. Once marks are selected, click on the ellipsis to the right of the layer to open menu of actions that can be performed with selected marks. Using this menu, selected marks can be exported into csv, JSON, and GeoJSON formats.

Attribute Table
This button opens a table at the bottom of the screen that displays all the information available on each mark.

Filters

By State, County, and PID: This tool allows the user to filter the marks on the map down to specific states, counties, or PIDS. After selecting the filter option, click on the switch at the top right of the filter box and the map will pan and zoom to the selected area. If the marks do not appear on the map, try zooming in until they appear.

Figure 13 provides four different options of the icons on the bottom of the web map. They include the “Legend,” Layer List,” Select Priority List,” and Filter by State. These options help the user focus on a particular area of interest. I would encourage the user to familiar themselves with each of these options because they help will make it easier to navigate the map and identify priority stations.

Figure 13: Example of Several Options on the Legend on GPS on Bench Marks Web Map Application Site (Photo: National Geodetic Survey) — Figure 13: Example of Several Options on the Legend on GPS on Bench Marks Web Map Application Site. Examples below are for the “Legend,” Layer List,” Select Priority List,” and Filter by County.” (Photo: National Geodetic Survey)

Another important icon located at the bottom of the Web Map opens an attribute table of the bench marks. (See Figure 14). Once you open the Attribute table tool (see the red arrow in the box), a table of attributes of the stations appears at the bottom of the screen. If you click on a station in the table, the station gets highlighted on the map (see the blue arrow in the box). NGS’ Web Map Application makes it very easy to locate potential stations in a user’s area of interest.

Figure 14: Example of the Attribute Table on the Legend on GPS on Bench Marks Web Map Application Site (Photo: National Geodetic Survey)

When the user clicks on the Layer List tool, they can select which priority list they would like to see plotted on the map. They can click on the “More Info” button to obtain the latest NGS Datasheet. Figure 15 provides an example of A and B stations from the 10 km priority list in the Loudoun County, Virginia, region. The map highlights priority A and B stations; the user can than find more information about a specific station by clicking on the map.

Figure 15: Excerpt from GPS on Bench Marks Web Map Layer List - Priority List 10 km (Photo: National Geodetic Survey) — Figure 15: Excerpt from GPS on Bench Marks Web Map Layer List – Priority List 10 km (Photo: National Geodetic Survey)

A very interesting feature is that once a station is classified as done in a 10 km hexagon, the hexagon is colored green and flagged as done. There is no longer a requirement to occupy a station in that hexagon to assist the 2022 transformation tool for the National level of accuracy. See Figure 16 to see a 10-km hexagon labeled as “Done.” Note that the station considered “Done” is labeled with a white circle.

Figure 16: An Example of a 10-km Hexagon in the Montgomery County, Maryland, and Loudoun County, Virginia, Region (Photo: National Geodetic Survey)

Now the user can focus on the 2-km hexagon boxes to identify stations to improve the local accuracy of the 2022 transformation tool in their area of interest. Figure 17 provides an example of the 2-km hexagons with priority marks plotted within each 2-km hexagon. Once again, the symbology indicates A and B stations, and the 2-km hexagons that need more observations and the hexagons that are labeled as “Done.”

Figure 17: An Example of 2-km Hexagons in the Montgomery County, Maryland, and Loudoun County, Virginia, Region (Photo: National Geodetic Survey)

NGS’ goal is to update the Interactive Web Map in “Near Real Time.” Of course, there’s always going to be some lag time from the time the user uploads their data into the OPUS Shared solution database to when the NGS 2022 Transformation Team reviews the data to ensure the results meet NGS’ criteria. Once again, NGS wants to minimize the amount of duplicate work performed by surveyors and maximize the number of stations contributing to the development of the 2022 transformation tool.

This newsletter highlighted the next phase of NGS’ GPS on Bench Marks program; that is, the development of the 2022 transformation model. The newsletter provided examples to explain the symbology and use of the new version of the GPS on Bench Marks program. It provided web links to material explaining the new GPS on Bench Marks program such as NGS’ July 2019 webinar on the latest GPS on Bench Marks program for developing the 2022 Transformation tool. NGS has done a tremendous job of explaining the importance, process, and results of the GPS on Bench Marks Program. Several of my previous newsletters have highlighted the NGS GPS on Bench Marks program and how users have supported the development of the hybrid Geoid18 model: Hopefully, this support will continue to develop the best possible 2022 Transformation Tool.

August 7, 2019

How to use the NGS Beta GEOID18 web map
This column discusses the results of the National Geodetic Survey (NGS) beta hybrid Geoid18 model and the differences between the beta model and the official hybrid geoid model, Geoid12B. It provides examples to explain the symbology of the Beta Geoid18 Web Map. GEOID18 will be the last hybrid geoid model that NGS will create before NAVD 88 is replaced by the North American-Pacific Geopotential Datum of 2022 (NAPGD2022). I encourage users to access, investigate and become familiar with the web map.

My last column included links to the NGS website that provides the beta coordinates and information about the latest Multi-Year CORS solution (MYCS 2). The column also noted that in late February 2019, NGS released a beta version of the latest hybrid geoid model. See Figure 1, “National Geodetic Survey’s Home Web Page.” This column discusses the Beta Geoid18 Web Map, the results of the hybrid Geoid18 model, and the differences between the beta model and the official hybrid model, Geoid12B.

Figure 1. National Geodetic Survey’s Home Web Page. (Screenshot: National Geodetic Survey)

The Geoid18 hybrid geoid model can be accessed here. See Figure 2, Excerpt from Beta Geoid18 Website. The site provides an opportunity for users to compute a Beta Geoid18 value for a particular station. I would encourage all users to obtain an understanding of the new hybrid model. Once again, it should be noted that this model is a beta model for users to test their workflows and should never be used for official or production work. This allows users to identifies potential issues and differences between Geoid12B and Geoid18, and then contact NGS if they have a question. NGS has done a tremendous job of explaining the Geoid18 process and results, and would appreciate users helping to evaluate the new hybrid model. Several of my previous columns have highlighted the NGS GPS on Bench Marks (GPS on BMs) program and how users have supported the development of the hybrid Geoid18 model: Part 5, Part 6, Part 7, Part 8 and Part 9.

The NGS Beta Geoid18 website provides access to GIS tools that allow users to identify changes between Geoid12B and Geoid18 in their area of interest. The site also states that the hybrid geoid model, Geoid18, will be the last hybrid geoid model that will be created before the new geopotential datum, NAPGD2022, is adopted as the official datum. This is the opportunity for users to be involved in the analysis of the Beta hybrid geoid model. NGS will consider changes to the Beta model until it becomes an official published product. This hybrid geoid model is slightly different from the previous hybrid geoid model, Geoid12B. Similar to Geoid12B, the majority of the design of the hybrid model comes from the relationship between the NGS’ GNSS-derived ellipsoid-derived heights and the leveling- derived orthometric NAVD 88 heights. In other words, the hybrid model is designed to fit to the NAVD 88 orthometric heights.

That said, since the creation of hybrid Geoid12b, there have been improvements in the underlying gravimetric geoid model used in Geoid18. These improvements include:
- Better elevation data and improved digital elevation modelling techniques,
- New gravity data from satellite gravity missions,
- New airborne gravity data from the NGS GRAV-D program, and
- Improved geoid modeling techniques.
My previous columns have focused on procedures and routines for establishing GNSS-derived orthometric heights. As I’ve mentioned in these columns, there are many ways to analyze and investigate GNSS data and adjustment results. I have provided basic concepts that I believe are important for users to understand. My October 2016 column focused on the NGS “GPS on BMS (GPSBM)” dataset that was used to create the last hybrid geoid model, Geoid12B.

As mentioned in my October 2015 column, the hybrid geoid model is designed to fit the published NAVD 88 leveling-derived orthometric heights. I highlighted that the GPS on BMs dataset can be used to identify potential issues in the NAVD 88 published orthometric heights. The October 2016 column provided tools and routines that can be used to identify potential issues in NAVD 88 heights and/or NAD83 (2011) published ellipsoid heights. In support of the Beta Geoid18, NGS performed a detailed analysis of the GPS on BMs stations that were used in the creation of Geoid18.

Figure 2. Excerpt from Beta Geoid18 Website. (Image: National Geodetic Survey)

If you click on the “Web Map button” on the Geoid18 web page (see arrow in Figure 2), you may see the statement highlighted in Figure 3. Clicking on the link will redirect you to the correct web site (see Figure 4.).

Figure 3. Result of Clicking on Web Map Button (Screenshot: National Geodetic Survey)

Figure 4. Web Map Option – Results after clicking https://arcg.is/vSn8K (Top Level of Beta Geoid18 Map) [Screenshot: National Geographic, Esri, Garmin, HERE, UNEP-WCMC, USGS, NASA, ESA, METI, NRCAN, GEBCO, NOAA, increment P Corp. | National Oceanic and Atmospheric Administration (NOAA), National Ocean Service (NOS), National Geodetic Survey (NGS)]
This data layer provides the value of the post-modeled residuals for all of the GPS on Bench Marks that were part of the evaluation of the Beta GEOID18 model. This Feature Layer is used to populate several layers in the Beta GEOID18 Web Map including the layers called Residuals and GPSonBM. The data for this web map can be found here.The top level of the Beta Geoid18 Map depicts a high-level picture of the residuals. The residuals are in centimeters and represented by different colors. The larger green and yellow circles represent the number of features in the region. The individual GPS on BMs station information appear as the user zooms down. There is a lot of information provided on the Web Map site. The legend changes to provide more detailed information as the user zooms down on the map. I have highlighted four sections on the legend in Figure 5 and provided an explanation of the layers below:
1. This data layer provides the value of the post-modeled residuals for all of the GPS on Bench Marks that were part of the evaluation of the Beta GEOID18 model. This Feature Layer is used to populate several layers in the Beta GEOID18 Web Map including the layers called Residuals and GPSonBM. The data for this web map can be found here.
2. This data layer denotes whether the GPS on Bench Mark was used or rejected in the development of the Beta hybrid geoid GEOID18. The data for this web map can be found here.
3. This data layer denotes whether the GPS on Bench Mark was used or rejected in the development of the hybrid geoid GEOID12B. This has all of the same attributes as the spreadsheet provided on the NGS GEOID12B web page. More information can be found here.
4. This is a tile package that displays the difference between GEOID18 and GEOID12B in CONUS. It contains two overlayed raster files, one of which is the estimated error and the other is its hill shade. The data for this web map can be found here.
Figure 5. Legend of Beta Geoid18 Web Map (Screenshot: National Geodetic Survey)

Clicking on the “Content” link provides the data layers (see Figure 6). The user can turn these layers on and off depending on what they’re interested in analyzing.

