Tag: NGS

NGS 2018 GPS on BMs program in support of NAPGD2022 — Part 5

My last column highlighted two components of the North American-Pacific Vertical Datum of 2022 (NAPGD2022) — the geoid undulation model of GEOID2022 and gravity model of GRAV2022. It expressed that these two models will be very important to future surveyors and mappers that are incorporating geodetic data into NAPGD2022. The last column also emphasized the significant differences between NAPGD2022 and the U.S. National Vertical Datums of NAVD 88 and NGVD 29. A year ago, my February 2017 column provided information on strategically occupying benchmarks to support NGS 2017 GPS on BM Program. The column focused on addressing the following questions: (1) Is the large GPS on BM residual due to an issue with the NAVD 88 orthometric height or the NAD 83 (2011) ellipsoid height? and (2) Should stations with large GMS on BM residuals be included in the development of NGS’ hybrid geoid models? The column provided suggestions on how users can assist NGS in determining the reason for the large difference between the modeled hybrid geoid value and computed GNSS/leveling geoid computed value. My October 2016 column demonstrated how to use the GPS on BMs dataset to identify potential issues in published NAVD 88 and NAD 83 (2011) heights. It focused on analyzing the NGS’ GPS on BM data set that was used to create NGS’ GEOID12B hybrid geoid model. It provided procedures that users could employ when analyzing the differences between the modeled geoid values and the computed geoid values using GNSS/Leveling data (GNSS-derived ellipsoid height minus leveling-derived orthometric height). The October 2016 column provided several examples of large relative differences in residuals between neighboring stations.

It should be noted that many of these large GPS on BM residuals could be due to an invalid NAVD 88 published height because the bench mark moved since the last time the height of the bench mark was adjusted and published, and/or an undetected error in an ellipsoid height due to a weak GNSS project design. Either way, in my opinion, most of these stations with large GPS on BMs residuals don’t accurately represent a bench mark with a current NAVD 88 height (or what I call a valid NAVD 88 height). When performing a geodetic survey, these stations would be identified as bench marks with invalid heights when following the appropriate Federal geodetic survey guidelines, procedures, and specifications. These bench marks should not be used in the hybrid geoid model just like they would not be used in controlling geodetic surveys. NGS’ goal is to create a hybrid geoid model that is consistent with published valid NAVD 88 values. User participation in NGS’ GPS on BMs Program is critical to creating a hybrid geoid model consistent with a current NAVD 88.

Recently, NGS performed a detailed analysis of the latest GPS on BMs data file using the published NAD83 (2011) ellipsoid heights, NAVD 88 orthometric heights, and the latest experimental geoid model height, xGeoid17b, to compute a new set of GPS on BMs residuals. At this time, the analysis has only included the 48 conterminous States, District of Columbia, Puerto Rico, and Virgin Islands. These data included NAD 83 (2011) ellipsoid heights from all submitted GNSS projects and OPUS Shared results. The goal of the detailed analysis was to create a statistical ranking of the marks based on a quantitative analysis of the leveling and GPS data. The following attributes were considered during the analysis:

Total number of GPS observations to and from the station
Date of last GPS observation to and from the station
Whether or not the GPS station has repeat baselines between closely spaced neighboring GPS on BMs stations
Total number of times the mark has been leveled to
Date of latest leveling
Quality of leveling (single run; double run; or single run, double simultaneous)

The analysis of this data set was used to identify stations that should not be used in the creation of a hybrid geoid model or a NAPGD2022 Transformation tool. The stations identified as outliers and labeled as “Do Not Use” in a hybrid geoid model were based on issues associated with the NAVD 88 published orthometric height and/or the NAD83 (2011) ellipsoid height. I have described some of these issues in previous columns (August 2015 column, June 2016 column, October 2016 column and February 2017 column) so I won’t go into details in this column. NGS used the detailed analysis of the latest GPS on BMs 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 results on bench marks in areas NGS said that they needed additional stations. Saying that, NGS realizes that everyone is busy and has limited resources to collect GNSS data on bench marks to support the next hybrid geoid model. NGS has 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 they would like occupied with GNSS during the 2018 GPS on BMs program. This column provides 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 (see box titled “Personal Communication received from Galen Scott, Project Lead of NGS’ GPS in BM Program.”).