Figure 6. Contents of Beta Geoid18 Web Map (Screenshot: National Geodetic Survey)

As previously stated, additional details are available as the user zooms into an area of interest (see Figure 7). Five stations have been highlighted in this figure to explain the symbology used on the Web Map site. See Figure 8 for these explanations.

Figure 7. Example of the details available in an area in Eastern North Carolina (Screenshot: National Geodetic Survey)

Figure 8. An Explanation of Stations Highlighted in box titled Example of the details available in an area in Eastern North Carolina (Screenshot: National Geodetic Survey)

When the user clicks on a station’s icon, another window appears that provides specific information about that station. See Figure 9. If the user clicks on the “More Info” button, the routine retrieves the NGS datasheet from the NGSIDB (see Figure 10). As the NGS datasheet states at the end of the description for station Y 247, the station has been obliterated by a mower, which is why it probably was not used in Geoid18.

Figure 9. Example of Information Available for Individual Stations (Screenshot: National Geodetic Survey)

Figure 10. NGS Datasheet for Station Y 247 (PID EX0083) (Screenshot: National Geodetic Survey)

Figure 11 provides all the information available for station Y 247. It should be noted that the station was used in Geoid12B and not used in Geoid18. This means that there will be differences between Geoid12B and Geoid18 in areas where a station was used in Geoid12B but not used in Geoid18. The amount of the difference will depend on the size of the post-modeled residual. In this example, the post-model residual is 7.39 cm.

Figure 11. Example of Geoid18 Information Available for Station Y 247 (Screenshot: National Geodetic Survey)

GPS on BMs data are usually based on different epochs of data; that is, the leveling data is usually observed at a different epoch than the GNSS data. This means, if the station has moved since the last time it was leveled, then the GNSS-derived ellipsoid height minus the leveling-derived orthometric height will not be equal to the geoid height. The procedure for computing GPS on BMs residuals was described in my February 2018 column. To determine if a bench mark had moved since it was last leveled, the analyst needs several nearby bench marks occupied by GNSS.Users have been very important to the development of Geoid18 by participating in NGS’ GPS on BMs program. These data have been used to improve the reliability of the hybrid geoid model. Users can now help by evaluating areas that have large changes between Geoid12B and Geoid18 (see box titled Figure 12). To help ensure that the appropriate stations were used to create the hybrid geoid model Geoid18, users could occupy nearby stations in the area to evaluate the reliability of the model. This will help NGS improve the reliability of the model in that region.

Figure 12. Example of a Large Difference Between Geoid12B and Geoid18 in Western North Carolina (Screenshot: National Geodetic Survey)

I described the NGS’ published height codes in my October 2016 column. In the case of Mitchell 2, there’s no leveling data in NGS’ database in the area surrounding Mitchell 2. There may be leveling projects that have been performed by other agencies such as the USGS but the leveling data have not been processed and loaded into NGS’ database. Users could help by performing GNSS observations on bench marks in the region that are in NGS’ database and/or by performing leveling observations between the GPS on BMs station and the nearest bench mark that has leveling data in NGS’ database.In the example of a large difference between Geoid12B and Geoid18 in Western North Carolina, station Mitchell 2 (PID FB2737) was used in Geoid12B but not used in Geoid18. It wasn’t used in Geoid18 because the NAVD 88 height was not based on an adjustment. According to the description, the leveling tie was performed by a field party that was performing a horizontal survey project (see Figure 13). The field party performed the appropriate leveling procedures but, in this case, the leveling data have not been placed in computer-readable form, so the orthometric height cannot be verified.

Figure 13. NGS Data Sheet for Station Michell 2 (PID FB2737) (Screenshot: National Geodetic Survey)

I encourage users to access the web map and investigate stations that have large post-modeled residuals and/or stations that were used in Geoid12B but were not used in Geoid18. The NGS analyst rejected stations based on pre- and post-modeled residuals but many times there wasn’t enough redundant information available to ensure the station should be rejected or used in the creation of the hybrid geoid model. Users should be commended for their participation in the GPS on BMs program. Hopefully, users will continue their support by evaluating the beta hybrid geoid model.
June 5, 2019
NGS releases beta coordinates and multi-year CORS solution
My last column discussed the preliminary results of NGS’ second Multi-Year CORS Beta Solution of the National CORS. Since my last column, NGS announced the release of the beta version of the hybrid geoid model GEOID18 and, on Feb. 15, NGS officially released the Beta CORS ITRF2014 coordinates and velocities.

This column provides the official links to NGS website that provide the beta coordinates and information about the latest multi-year CORS solution. Below is the NGS announcement of the beta release of the updated coordinates.

Excerpt from Feb. 15, 2019, “NOTICE: New BETA Coordinates Available for CORS and OPUS”. (Screenshot: NGS)

NGS also provides a notice of the new beta coordinates on the National Geodetic Survey homepage, with a link to the Beta CORS ITRF14 coordinates (see the highlighted section below).

National Geodetic Survey homepage. (Screenshot: NGS)

Clicking on the hyperlink labeled BETA CORS ITRF14 Coordinates directs you to the Multi-Years CORS Solution informational homepage.

Information on Multi-Year CORS Solution 2. (Screenshot: NGS)

By clicking on the CORS Home button, the user is directed to the Beta CORS page.

Beta CORS release page. (Screenshot: NGS)

This page clearly states that the ITRF2014 reference frame for CORS is available as a beta product. It also implies that these coordinates are being used in other beta products such as OPUS. I’ll address this later in this column.

Users can obtain information about the MYCS and other related products and services such as Beta OPUS by clicking on links provided on the Beta CORS homepage.

Accessing information about ITRF2014 frame in NGS beta products. (Screenshot: NGA)

It should be noted that these values are considered “beta” and are available to users for testing and feedback. NGS provides a statement about its beta release products. Basically, it states that users should only use beta products to test their workflows and never for official or production work.

The NGS beta release statement. (Screenshot: NGS)

To facilitate testing of the beta CORS coordinates and velocities, NGS provides links to other beta products that will use the MYCS 2 coordinates and velocities.

By clicking on the link labeled BETA OPUS on the beta CORS homepage, the user is directed to the BETA OPUS webpage. This page clearly states that the beta OPUS routine uses the new ITRF2014 reference frame for CORS.

The NGS beta OPUS webpage. (Screenshot: NGS)

NGS also provides a link to Beta OPUS Projects that use the MCYS2 coordinates and velocities.

Beta OPUS Projects webpage. (Screenshot: NGS)

Once again, the Beta OPUS Projects website clearly states that the beta version is using the CORS coordinates and velocities from the MYCS2. It also states that, at this time, NGS will not accept ITRF2014 submissions for publication. As previously stated, NGS’ beta products are for users to test their workflows and should never be used for official or production work.

The Beta CORS webpage provides a lot of valuable information on the processing and establishment of the multi-years CORS solution. I’ve highlighted several of the sections below.

First, by clicking on the link MYCS2 Processing, the user is directed to the section that describes the data used and the processing strategy.

Excerpt from Beta CORS Webpage – MYCS2 Processing. (Screenshot: NGS)

The following are highlights from the section:
- The processing included data spanning 1996 to 2016 and involved around 3050 CORS, IGS and other (e.g., NGA) stations.
- The corresponding input and output data occupied about 25 TB on the NGS computers.
- The residual time series in the early 1990s showed exceptionally noisy behavior at times, which were deleted in the alignment/velocity computation stage.
- The processing was performed in 3 steps:
1. The global processing step solves for orbits, Earth Orientation Parameters (EOPs), hourly tropospheric delay parameters and weekly global (IGS) station positions in an IGS-NNR frame.

2. The CORS processing step ties the remaining CORS to global, backbone, sites holding fixed estimated orbits, troposphere, EOPs and IGS station coordinates. This leads to estimated CORS coordinates in a no net rotation (NNR) frame.

3. The last step is the alignment of the estimated coordinates with ITRF2014 and velocity estimation. This process was done in 15 iterations to achieve rigorous quality control and discontinuity detection.

Linear velocities for all stations are estimated in the NGS realization of ITRF2014. NGS explains how this was implemented in the section titled “The velocity field relative to ITRF2014” (see box titled “Section Describing the Velocity Field Relative to ITRF2014”). The website provides figures that depict the horizontal and vertical velocities used in the processing.

The following are a few highlights from the section:
- Unless an earthquake or a post seismic adjustment occurred, the velocities of a station in between discontinuities are constrained to have the same value.
- Stations that experience earthquakes, post seismic adjustment and in a few cases, non-uniform vertical motion, are allowed to have different velocities in between events as dictated by the data.
- The webpage provides figures that depict the estimated horizontal and vertical CORS velocities.
Section describing the velocity field relative to ITRF2014. (Screenshot: NGS)

What users usually want to know is how much the coordinates have changed and what it means to their surveying activities. The section titled “Main Changes Compared to Previous Reference Frames” provides information and plots that depict the changes of coordinates.

Section on changes in coordinates. (Screenshot: NGS)

This section provides NAD83 (MYCS2) coordinate values minus NAD83 (MYCS1) coordinate values.

The following are a few highlights from the section:
- The ITRF2014 coordinates of all computed CORS coordinates from MYCS2 processing are converted to NAD83 (2011) using HTDP.
- The resulting NAD83 (2011) coordinates are then compared to those obtained from MYCS1 at all common sites.
- The coordinate differences are compared at epoch 2010.0 (MYCS2 – MYCS1).
- The differences are less than 5 mm in most areas with some exceptions.
- The largest differences are seen in southern Alaska.
- Other visible changes are seen in areas of significant and real subsidence and in places where the time series are too short, such as in Iowa where almost all time series are three years long.
- Vertical coordinates (ellipsoidal heights) are compared using the same criteria.
- The stations with the HTDP estimated velocities from MYCS1 (no vertical velocities) show the largest differences. In addition, non-secular subsidence areas also show larger differences.
By clicking on the plots, the user is directed to a larger figure that is easier to interpret. (See boxes titled “NAD83 (MYCS2) – NAD83 (MYCS1) Horizontal Position Differences” and “NAD83 (MYCS2) – NAD83 (MYCS1) Vertical Position Differences.”)