Personal Communication received from Galen Scott, Project Lead of NGS’ GPS on BM Program

In early 2019, NOAA’s National Geodetic Survey (NGS) will replace GEOID12B with GEOID18, a new hybrid geoid model to deliver improved GPS-derived NAVD 88-equivalent orthometric heights. This new model will serve as the official means for obtaining NAVD 88-equivalent heights via GPS. It will be the last hybrid geoid model that NGS will create before NAVD 88 is replaced by NAPGD2022.NGS will use available GPS on bench mark data to create the new model. Recent analysis of existing GPS on bench mark data and a prototype of the new hybrid geoid model created using that data has highlighted areas where additional data is needed to either confirm or update the local relationships between the ellipsoid, orthometric, and geoid heights.

This email provides a prioritized list of bench marks for which additional GPS data is needed to improve the hybrid model. Data submitted on these marks will also support the development of the transformation tools that will be developed as part of the transition to the new datums.

Data to support the hybrid geoid model will be accepted through August 31, 2018. NGS will continue to accept data to support the transformation tools through 2020. New prioritized lists of marks to support the transformation tools will be made available over the next few years as analysis of data requirements progresses.

For the marks included in the attached document, NGS is requesting support in two ways:

Attempt to locate the marks on the list and submit a mark recovery through DS World. Check this NGS page for more information on mark recovery.
Collect 4 or more hours (more is better) of GNSS data on the mark following NGS guidelines, submit the data to OPUS and select the option to Share.

More information, including training material, is available on the NGS GPS on Bench Marks (GPS on BM) website. Two matching, independent GPS observations are required for each mark. The list indicates how many observations we have so far on each mark (obs_cnt column). A tracking map showing the currently prioritized marks and the number of observations we have on each will be added to the GPS on BM website in the near future. To maximize efficiency, please check this map before observing a mark to ensure that the required data has not already been submitted.

Please note: Marks on this list may be inaccessible, destroyed, or not GPS’able. If this is the case, please locate and observe another nearby NAVD 88 mark, within ~10 km.

The mark list is provided in three file formats, but all contain the same information, so choose the format you are most comfortable with: excel spreadsheet, esri shapefile, and Google Earth kmz.

The image below shows the changes between GEOID12B and the prototype hybrid geoid model. While data is needed on all the marks in the list, you may further focus your data collection efforts by looking for areas in this image that show large changes in your region.

It is important for users to understand that NGS needs to have a high level of confidence that the OPUS Share results are accurate; therefore, they are requiring that “two matching, independent GPS observations are required for each mark.” The list of stations that they would like observed includes a count of the number of times that station has already been observed. NGS will be updating a website as stations are submitted so participants will not be wasting resources observing a station that has already been observed by someone else. It should be noted that if a station is only occupied once, it will still be useful for validating the hybrid geoid model; but stations occupied twice can be used in defining the hybrid geoid model.

The attached file includes the list of stations that NGS would like observed to support the next geoid model. The information is provided in three different formats — excel spreadsheet, esri shapefile, and Google Earth kmz (See the box titled “List of Files for the 2018 GPS on BMs Program.”)

List of Files for the 2018 GPS on BMs Program

The data set also contains a folder titled “GEOID Model Changes by Region” which contains plots that depict changes between GEOID12B and the Prototype Hybrid Geoid Model (Note: at this time, NGS is denoting this prototype hybrid geoid model as GEOID18v2.2).

List of Files from Folder Titled “GEOID Model Changes by Region”

Figure 1 is a plot of the change between the prototype GEOID18v2.2 and GEOID12B in the Mid-Atlantic States. Looking at figure 1, the reader can see that there are some significant differences between the prototype hybrid geoid model values and the published GEOID12B values. On figure 1, all of the dark blue values are differences at the -10 cm level and the dark orange values are differences at the 10 cm level. There are several reasons for these changes including newly observed gravity data observations (especially in area with new GRAV-D data), improved data and models from satellites programs, new and improved algorithms for processing gravity data and estimating topographic effects, additional OPUS Share results in areas where GEOID12B didn’t have observations, and differences based on stations that were included in GEOID12B but rejected in the prototype model based on the latest detailed analysis.

Figure 1 – Changes between Prototype GEOID18v2.2 and GEOID12B in the Mid-Atlantic States (units = meters).