NAD83 (MYCS2) – NAD83 (MYCS1) Horizontal Position Differences. (Screenshot: NGS)

NAD83 (MYCS2) – NAD83 (MYCS1) Vertical Position Differences. (Screenshot: NGS)

NGS has done a tremendous job of explaining the MYCS2 process and results. As the results indicate, most differences between the MYCS1 and MYCS2 are small. Saying that, I would encourage all users to look at the NGS Beta webpages and obtain an understanding of the MYCS2 process and results. Users should also use the beta products and compare their results to the current production products to evaluate the CORS beta coordinates and velocities in their region of interest.

Notice announcing beta version of Geoid18 on NGS homepage. (Screenshot: NGS)

It should also be noted that in late February, NGS released a beta version of the latest hybrid geoid model, Geoid18. This model can be accessed here; the site provides an opportunity for users to compute a beta Geoid18 value for a particular station.

Excerpt from beta Geoid18 website. (Screenshot: NGS)

I would encourage all users to obtain an understanding of the new hybrid model. Once again, it should be noted that this model is a beta model for users to test their workflows and should never be used for official or production work.

My next column will discuss the beta hybrid Geoid18 model, and the differences between the beta model and the official hybrid geoid model, Geoid12B.
April 3, 2019

NGS to perform another Multi-Year CORS Solution

The National Geodetic Survey (NGS) is performing another Multi-Year CORS Solution (MYCS) of the National CORS. This will mean the CORS coordinates will be updated to be consistent with the latest International Terrestrial Reference Frame of 2014 (ITRF2014). NGS has provided preliminary information from the new MYCS at this website. It should be noted that these values are considered beta so they are not final. They may change before they are adopted by NGS for publication. This column will provide potential changes in ellipsoid heights based on the updated beta CORS coordinates (downloaded on January 11, 2019).

NGS has a website that describes the CORS coordinates and how they are established. (See box titled “Excerpt from web page https://www.ngs.noaa.gov/CORS/coords.shtml.)

Excerpt from NGS website

Screenshot: NGS website

What is a Multi-Year CORS Solution? NGS provides a short explanation on their web page (see box titled “Description of MYCS1.”)

Description of MYCS1

(https://geodesy.noaa.gov/CORS/coords.shtml#MYCS1)

Multi-Year CORS (MYCS1) Solution

To obtain the new coordinates that were just described the CORS team completed a full reanalysis of all data from CORS and from a set of global sites with the goal of simultaneously computing a fully consistent set of coordinates, GPS satellite orbits and Earth Orientation Parameters (EOP). This initial Multi-Year CORS (MYCS1) effort is the first of a series of reprocessing projects that will occur periodically in the coming years. The last time a reanalysis of CORS data occurred was in 2002 and numerous inconsistencies and changes have occurred in our processing techniques since that date. The concern over the overall quality of the solutions was not limited to NGS, but also to other geodetic groups, in particular IGS. Thus, IGS requested participation in a reanalysis of all data collected since 1994 to establish a new consistent set of GPS orbits, clocks and EOPs. This project was called IG1/repro1. NGS elected to contribute to this effort as an IGS Analysis Center and used this opportunity to simultaneously reprocess all its CORS data to provide a single consistent set of coordinates for all sites computed using the best available methods.

For regional and site specific plots and many details about the MYCS1 please consult the FAQ. The FAQ also includes comparison between the current and previous frame coordinates.

As noted in the explanation, MYCS1 was the first of a series of reprocessing projects that will occur periodically in the coming years. The current CORS coordinates are referenced to IGS08 epoch 2005.00 (see box titled “Description of IGS08 epoch 2005.00 Coordinates”). The updated coordinates will be consistent with the International Terrestrial Reference Frame of 2014. ITRF2014 is the latest frame realization of the International Earth Rotation and Reference Systems Service (see box titled “Description of ITRF2014”). What will be important to users is how much have the coordinates changed due to the reprocessing.

Description of IGS08 epoch 2005.00 Coordinates

(https://geodesy.noaa.gov/CORS/coords.shtml#IGS08)

IGS08 epoch 2005.00 Coordinates

Since April 17, 2011, the National Geodetic Survey (NGS) and the other Analysis Centers of the International GNSS Service (IGS) have been providing GPS satellite orbits (ephemerides) that are referred to a new terrestrial reference frame, called IGS08 and defined by the IGS. This new frame is based on GPS observations and was designed to be consistent with the International Terrestrial Reference Frame of 2008 (ITRF). ITRF2008 is the latest frame realization of the International Earth Rotation and Reference Systems Service (IERS) and is a multi space-based geodetic technique solution, combining Very Long Baseline Interferometry (VLBI), Satellite Laser Ranging (SLR), Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) and GPS data. Although, the best fitting Helmert transformation between IGS08 and ITRF2008 for a set of well-established, international GNSS satellite tracking sites is the identity function, the transformed ITRF2008 positions have a site specific “correction” applied to them to create IGS08 positions (for additional details on IGS08 consult the following IGSMAILs 6354, 6355, 6356, 6374). Thus the IGS08 position for a particular site may differ from its corresponding ITRF2008 position; however, the velocities remain identical. By using IGS08 coordinates and the associated absolute antenna calibrations in combination with IGS orbits a consistent frame is realized. In addition, NGS has updated the IGS orbits from January 1, 1994 to April 16, 2011 in its online storage with the recently released IGS reprocessed (repro1) orbits that are all aligned consistently with IGS05. For most non-research applications, users can freely mix IGS05 and IGS08 orbits to compute coordinates for control points. Additional information is available in the following IGSMAIL 6475.

On October 7, 2012, the IGS introduced an update to IGS08, called IGb08 (see IGSmail 6663).
This change is transparent/invisible to most users as it focused on introducing positions for: 3 new stations at multi-technique colocations, and 33 IGS reference frame stations with IGS08 coordinates invalidated by positional discontinuities. Coordinates for stations with velocity discontinuities were not updated.

Description of ITRF2014

(http://itrf.ign.fr/ITRF_solutions/2014/)

How does this fit into what surveyors use, that is NAD 83 (2011, MA11, PA11)? NGS provides a description of CORS coordinates and NAD83 (2011, MA11, PA11) on their web page (see box titled “Description of NAD83 (2011, MA11, PA11)”). NGS provides CORS coordinates in both IGS08 epoch 2005.00 and NAD83 (2011, MA11, PA11) epoch 2010.00. See box titled “Current CORS Coordinate Listing for a CORS in Monroe, NC (NCMR)” for an example of a CORS in North Carolina. The beta website contains links to the updated CORS coordinates based on ITRF 2014 (see box titled “Updated CORS Coordinate Listing for a CORS in Monroe, NC (NCMR)” – ftp://www.ngs.noaa.gov/cors/coord/beta_coord_14/ncmr_14.betacoord.txt).

Description of NAD83 (2011, MA11, PA11)

(https://geodesy.noaa.gov/CORS/coords.shtml#NAD83)

NAD83(2011,MA11,PA11) epoch 2010.00 Coordinates

On September 6, 2011, NGS updated the National Spatial Reference System NAD 83 (CORS96, MARP00, PACP00) positions and velocities for all CORS sites, to NAD 83 (2011, MA11, PA11). The NAD 83 (2011) frame, which is relative to the fixed North American plate, is used to define the coordinates for sites located in the CONterminous United States (CONUS), Alaska and US territories in the Caribbean. The NAD 83 (MA11) frame is realized with respect to the fixed Marianas plate and is used to define coordinates in the Marianas. The NAD 83 (PA11) is a Pacific plate fixed frame and is used to define coordinates in Hawaii, American Samoa, the Marshall Islands and other US territories residing on the Pacific Plate. For informative articles about NAD 83 see Snay and Soler, 2000, Snay, 2003. The new realization of NAD 83 involves no datum change, which means that, the origin, scale and orientation of NAD 83(2011) are identical to those of NAD 83(CORS96), and the same for the two other frames. The coordinates are not the same in the old and new realizations for multiple factors including the switch to absolute antenna calibrations, new/revised processing algorithms, improved discontinuity identification, several years of additional GPS data, change in reference epoch, and an improved definition of the global reference frame, IGS08. For a description of how NAD 83 is related to the global reference frame see Craymer et al., 1999, Snay and Soler, 1999. Users working in Canada should consult Craymer, 2006 for a review of how NAD 83 is implemented in Canada. Concisely, the two biggest changes are caused by the change in reference epoch and the move from relative to absolute antenna calibrations.

Current CORS Coordinate Listing for a CORS in Monroe, NC (NCMR)

(ftp://www.ngs.noaa.gov/cors/coord/coord_08/ncmr_08.coord.txt)

Updated CORS Coordinate Listing for a CORS in Monroe, NC (NCMR)

(ftp://www.ngs.noaa.gov/cors/coord/beta_coord_14/ncmr_14.betacoord.txt)

A March 2017 presentation by NGS employee Kevin Choi does an excellent job of explaining the CORS status and why NGS is reprocessing the CORS to be consistent with the latest ITRF. It is dated because it was presented in March 2017 but the reasons for reprocessing and recommendations are still valid today.

Slides from NGS Presentation titled “CORS, OPUS, and Reprocessing status”

by Kevin Choi

(https://www.ngs.noaa.gov/CORS/Presentations/NSPS_MAPPS/CORS-OPUS-Repro2-version1.pdf)

The text box titled “Slides from NGS Presentation, titled ‘CORS, OPUS, and Reprocessing status by Kevin Choi,’” contains four slides from Kevin Choi’s presentation that explains why NGS periodically performs a multi-year reprocessing of the National CORS. As stated in the slides: (1) some CORS coordinates are outside their allowable 2/4 cm (H/V) threshold, (2) some stations that had modeled velocities will now have computed velocities, (3) there are new CORS stations since the last reprocessing, and (4) there is an updated ITRF (ITRF2014 and a corresponding IGS14). What’s really important to the user of CORS is, how much have the coordinated changed and what does it mean to me. This column will focus on changes in ellipsoid heights. I downloaded the CORS coordinate information for both the updated ITRF2014 values (ftp://www.ngs.noaa.gov/cors/coord/beta_coord_14/nad83_2011_geo.comp.txt and ftp://www.ngs.noaa.gov/cors/coord/beta_coord_14/nad83_2011_geo.htdp.txt) and the current IGS08 values (ftp://www.ngs.noaa.gov/cors/coord/coord_08/nad83_2011_geo.comp.txt and ftp://www.ngs.noaa.gov/cors/coord/coord_08/nad83_2011_geo.htdp.txt) from NGS’ website. See box titled “Excerpt from ftp://www.ngs.noaa.gov/cors/coord/beta_coord_14/nad83_2011_geo.comp.txt” for a sample of the contents of the file of the stations with computed velocities and the box titled “Excerpt from ftp://www.ngs.noaa.gov/cors/coord/beta_coord_14/nad83_2011_geo.htdp.txt” for a sample of the contents of the file of the stations with modeled velocities. As previously stated, the ITRF2014 coordinates are “Beta” and can change before being officially published. I generated several plots that depict the difference between the two sets of ellipsoid heights referenced to NAD83 (2011).

Excerpt from ftp://www.ngs.noaa.gov/cors/coord/beta_coord_14/nad83_2011_geo.comp.txt

Data: National Geodetic Survey

Excerpt from ftp://www.ngs.noaa.gov/cors/coord/beta_coord_14/nad83_2011_geo.htdp.txt

The column labeled “Site Status” indicates whether the site is (1) Decommissioned, (2) IGS station and not a National CORS, (3) Non-Operational, and (4) Operational. A summary of the status of the Stations is listed in the text box titled “A Summary of ITFR2014 CORS Stations.”

A Summary of ITFR2014 Station

The file also provides the velocity of the station (modeled or computed) in the north (Vn – units mm/yr), east (Ve – units mm/yr), and up (Vu – units mm/yr) component. The box titled “A Summary of the Velocity Values of ITRF2014 Stations” provides a statistically summary of the velocity components of the stations.

A Summary of the Velocity Values of ITRF2014 Stations

The text box titled, “Differences between 2014 Reprocessed CORS Ellipsoid Heights and Published CORS” depicts the differences in NAD83 (2011) ellipsoid heights between all common CORS between the two set of values in conterminous United States. There appears to be several CORS that have very large changes in ellipsoid heights.

Differences between 2014 Reprocessed CORS Ellipsoid Heights and Published CORS

The box titled “Differences between 2014 Reprocessed CORS Ellipsoid Heights
and Published CORS – Less Than +/- 1 cm” depicts the differences that are between -1 cm and 1 cm.

Differences between 2014 Reprocessed CORS Ellipsoid Heights and Published CORS – Less Than +/- 1 cm

The box titled “Differences between 2014 Reprocessed CORS Ellipsoid Heights and Published CORS – Greater Than +/- 1 cm” depicts the differences that are less than – 1cm or greater than 1 cm. As the plots indicate, most of the differences are between +/- 1 cm.

Differences between 2014 Reprocessed CORS Ellipsoid Heights and Published CORS – Greater Than +/- 1 cm

The box titled “Differences between 2014 Reprocessed CORS Ellipsoid Heights and Published CORS – Greater Than +/- 2 cm” depict the differences that are greater than absolute 2 cm (that is, less than – 2 cm and greater than 2 cm). The plot clearly indicates that most of the station coordinates will change less than +/- 2cm.

Differences between 2014 Reprocessed CORS Ellipsoid Heights and Published CORS – Greater Than +/- 2 cm

The text box titled “Differences between 2014 Reprocessed CORS Ellipsoid Heights and Published CORS in North Carolina” depicts the differences in NAD83 (2011) ellipsoid heights between all common CORS between the two set of values in North Carolina. Most of the differences are less than a centimeter.

Differences between 2014 Reprocessed CORS Ellipsoid Heights and Published CORS in North Carolina

The box text titled “Differences between 2014 Reprocessed CORS Ellipsoid Heights and Published CORS in North Carolina – Greater Than +/- 5 mm” depict the differences greater than absolute 5 mm. The plot clearly shows that the coordinates of most stations in North Carolina will change less than 5 mm.

Differences between 2014 Reprocessed CORS Ellipsoid Heights and Published CORS in North Carolina – Greater Than +/- 5 mm

The box titled “Differences between 2014 Reprocessed CORS Ellipsoid Heights and Published CORS in Alaska” depict the differences in Alaska. Most of these differences are less than a couple of cm but there are a few large differences.

Differences between 2014 Reprocessed CORS Ellipsoid Heights and Published CORS in Alaska

The box titled “Differences between 2014 Reprocessed CORS Ellipsoid Heights and Published CORS in Anchorage, Alaska, Region” depict the differences in the Anchorage, Alaska, region. There are a couple of stations in the Anchorage region that their ellipsoid heights will change over 10 cm.

Differences between 2014 Reprocessed CORS Ellipsoid Heights and Published CORS in Anchorage, Alaska, Region

Sources: Esri, DeLorme, USGS, NPS, NOAA

This column discussed the preliminary results of NGS’ second Multi-Year CORS Beta Solution of the National CORS. The CORS coordinates will be updated to be consistent with the latest International Terrestrial Reference Frame of 2014 (ITRF2014). NGS has provided preliminary information from the new MYCS at the following website: ftp://www.ngs.noaa.gov/cors/coord/beta_coord_14. It was noted that these values are considered “Beta” so they are not final. It was emphasized that they may change before they are adopted by NGS for publication. This column provided potential changes in ellipsoid heights based on the updated beta CORS coordinates (downloaded on January 11, 2019). Future column will provide more details after NGS completes their analysis and adopts the final coordinates for the MYCS.

February 6, 2019

Blue Marble releases 2019 Geographic Calculator

Blue Marble Geographics has launched its 2019 Geographic Calculator, which features a universal copy and paste function, a new angular unit conversion tool, support for NADCON 5.0 and updated seismic survey conversion functionality.

According to Blue Marble, the foundation of the Geographic Calculator’s geodetic data processing functionality is the embedded GeoCalc datasource, which is continually revised and improved with updates through the online GeoCalc Geodetic Registry. The datasource included in the 2019 release mirrors the most current EPSG database definitions.

The calculator’s copy and paste function can be used to quickly capture data for use in a third-party application or to insert new coordinate values in an existing job. The latest version also includes a new tool for accurately converting between various angular units and offers expanded seismic survey conversion capability with improved P1/11 format support and additional SEG-Y format handling for coordinate scalar values, the company added.

The 2019 Geographic Calculator also includes support for NADCON 5.0 from the National Geodetic Survey, providing 3D coordinate transformations within the National Spatial Reference System, as well as several new projection methods.

The calculator also boasts a refreshed interface with new icons and graphic elements for various jobs and tools, as well as optimized architecture for the Windows 10 environment.

“With the imminent approach of the North American Terrestrial Reference Frame in 2022 the need for highly accurate geodetic tools has become more important than ever,” said Patrick Cunningham, president at Blue Marble. “Geographic Calculator continues to lead the way in this field and as is evident in release of the 2019 edition, we are continually raising the bar by providing ever more powerful tools combined with the world’s most expansive geodetic database.”

Blue Marble Geographics is a mapping software company based in Hallowell, Maine. The company offers a number of GIS software solutions, including data conversion, software development kits, low cost GIS, educational resources, 3D analysis, coordinate transformation and GPS.

December 10, 2018

A look at NGS’ experimental and hybrid geoid models

On Aug. 10, the National Geodetic Survey (NGS) released its latest experimental geoid model, xGeoid18. In early 2019, NGS is scheduled to release its next hybrid geoid model, Geoid18.

NGS’ 2018 experimental geoid model, xGeoid18, and the next hybrid geoid model, Geoid18, are not the same. This column will address the latest experimental geoid model, xGeoid18, and the future hybrid geoid model, Geoid18, and why it’s important to understand that they are very different and cannot be interchanged.

In my October 2015 column, I described the differences between NGS’ hybrid geoid models and their experimental geoid models. It has been three years since I wrote the newsletter that addressed the differences between the experimental geoid model and hybrid geoid models. NAPGD2022 is now only about three years away. There will be significant differences between NAVD 88 and NAPGD2022 height.

My June 2017 column provided an estimate of the differences based on the 2016 experimental geoid model, xGeoid16b. These differences between NAVD 88 and NAPGD2022 will vary from state to state, as well as within an individual State. Products referenced to NAVD 88 will be different from products referenced to NAPGD2022. Users will need to prepare for the NAPGD2022 and develop implementation plans. Users should obtain an understanding of the differences between NAPGD2022 and NAVD 88.

NGS has a webpage that provides information on all of their experimental geoid models. It page provides information on the development of the program and information on each of the experimental geoid models.

NGS’ Experimental Geoid Website

Photo: National Geodetic Survey. Click to enlarge.

If the user clicks on the xGeoid18 button (see orange arrow in the box titled “NGS’ Experimental Geoid Web Site”), the experimental geoid model xGeoid18 web page appears (see box titled “NGS’ Experimental Geoid Models 2018 Web Site”).

NGS’ Experimental Geoid Models 2018 Website

Once users get to the xGeoid18 web site, they can obtain estimates of xGeoid18 values for any latitude and longitude by clicking on the button titled “Interactive Geoid Computation.” See red arrow in box titled “NGS’ Experimental Geoid Models 2018 Web Site.”

Input Page of xGeoid18 Interactive Web Page Using the Sample Dataset

Users should note that the output of the xGeoid18 interactive web service provides the results in IGS08 epoch 2022.00. The output provides an estimate of the NAVD 88 orthometric height based on GEOID12B, an estimate of the NAPGD2022 orthometric height based on xGeoid18b, and the difference between NAPGD2022 and NAVD 88. The box titled “Output from xGeoid18 Interactive Web Page Using the Sample Dataset” shows the output from the interactive web service using the sample dataset provided by the web service.

The sample dataset has four stations — a station in California, Louisiana, Michigan and Maine. The results indicate that the differences will vary from state to state — the difference between NAPGD2022 and NAVD 88 in California using xGeoid18b is -0.722 meters, in Louisiana the difference is -0.274 meters, in Michigan the difference is -0.646 meters, and in Maine the difference is -0.307 meters (see box titled “Output from xGeoid18 Interactive Website Using the Sample Dataset”). More detailed estimates of differences between NAPGD2022 and NAVD 88 based on xGeoid16b can be found in my June 2017 column.

Output from xGeoid18 Interactive Website Using the Sample Dataset

Note: The GRS80 ellipsoid is used for both NAD83 and IGS08.

Users can find technical information on xGeoid18 by clicking on the link labeled as Technical Details on the xGeoid18 website (see blue arrow in box titled “NGS’ Experimental Geoid Models 2018 Web Site”). The box titled “Excerpt from Technical Details for xGEOID18 Models” provides an excerpt of the technical details of xGeoid18.

Excerpt from Technical Details for xGEOID18 Models

Summary
xGEOID18 is identical to xGEOID17 in the area bordered by 5˚ ≤ φ ≤ 85˚, 170˚ ≤ λ ≤ 350˚, which includes CONUS, Alaska, Hawaii, and Puerto Rico. Therefore, for information on xGEOID18 in those areas, the user should refer to the Technical Details of xGEOID17.

For extended areas down to the equator and above latitude 85˚ north, the geoid is computed from the NGA’s Preliminary Geopotential Model 2017 (PGM17).

The geoid models for Guam/central Northern Marianas Islands and American Samoa are computed in the closest way as xGEOID17 using the shipborne gravity, altimetric gravity and the reference gravity model PGM17.

The deflections of the vertical are computed from all the geoid grids and the plumb curvature correction is applied by using the classical Bouguer reduction.

As the technical detail webpage states, xGEOID18 is identical to xGEOID17 in the area bordered by 5˚ ≤ φ ≤ 85˚, 170˚ ≤ λ ≤ 350˚, which includes CONUS, Alaska, Hawaii and Puerto Rico. Therefore, for information on xGEOID18 in those areas, the user should refer to the Technical Details of xGEOID17. The box titled “Excerpt from Technical Details for xGEOID17 Models” provides an excerpt of the technical details of xGeoid17. This link provides figures that show the contribution of the airborne gravity data to the geoid models. See boxes titled “Excerpt from Technical Details for xGEOID17 Models” and “Figure (2,3,4,5) from Technical Details for xGEOID17 Models.” As stated in the technical details, users can examine each of the regional plots to see where the incorporation of GRAV-D data has changed the values of the xGeoid17B model.

Excerpt from Technical Details for xGEOID17 Models

GRAV-D Airborne Gravity Contribution

The xGEOID17A and xGEOID17B models are identical except that xGEOID17B includes the available GRAV-D airborne gravity data. The difference between the two models shows the contribution of the airborne gravity data to the geoid models. Since the differences are only in areas where the GRAV-D airborne gravity data has been used, examining the regional plots given below will illustrate the varying levels of improvement due to GRAV-D, seen in different parts of the country.

Figure 1. CONUS – Contribution of GRAV-D airborne gravity [units in cm]

Figure 2 from Technical Details for xGEOID17 Models

Figure 2. Alaska – Contribution of GRAV-D airborne gravity [units in cm]

Figure 3 from Technical Details for xGEOID17 Models

Figure 3. Gulf Coast – Contribution of GRAV-D airborne gravity [units in cm]

Figure 4 from Technical Details for xGEOID17 Models

Figure 4. Northeast – Contribution of GRAV-D airborne gravity [units in cm]

Figure 5 from Technical Details for xGEOID17 Models

Figure 5. Pacific Coast – Contribution of GRAV-D airborne gravity [units in cm]

What does mean to a user today? A station can now have a published ellipsoid height, modeled GEOID12B value, a published NAVD 88 orthometric height, and several xGeoid modeled values. This can lead to confusion if the user is not careful about providing the correct metadata associated with their data and results.

The box titled “Excerpt from The NGS Data Sheet for Station E 116 (PID GA0589)” provides the output from NGS data sheet retrieval program. The first item to note is that if you compute the GNSS-derived orthometric height (H_GNSS) using the formula:

Equation: National Geodetic Survey

the computed value does not equal the published NAVD 88 leveling-derived orthometric height. In this example, the two heights differ by 2.3 cm. As explained in a previous column, GEOID12B is a hybrid geoid model that is distorted to be consistent with NAVD 88 published heights. It is a model and the documentation states that “The relative accuracy of GEOID12B to NAVD88 is characterized by a misfit of +/-1.7 centimeters nationwide.” The box titled “Excerpt from The NGS Data Sheet for Station E 116 (PID GA0589)” provides the computations and the results.

Excerpt from The NGS Data Sheet for Station E 116 (PID GA0589)

Users can also obtain a xGeoid18B value for the station. The box titled “xGeoid18 Output for Station E 116 (PID GA0589)” provides the output of the xGeoid18 using NGS’ xGeoid18 interactive web service. It should be noted that the xGeoid18 output only provides the NAVD 88 orthometric height using GEOID12B; it does not include the published NAVD 88 orthometric height from the NGS Datasheet.

xGeoid18 Output for Station E 116 (PID GA0589)

Note: The GRS80 ellipsoid is used for both NAD83 and IGS08.

The box titled “Different Height Values for Station E 116 (PID GA0589)” provides three different height values that are currently available from NGS web services. These different heights could lead to confusion if users are not careful. Most users won’t be using the experimental geoid interactive web service to compute an estimate of an orthometric height but all users should provide the appropriate metadata to avoid any confusion.

Different Height Values for Station E 116 (PID GA0589)

Chart: National Geodetic Survey

The hybrid geoid model GEOID18 is currently being developed and is not ready to be published, but there is a web page that highlights that it will replace GEOID12B in early 2019 [see box titled “Hybrid GEOID18 Website“] GEOID18 values will be similar to GEOID12B because both hybrid geoid models are made to be consistent with published NAVD 88 values. Saying that, there will be differences especially in areas where the GPS on BMs program identified stations that have moved since the last time they were leveled and, therefore, they were not used in GEOID18.

Hybrid GEOID18 Website

Photo: National Geodetic Survey

My last column provided an update and status report on stations observed in support of the 2018 GPS on BMs program. Many stations with potential invalid published orthometric heights have been identified by the GPS on BM program. This information will be very useful to the surveying and mapping community as well as to NGS. Once NGS publishes the next hybrid geoid model, GEOID18, OPUS results will probably provide an estimate of the NAVD 88 orthometric height computed using GEOID18 similar to what it does now using GEOID12B. In my opinion, the results of GEOID18 will be better than GEOID12B in most areas of the United States and will be helpful in identifying stations that have moved since they were last leveled.

NGS’ official date for accepted data for inclusion in the next hybrid geoid model, GEOID18, ended September 21, 2018. Continuing to submit your results to OPUS Shared will provide a way for others to analyze the results to determine whether a station has an issue that requires attention. New OPUS shared results will be very useful for evaluating the reliability of the model. After the hybrid geoid model, GEOID18, is published, NGS’ GPS-on-Bench-Mark Program will expand to include other regions and will focus on data to improve NGS datum transformation tools. Further columns will address differences between GEOID12B and GEOID18 after GEOID18 officially replaces GEOID12B.

December 5, 2018

NGS 2018 GPS on BMs program in support of NAPGD2022 — Part 9

The number of GPS on Bench Mark (BM) stations highlighted as complete on the National Geodetic Survey (NGS) GPS tracking page as of Sept. 25 represents 43 percent of the total number of stations that need to be observed (2451 of 5862 Priority Marks Completed).

These new GPS on BMs observations will be helpful in identifying invalid GPS on BM stations that may have been used in the next hybrid geoid model.

Now that the 2018 GPS on BM program has officially ended for data included in the hybrid model GEOID18, NGS’ GPS on Bench Mark Program will soon be expanded to include other regions and will focus on data to improve NGS datum transformation tools.

NGS has aided users that are submitting data using OPUS through their GPS on BM website service. Previous columns have highlighted the website. This column will highlight a new feature on the NGS GPS on BMs webpage that displays the progress of priority marks and its associated statistics. This webpage can be accessed through a link on the GPS on BMs Program main webpage — (see highlighted section in box tilted “GPS on BM Project Webpage”). The new webpage provides statistics by state as well as which agencies are submitting the most GPS on BMs data (see the box titled “NGS Webpage of Priority Marks Progress Update”).

GPS on BM Project Webpage

(Source: NGS website)

Image: National Oceanic and Atmospheric Administration

NGS Webpage of Priority Marks Progress Update

(Source: NGS website)

The right side of the webpage provides the percent of the goal reached, a link to the progress tracking map, and a link to progress by state (see box below). The first thing to notice that it provides a current percent of goal reached to date. In this example, the GPS on BM program is at 45 percent complete.

Right Side of Priority Marks Progress Update Webpage

(Source: NGS website)

Clicking on the “Progress Tracking Map” picture will bring up the latest map update (see box below). As depicted in the box, as of Sept.25, 2,451 of 5,862 priority marks have been completed. The “Progress Tracking Map” provides information based on the last time the map was updated, and the “Percent of Goal Reached” is based on the most current OPUS Shared solutions submitted. NGS is working toward generating the map and solutions in near real time.

2018 Progress Tracking Web Map

(Source: NGS website)

Clicking on the “View Progress by State” picture will bring up a table of progress of priority marks by state (see box titled “View by State Webpage”). As depicted in the box, the following statistics are provided for every state:

Source: National Oceanic and Atmospheric Administration

View by State Webpage

(Source: NGS website)

Source: National Oceanic and Atmospheric Administration

The following states have officially completed 100 percent of their priority A and B stations: Connecticut, Minnesota, North Carolina, New Jersey and U.S. Virgin Islands. Congratulations to these states (see the box titled “Priority A & B Progress – states with 100 percent complete”).

Priority A & B Progress — States with 100 percent complete

(Source: NGS website)

It should also be noted that there are 15 states that have completed at least 75 percent of their priority A and B stations (see box below). This is a tremendous amount of work, and everyone should be commended for participating in the GPS on BM program.

Priority A & B Progress – States with at 75 percent Completed

(Source: NGS website)

For completeness, the box below provides a list of the States sorted by percent complete.

Priority A & B Progress – Sorted by Total % Complete

(Source: NGS website)

Source: National Oceanic and Atmospheric Administration

The left side of the webpage provides information on the top submitting agencies. As indicated in the box below, the Illinois Department of Transportation (DOT) and Montana DOT are the two top leaders in submitting GPS on BMs data. They have submitted well over 200 OPUS Shared solutions. The New Jersey and Oregon DOTs are close behind, providing about 200 OPUS Shared solutions.

Left Side of Priority Marks Progress Update Webpage

(Source: NGS website)

Source: National Oceanic and Atmospheric Administration

It’s not surprising to see that state agencies have provided the most submissions to the GPS on BM project (73 percent). It’s very encouraging to see that the private sector has provided 13 percent. Having an accurate and reliable hybrid geoid model will assist surveyors in performing their jobs as well as improve their efficiency in performing geodetic surveys requiring heights.

This column provided an update and status report on stations observed in support of the 2018 GPS on BMs program, and highlighting a new NGS GPS on BMs webpage that displays the progress of priority marks and its associated statistics. The number of GPS on Bench Mark stations completed as of Oct. 1 represents 45 percent of the total number of stations that need to be observed.

As I have explained in previous columns, there were many stations with invalid heights that could be used in the next hybrid geoid model unless more bench marks with valid NAVD 88 heights were observed with GNSS.

Many stations with potential invalid published orthometric heights have been identified by the GPS on BM program. This information will be very useful to the surveying and mapping community as well as to NGS.

NGS’ official date for accepted data for inclusion in the next hybrid geoid model, GEOID18, was Sept. 21. However, any OPUS Shared observations submitted before the final version of GEOID18 has a possibility of being included in the model. Even if it’s not included in the hybrid model, it will be very useful for evaluating the reliability of the model.

After the hybrid geoid model, GEOID18, is published, NGS’ GPS on Bench Mark Program will expand to include other regions and will focus on data to improve NGS datum transformation tools. I encourage everyone to continue supporting the GPS on BMs program — not only for improving the development of the 2022 transformation tool, but to assist in identifying bench marks in your local area that have invalid published orthometric heights due to movement.

Once NGS publishes the next hybrid geoid model, GEOID18, OPUS results will probably provide an estimate of the NAVD 88 orthometric height computed using GEOID18 similar to what it does now using GEOID12B.

In my opinion, the results of GEOID18 will be better than GEOID12B in most areas of the United States and should be helpful in identifying stations that have moved since they were last leveled. Submitting your results to OPUS Shared will provide a way for others to analyze the results to determine whether a station has an issue that requires attention.

October 3, 2018

NGS 2018 GPS on BMs program in support of NAPGD2022 — Part 8

My last two columns (NGS 2018 GPS on BMs program in support of NAPGD2022 — Part 6 and NGS 2018 GPS on BMs program in support of NAPGD2022 — Part 7) described the National Geodetic Survey’s (NGS) GPS on BMs 2018 interactive web map, and provided an update and status report on stations observed in support of the 2018 GPS on BMs Program. This column will provide another update and status report on stations observed in support of the 2018 GPS on BMs program and provide an example of how the OPUS-shared results filled in a void area in West Virginia that will benefit the development of the hybrid geoid model GEOID18. The column will also provide an example of how OPUS Shared results identified a reset station that has an invalid NAVD 88 height, and the importance of having a least two OPUS Shared results to ensure the reliability of the OPUS solutions.

As mentioned in the last column, the GPS on BMs 2018 web page contains a link to a web map where users can determine which bench marks NGS would like users to occupy before the August 31, 2018, deadline. The box titled “2018 Web Map” depicts the map update as of July 27, 2018 (1738 priority marks completed). My last column reported that as of May 29, 2018, there were 1067 priority marks considered completed. During the past two months, 671 more priority stations have been reported completed. This is progress but this still only represents about 30 percent of the priority marks. Hopefully, this will increase dramatically during the month of August. Remember, the cut-off date for data to be included in the creation of the hybrid geoid model GEOID18 is August 31, 2018.

2018 Web Map

(Source: NGS website)

Image: National Geodetic Survey

NGS periodically provides an update on the GPS on Bench Marks Program. On July 3, 2018, NGS sent an email to everyone that shared GPS data on NGS bench marks via OPUS or registered for NGS’ February 2018 webinar about GPS on Bench Marks. The email provided an update on the GPS on Bench Marks Program (see box titled “July 3, 2018, NGS Email on GPS on BMs Update”). The map provided in the update indicated that some of the new observations may generate changes between +/- 8 cm.

July 3, 2018, NGS Email on GPS on BMs Update

(Source: Email from National Ocean Service, NOAA; [email protected] to Dave Zilkoski)

Update: GPS on Bench Marks

Over 1,420 marks completed, and two months left to improve GEOID18 accuracy in your area!

Image: National Geodetic SurveyYour observations are making a difference! The color ramp in the map above reflects accuracy improvements in a hybrid geoid model from your recently submitted GPS observations. The improvements will be realized when NGS releases GEOID18.

In case you missed it

In early 2018, NGS released a list of priority bench marks where GPS data is needed to improve GEOID18, NGS’ last planned hybrid geoid model before The North American Vertical Datum of 1988 (NAVD 88) is replaced by the North American-Pacific Datum of 2022 (NAPGD2022). Data to support GEOID18 will be accepted until the end of August 2018. After that, GPS on Bench Marks (GPS on BM) efforts will expand to include other regions and will focus on data to improve future transformation tools.

How can I help?

Following the guidance provided on the NGS GPS on BM website, you can help by collecting static GPS data on adjusted NAVD 88 bench marks and submitting the data to NGS via OPUS Share. To improve efficiency and reduce unnecessary redundancy, we have created a GPS on Bench Marks 2018 web map to help contributors know where we have the data we need and where we still need GPS observations.

Thank you to our contributors

Over 1,700 observations have been submitted to date, completing the required observations for over 1,420 marks from our prioritized list. Each observation requires at least 4 hours of data collection with a survey grade GPS receiver, plus additional time for planning, travel, and data submission, so each one is a significant contribution. Visit the GPS on BM website for updates on our biggest data contributors and each state’s progress toward the goals.

Why are you receiving this email?

• You shared GPS data on NGS bench marks via OPUS, or
• You registered for our February 2018 webinar about GPS on Bench Marks.

We anticipate sending quarterly updates about these and related efforts. If you’d like to opt-out, click the “Manage Subscriptions” at the bottom of this email.

NOAA’s National Geodetic Survey
geodesy.noaa.gov

NGS is tentatively planning another webinar on the GPS on Bench Marks program for August 9, 2018 (2 pm to 3 pm eastern time). NGS will provide an update on the GPS on Bench Mark program and probably will highlight potential improvements between the current hybrid geoid model GEOID12B and the latest prototype version of the future hybrid geoid model GEOID18. I would encourage everyone to sign up for the NGS webinar series.

Source: Plot Generated Using ArcGIS

Users can subscribe to any or all of NGS four public subscription lists — news, webinar, training, and GPS on Bench Marks — by visiting the NGS subscription services web page and submitting their email address for the type(s) of notices they want to receive. (https://www.ngs.noaa.gov/INFO/subscribe.shtml)

As indicated in the figure provided in NGS’ July 3rd update on the GPS on Bench Marks program email, there are many areas of the country that have already benefitted from users participating in NGS’ GPS on BMs program. This column will highlight an area near Charleston, West Virginia, were users have been very active in providing OPUS Shared results. The box titled “GPS on Bench Marks near Charleston, West Virginia” depicts the marks that meet NGS’ criteria and will be involved in the development of the hybrid geoid model GEOID18. As you can see from the plot, there are several new stations that will be used in the development of the model which will help to improve the reliability of the product.

GPS on Bench Marks near Charleston, West Virginia

(Source: NGS Website)

Image: National Geodetic Survey

The box titled “An Example of OPUS Shared Stations in Charleston, West Virginia, Region” provides the stations’ PID and OPUS designation. The six OPUS Shared stations cover approximately a 50 km square area. Most of the stations are only 10 km apart. These stations will definitely help to improve the reliability of the hybrid GEOID18 model.

An Example of OPUS Shared Stations in Charleston, West Virginia, region

(Source: Plot Generated Using ArcGIS)

Source: Plot Generated Using ArcGIS

When using OPUS Shared results, users should always check to see if a station has been observed more than once. The box tilted “Differences in OPUS Shared Ellipsoid Heights in Charleston, WV, Region” lists the pairs of OPUS observations for the stations depicted in the previous plot. The column labeled “Difference in Ellipsoid Heights” provides the differences in ellipsoid heights based on the two different OPUS Shared results. All differences are less than 1.5 cm and most are less than 1.0 cm. This is indicating good repeatability to the cm level but this may not be indicating accuracy. These stations were observed one day apart but observed at about the same time of the day. They could have the same systematic errors effecting the results such as multipathing and satellite geometry. When performing the second OPUS Shared observation, users should select a different time of day to improve the chances of detecting, reducing, and/or eliminating the effects of remaining systematic errors.

Differences in OPUS Shared Ellipsoid Heights in Charleston, West Virginia, region

Source: National Geodetic Survey

The box titled “Differences in OPUS-Shared GNSS-Derived Orthometric Heights Using GEOID12B and Published NAVD 88 Heights” provides the differences between the GNSS-derived orthometric heights using GEOID12B and the published NAVD 88 values. This table indicates that there is a large difference (23.4 cm) for station HX2382 (L105 Reset 1962). Since the two ellipsoid heights only differ by 1.0 cm, this is an indication that the station probably moved since it was Reset or the reset observations were performed incorrectly. Either way, this station should not be used in the development of the hybrid model or used by anyone for geodetic control.

Differences in OPUS-Shared GNSS-Derived Orthometric Heights using GEOID12B and Published NAVD 88 Heights

Source: National Geodetic Survey

Since GEOID12B is a hybrid geoid model that was designed to be consistent with NAVD 88 values, I always compute differences between GNSS-derived orthometric heights using the experimental geoid model and published NAVD 88 height values. I described this process in my October 2015 column (http://stage.globalpositioningnews.com/establishing-orthometric-heights-using-gnss-part-3/). The box titled “Differences in OPUS-Shared GNSS-Derived Orthometric Heights Using xGeoid17b and Published NAVD 88 Heights” provides the differences between the GNSS-derived orthometric heights estimated using IGS08 (2005) ellipsoid heights with the xGeoid17b geoid model and published NAVD 88 heights. The values in the column labeled “GNSS-Derived Orthometric Height minus Published NAVD 88” represent an approximate difference between NAPGD2022 and NAVD 88. The box titled “OPUS-Shared GNSS-Derived Orthometric Heights Using xGeoid17b minus Published NAVD 88 Heights” provides a plot that depicts these differences.

Differences in OPUS-Shared GNSS-Derived Orthometric Heights Using xGeoid17b and Published NAVD 88 Heights

Source: National Geodetic Survey

OPUS-Shared GNSS-Derived Orthometric Heights Using xGeoid17b minus Published NAVD 88 Heights

(Source: Plot Generated Using ArcGIS)

Source: Plot Generated Using ArcGIS

Once again, it should be noted that PID HX2382 value is much different from the other values. To look for outliers, a mean difference was removed from the results. The box titled “OPUS-Shared GNSS-Derived Orthometric Heights Using xGeoid17b minus Published NAVD 88 Heights with a Mean Value Removed” makes it easier to see that station HX2382 is an outlier. The station is approximately 25 cm different from its neighboring stations that are only 10 km away. As previously mentioned, this station apparently moved since being Reset in 1962 or the reset observations were performed incorrectly. Identifying stations that have moved since the last time they have been leveled is one of the benefits of participating in the GPS on BMS program.

OPUS-Shared GNSS-Derived Orthometric Heights Using xGeoid17b minus Published NAVD 88 Heights with a Mean Value Removed

(Source: Plot Generated Using ArcGIS)

Source: Plot Generated Using ArcGIS

For completeness, both a bias and trend were removed from the differences since IGS08 (2005) GNSS-derived orthometric heights and NAVD 88 heights indicate that there’s an apparent long-wavelength trend between the two sets of values. The box titled “OPUS-Shared GNSS-Derived Orthometric Heights Using xGeoid17b minus Published NAVD 88 Heights with Bias and Trend Removed” depict the differences with a bias and trend removed. As in the other figures, PID HX2382 clearly indicates that it is an outlier relative to its neighbors. This station would be rejected by the geoid team when creating the next hybrid geoid model.

It should be noted that except for the Reset station, all of the differences are less than 2 cm. Although, some relative differences between closely-spaced stations approach 4 cm. For example, the differences between stations HX1746 and HX2496 is -3.7 cm (-1.8 cm – 1.9 cm). The differences in ellipsoid heights from the OPUS Shared solutions are all less than 1.5 cm, even the differences between ellipsoid heights for station HX2382 is only 1 cm. This is an indication that the reset station, HX2382, does not have a valid NAVD 88 published height and should not be used as control. Surveyors that adhere to the FGCS specifications and procedures would realize that this station did not have a valid NAVD 88 height and would not use the published NAVD 88 as control in their project. For example, surveyors performing a leveling project would perform a 2- or 3- mark leveling tie and the results would indicate that the station had moved since it was last leveled.

OPUS-Shared GNSS-Derived Orthometric Heights Using xGeoid17b minus Published NAVD 88 Heights with Bias and Trend Removed

(Source: Plot Generated Using ArcGIS)

Source: Plot Generated Using ArcGIS

This type of validation procedure should also apply for OPUS users. If a user obtains one OPUS solution and proceeds to perform a survey from that station, the user does not know whether the OPUS height value is reliable or accurate. One solution does not provide any indication of reliability.

(Source: Merriam-Webster dictionary)

The OPUS Shared station PID SV0942 (A 25) is an example of two OPUS Shared results generating ellipsoid height values that differ by 10 cm. (See yellow highlighted section in the box titled “Differences in OPUS Shared Ellipsoid Heights for PID SV0942.”) This large difference is significant when you performing a survey where you need heights to better than 3 cm (0.1 foot). This is one reason that NGS requires two OPUS Shared solution for every mark used in the development of the hybrid geoid model.

Differences in OPUS Shared Ellipsoid Heights for PID SV0942

Source: National Geodetic Survey

In the OPUS Shared solutions of PID SV0942, the latest OPUS Shared GNSS-derived orthometric heights (2018-07-14) agrees to about a cm with the published NAVD 88 height, while the 2014 Opus Shared GNSS-derived orthometric height is -11.4 cm different from the published NAVD 88 value. (See yellow highlighted section in box titled “Differences in OPUS-Shared GNSS-Derived Orthometric Heights Using GEOID12B and Published NAVD 88 Heights for PID SV0942.”)

Differences in OPUS-Shared GNSS-Derived Orthometric Heights Using GEOID12B and Published NAVD 88 Heights for PID SV0942

Source: National Geodetic Survey

It should be noted that the error estimates provided in the Opus Shared output indicate the ellipsoid heights are good to about +/- 1 cm. (See highlighted section in box titled “Two OPUS Shared Solution for PID SV0942.”) Saying that, the two NAD 83 (2011) ellipsoid heights disagree with each other by 10.2 cm. I like a quote that is attributed to Mark Twain – “It ain’t what you don’t know that gets you into trouble. It’s what you know for sure that just ain’t so.” (Obtained from http://lukefostvedt.com/famous-quotes-about-statistics/). I’m not suggesting that Opus Shared solutions results are incorrect. I’m attempting to provide an example of why users need to repeat all observations and to demonstrate how error estimates can be misleading.

“It ain’t what you don’t know that gets you into trouble.It’s what you know for sure that just ain’t so.”

Mark Twain

(Source: http://lukefostvedt.com/famous-quotes-about-statistics/).

Two OPUS Shared Solution for PID SV0942

(Source: NGS website)

07/14/2018 OPUS Solution

12/09/2014 OPUS Solution

The number of GPS on Bench Mark stations completed as of July 27, 2018, represents about 30 percent of the total number of stations that need to be observed. As I have explained in previous columns, there are many invalid GPS on BMs stations that may be used in the next hybrid geoid model unless more bench marks with valid NAVD 88 heights are observed with GNSS. NGS will accept data for inclusion in the next hybrid geoid model, GEOID18, until the end of August 2018. After that, NGS’ GPS-on-Bench-Mark Program will expand to include other regions and will focus on data to improve NGS datum transformation tools. This column provided an update and status report on stations observed in support of the 2018 GPS on BMs program, provided an example of how the OPUS Shared results can be used to identify a station that may have moved since it was last leveled, and the importance of repeating OPUS observations. I would encourage users to register for NGS’ next webinar on the GPS on Bench Mark Program scheduled for Thursday, Aug. 9th to hear the latest status of the program.

August 1, 2018

NGS 2018 GPS on BMs program in support of NAPGD2022 — Part 7

My last column described the National Geodetic Survey’s (NGS) GPS on Bench Marks (BM) 2018 interactive web map, and provided an update and status report on stations observed in support of the 2018 GPS on BMs Program. It mentioned that all new data received by the cut-off date of Aug. 31 will be analyzed by NGS and, if appropriate, the results will be included in the next hybrid geoid model. This is a great opportunity to provide data that will help to improve the hybrid geoid model in your region. This column will provide an update and status report on stations observed in support of the 2018 GPS on BMs program and provide an example of how OPUS-shared results identified a station that may have moved since it was last leveled.

As mentioned in the last column, the GPS on BMs 2018 web page contains a link to a web map where users can determine which bench marks NGS would like users to occupy before the August 31, 2018, deadline. The web map also provides a list of the stations observed to date to ensure users are not wasting their time observing stations that already have enough repeat observations. NGS is updating the map weekly to reduce users occupying stations that already have enough redundant observations. The box titled “2018 Web Map” depicts the map update of May 25, 2018. The web map has a search feature so if the user knew a station’s PID, they could locate the station on the map. The box titled “An Example of Using the Web Map Search Feature” depicts the search feature using PID AW0690 (see highlighted section in the box).

2018 Web Map

Click to enlarge.

The box titled “Map After Searching for PID KW0690” depicts the map after searching for PID KW0690. As indicated by the symbol, the station meets the current criteria. That is, it has two GNSS-derived ellipsoid heights that agree within NGS’ criteria for use in evaluating and generating the next hybrid geoid model.

Map After Searching for PID KW0690

Click to enlarge.

The user can continue to check on the link labeled “Datasheet” to obtain the latest data sheet for the station (see the box titled “NGS Data Sheet for KW0690”).

NGS Data Sheet for KW0690

Click to enlarge.

Next, let’s look at the OPUS shared results for the station (KW0690 – G 171). OPUS shared solutions can be found at this website. (see box tilted “OPUS Shared Solutions Web Page”).

OPUS Shared Solutions Web Page

Click to enlarge.

The user can search for a particular OPUS shared solution by checking on the PID option (see highlighted section on the box titled “Web Page After Clicking on PID Option.”

Web Page After Clicking on PID Option

Click to enlarge.

The box titled “An Example of Selecting an OPUS Shared Solution for a PID” depicts the output after clicking on the button labeled “List Marks.”

An Example of Selecting an OPUS Shared Solution for a PID

Click to enlarge.

The box titled “The OPUS Shared Solution for KW0690 (2018-03-20)” provides the OPUS Shared solution for station KW0690 performed on March 20, 2018. The output provides the NAD 83 (2011) 2010.0 coordinates with error estimates.

The OPUS Shared Solution for KW0690 (2018-03-20)

Click to enlarge.

When there is more than one observation, the output file provides a link to the other observations. In this case, there was another shared solution on March 31, 2014 (see box titled “The OPUS Shared Solution for KW0690 (2014-03-31).”) The two solutions indicate the ellipsoid heights agree to 8 mm (129.269 m – 129.261 m). This is an indication that the station is a valid candidate to be considered for the development of the hybrid geoid model.

The OPUS Shared Solution for KW0690 (2014-03-31)

Click to enlarge.

The second OPUS Shared solution also indicates that there is a third observation (2005-03-19). Clicking on that link provides the NGS data sheet (see box titled “Excerpt from NGS Data Sheet for KW0690”).

Excerpt from NGS Data Sheet for KW0690

Click to enlarge.

It should be noted that this station doesn’t have a published NAD 83 (2011) coordinate. The OPUS shared solutions provide the NAD 83 (2011) ellipsoid height and the NGS data sheet provides the published NAVD 88 orthometric height. Comparing the GNSS-derived orthometric height using the OPUS shared ellipsoid heights and GEOID12B indicate the station is inconsistent with published NAVD 88 orthometric height. The box titled “Table of GPS on BMs Residuals for KW0690” provides the GPS on BMs residuals based on using the latest hybrid geoid model GEOID12B. It was noted that the two ellipsoid heights agree to within 8 mm but the GNSS-derived orthometric heights using GEOID12B indicate that the two stations disagree with the published NAVD 88 height by almost 10 cm. This may be an indication that the station may have moved since the last time it was leveled. The question that needs to be addressed is should this station be used in the development of the next hybrid geoid model. In my mind, there are basically two school of thought on this topic. One, users that have used this individual station as control would like the hybrid geoid model to provide a GNSS-derived orthometric heights consistent with the published height of this station. If a surveyor followed the appropriate precise leveling procedures to check the validity of the station, that is, performed at least a two-mark leveling tie to ensure that the monument did not move, then they would want the model to be consistent with the published value. Two, if the station moved since it was last leveled, the hybrid geoid model would not provide the most accurate NAVD 88 height.

Table of GPS on BMs Residuals for KW0690

Click to enlarge.

The next step is to analyze the GNSS-derived orthometric height using the latest experimental geoid model. Evaluating GPS on BMs stations nearby station KW0690 will help in determining if the station KW0690 has moved since the last time it was leveled. One way that users can determine stations nearby is to use NGS data sheet retrieval program using the option to retrieve stations by point radius. See box titled “Using NGS Data Sheet Point Radius Retrieval Option for KW0690.” The user enters a latitude and longitude value and a radius search in miles.

Using NGS Data Sheet Point Radius Retrieval Option for KW0690

Click to enlarge.

In this case, I entered the latitude and longitude of station KW0690, a radius of 20 miles (approximately 30 kilometers) and selected the option “GPS Stations Only.” The box titled “Output of NGS Data Sheet Point Radius Retrieval Option for KW0690” provides the output of the search. I sorted the stations by vertical control (“V”)

Output of NGS Data Sheet Point Radius Retrieval Option for KW0690

Click to enlarge.

The four bench marks that also have GNSS-derived heights are highlighted in yellow in the box titled “Select the Bench Marks Based on NGS Data Sheet Point Radius Retrieval.” They are all within 20 miles (approximately 30 km) of the station KW0690. By analyzing the GPS on BMs residuals of these nearby stations we can determine if station KW0690 is consistent with its neighbors.

Select the Bench Marks Based on NGS Data Sheet Point Radius Retrieval

Click to enlarge.

I retrieved the data sheets so I could get their published coordinates for the xGeoid17 web tool. See box titled “Excerpts from Data Sheets Based on NGS Data Sheet Point Radius Retrieval” for the data sheets.

Excerpts from Data Sheets Based on NGS Data Sheet Point Radius Retrieval

Once you have the stations that are located near the station you’re interested in you can proceed to the xGeoid17 website to obtain the latest information based on the scientific geoid model. I described this procedure in a previous column. See box titled “Using the xGeoid17 Web tool for Stations Nearby KW0690” for an example of the input to the tool.

Using the xGeoid17 Web tool for Stations Nearby KW0690

Click to enlarge.

The table titled “Table of GPS on BMs Residuals for KW0690 Using xGeoid17b” provides a summary of the results from the xGeoid17 web page. The procedure used to compute the GPS on BMs residuals has been described in a previous column.

Table of GPS on BMs Residuals for KW0690 Using xGeoid17b

Click to enlarge.

Looking at the column labeled “[GNSS-Derived Orthometric Height (using xGEOID17B) minus Published NAVD 88 Height] minus Average Difference” indicate that the large difference that we noticed using GEOID12B at station KW0690 is also seen using the latest experimental geoid model xGeoid17b. Once again, this is an indication that the station may have moved since it was last leveled.

As of May 29, 2018, 1067 of the 5760 priority marks were completed. The box title “Status of NGS 2018 GPS on BMs Program as of May 29, 2018“ is a plot the stations that are labeled as completed and the box titled “Count of Stations Completed by State “ provides the number of stations completed by state. The red triangles are priority A stations completed and the blue “X” are priority B stations labeled as completed.

Status of NGS 2018 GPS on BMs Program as of May 29, 2018

Click to enlarge.

Count of Stations Completed by State
May 29, 2018

The number of stations completed as of May 29, 2018, represents about 18.5 percent of the total number of stations that need to be observed. August 31, 2018, is only two months away. According to my latest search of the NGS website (June 3, 2018), 1098 stations are considered done. Hopefully, the number of completed stations will significantly increase during the next last two months. As I have explained in previous columns, there are many invalid GPS on BMs stations that may be used in the next hybrid geoid model unless more bench marks with valid NAVD 88 heights are observed with GNSS. This column provided an update and status report on stations observed in support of the 2018 GPS on BMs program and provided an example of how the OPUS Shared results as identified a station that may have moved since it was last leveled. This is your opportunity to help develop a current, valid hybrid geoid model in your area, and identify NAVD 88 bench marks that have moved since they were last leveled.

June 6, 2018
NGS 2018 GPS on BMs program in support of NAPGD2022 — Part 6
My last column described how the U.S. National Geodetic Survey (NGS) used the detailed analysis of the latest GPS on Bench Marks dataset to:
1. generate a prototype hybrid geoid model to evaluate the residuals at stations not used in the hybrid geoid model,
2. confirm that the stations recommended for re-observations should be observed again, and
3. identify void areas that need additional observations.
Since GEOID12B was created, users have been instrumental in providing OPUS with results on benchmarks in areas where NGS said that additional stations were needed. It showed how NGS used the detailed analysis to prepare material to assist users on strategically occupying stations to help support the GPS on Bench Marks Program and create a hybrid geoid model that accurately represents a current NAVD 88.

To eliminate confusion of where NGS would like new observations, NGS’ material contains a specific list of stations that it would like occupied with GNSS during the 2018 GPS on BMs program. My previous column provided a summary of the latest details of NGS’ 2018 GPS on BMs campaign, which will be used to create the next hybrid geoid model in 2019.

The analysis described in my column was the first cut at identifying stations that should not be used in a hybrid geoid model, and providing a list of specific stations that could help improve the hybrid geoid model. All new data received by the cut-off date of Aug.31, 2018, will be analyzed by NGS and, if appropriate, the results will be included in the next hybrid geoid model.

This is a great opportunity to provide data that will help to improve the hybrid geoid model in your region.

This column will describe NGS’ GPS on BMs 2018 interactive web map and provide an update and status report on stations observed in support of the 2018 GPS on BMs Program.

First, NGS has a web page dedicated to the 2018 GPS on BMs program. See the box titled “GPS on Bench Marks Web Page.”

GPS on Bench Marks Web Page

The GPS on BMs 2018 web page contains a link to a web map where users can determine which bench marks NGS would like users to occupy before the Aug.31 deadline. On the left-hand side of the web page there is a link titled “2018 Web Map” (see highlighted section of box titled “GPS on Bench Marks Web Page”). The next few boxes demonstrate how a user can use the web map tool to locate bench marks in their local area of interest. The box titled “2018 Web Map” depicts what the user will see when the link “2018 Web Map” is clicked.

2018 Web Map

The user can then click on the map and the tool will provide more details. The box titled “Map After Clicking on Priority Mark Cluster #488 in the Great Plains Region“ is a depiction of the map after clicking on a priority mark cluster.

Map After Clicking on Priority Mark Cluster #488 in the Great Plains Region

The user can continue to check on the map until the map depicts individual bench marks where the symbology indicates the status of the monuments. The symbology labels are fairly straightforward. The box titled “The Web Map Symbology” provides the five different categories of monuments.

The Web Map Symbology

NGS is updating the map weekly to reduce users occupying stations that already have enough redundant observations. Clicking on a station provides the status of the station. The box titled “An Example of a Priority A Station” depicts station (PID KZ1401) that is labeled as a Priority A station and requires two observations.

An Example of a Priority A Station

The user can obtain the datasheet for the station by clicking on the Datasheet button in the box (see box titled “Excerpt from the Datasheet for PID KZ1401”).

Excerpt from the Datasheet for PID KZ1401

The box titled “An Example of a Priority B Station” depicts a priority B station (PID PM0117) that NGS would like one more observation. Users should remember that priority A stations are more important than priority B stations but B stations are still important for the development and analysis of the hybrid geoid model.

An Example of a Priority B Station

The box titled “An Example of a Station that Meets Current Criteria” provides an example of a station that does not need any more observations. As previously stated, NGS will be updating this web map on a regular basis so users will not waste their time and resources.

An Example of a Station that Meets Current Criteria

The web map has a search feature, so if the user knew a priority A or B station’s PID, they could locate the station on the map. The box titled “An Example of Using the Web Map Search Feature“ demonstrates the search feature using PID JX1344 (see highlighted section in the box).

An Example of Using the Web Map Search Feature

The box titled “Output from Search Feature for PID JX1344“ is a depiction of the output using the search feature.

Output from Search Feature for PID JX1344

The last category of stations that are shown on the web map are monuments that are reported as unfounded or not GPSable. This is very useful information for NGS and others to have on datasheets. The box titled ” Output from Search Feature for PID JX1344 “ depicts bench mark PID JX1344 that is labeled as unfound or not GPSable. The datasheet for JX1344 indicates that the bench mark is set vertically in a rock ledge (see highlighted section in the box titled “Excerpt from the Datasheet for PID JX1344.”

Excerpt from the Datasheet for PID JX1344

As of March 30, 362 of the 5745 priority marks have been completed. The box titled “Status of NGS 2018 GPS on BMs Program as of March 30, 2018“ is a plot of the stations that are completed, and the box titled “Count of Stations Completed by State “ provides the number of stations completed by state. The red triangles are priority A stations completed and the blue “X” are priority B stations labeled as completed.

It appears that the central portion of the country has been very active. For example, there are 34 priority A stations completed in Missouri and 28 completed in Kansas. The State of Florida has completed 45 priority B and nine priority A stations for a total of 54 stations (see box titled “Count of Stations Completed by State “).

Status of NGS 2018 GPS on BMs Program as of March 30, 2018

Count of Stations Completed by State

March 30, 2018

The number of stations completed to date represents about 6 percent of the total number of stations that need to be observed. Aug. 31 is only five months away. Hopefully, the number of completed stations will significantly increase during the next several months.

If you have a GNSS receiver, please identify a priority monument nearby and occupy it. As I have explained in previous columns, there are many invalid GPS on BMs stations that may be used in the next hybrid geoid model unless more benchmarks with valid NAVD 88 heights are observed with GNSS.

Please encourage your fellow surveyors and friends to occupy a benchmark to support the next NGS hybrid geoid model. This is your opportunity to help develop a current, valid hybrid geoid model in your area.
April 4, 2018