As previously mentioned, the list of stations that NGS would like observed with GNSS are provided in three formats: excel spreadsheet, esri shapefile, and Google Earth kmz. The box titled “Sample Data Elements Extracted from the Excel File Titled “gpsonbm_priority_list_20180205.xlsx” provides a sample of the data from the excel file. The box titled “Definition of Columns of GPS on BMs data file” provide the columns and a brief definition of the data field.

Sample Data Elements Extracted from the Excel File Titled “gpsonbm_priority_list_20180205.xlsx”

The priority column has two entries – A or B. Priority A is more important than priority B. In other words, if the user has to make a choice, NGS would like the priority A station observed first. The obs_cnt field will be updated as users submit their OPUS Shared results. Remember, NGS is requiring two matching, independent GPS observations for the station to be included in the development of the hybrid geoid and transformation tool.

The near_pid provides the pid of the station that is near the original station. The selection of the near_pid was based on the original station’s position and a search of the NGS database for a station within 5 to 15 kilometers of the original station. NGS’ analysis indicated that the original GPS on BMs station may have moved so an additional observation on the same station will not help to generate a hybrid geoid model that represents the current NAVD 88. It would warp the geoid model to fit the published NAVD 88 height but if the station moved since it was last leveled to, then it does not have a valid NAVD 88 height. As previously stated, when performing a geodetic survey, these stations would be identified as bench marks with invalid heights when following the appropriate Federal geodetic survey guidelines, procedures, and specifications. The surveyor would then level to another bench mark until they met the survey’s specifications. These bench marks with invalid heights should not be used in the hybrid geoid model just like they would not be used in controlling geodetic surveys. If the near_pid column is “n-a” then NGS would like the original station observed.

The box titled “Number of Priority Stations in Each State” provides the number of priority A and B stations for every State in the lower 48, the District of Columbia, Puerto Rico, and the Virgin Islands. Overall, there are 6082 stations in the list – 3544 Priority A stations and 2538 Priority B stations.

Number of Priority Station in Each State

As an example of a State in eastern United States, the box titled “List of PIDs of Priority “A” and “B” Stations in North Carolina” provides the list of priority A and B stations that need to be observed in North Carolina. The box titled “List of PIDs of Priority “A” Stations in North Carolina” provides the list of priority A stations in North Carolina. Figure 2, titled “NGS 2018 GPS on BMs Program, Priority A and B Stations in North Carolina,” depicts the locations of these stations. Figure 3 depicts the location and PID of the priority A stations in western North Carolina. Figure 4 depicts the same stations with their Obs_Cnt value.

List of PIDs of Priority “A” and “B” Stations in North Carolina That Need to be Observed
Information extracted from Excel File Titled “full_priority_list.csv”

(Note: The stations in this table may not be the final list of priority A and B. The attached zip file contains the latest list of stations. The latest list was received too late to modify the table.)

List of PIDs of Priority “A” Stations in North Carolina

Figure 2 – NGS 2018 GPS on BMs Program – Priority A and B Stations in North Carolina.

Figure 3 – NGS 2018 GPS on BMs Program – Priority A Stations in Western North Carolina With the PID of the Station.

Figure 4 – NGS 2018 GPS on BMs Program – Priority A Stations in Western North Carolina With the Number of Observations.

For completeness, I will provide an example of a region in the western United States – California and Nevada. They are larger States than North Carolina and have more Priority A stations that need to be observed. Figure 5 depicts the Priority A and B stations in California and Nevada, and figure 6 depicts the Priority A stations in California and Nevada. It is recognized by NGS that managing how these stations are observed and who does what is a monumental task. Some state agency may undertake observing all of the Priority A stations; for example, Gary Thompson, Chief of the North Carolina Geodetic Survey, has committed to observing all of the Priority A stations (personal communication). Other States have County and City surveyors that will help observe and manage the process. All of the information provided in the 2018 GPS on BMs allow individuals to sort the data in ways that meet their needs. For example, the box titled “List of Priority “A” Stations by County in California” provide the list of stations in California by county.

Figure 5 – NGS 2018 GPS on BMs Program – Priority A and B Stations in California and Nevada.

Figure 6 – NGS 2018 GPS on BMs Program – Priority A Stations in California and Nevada.

It should be noted that NGS identified the priority stations based on hybrid geoid requirements. The NGS geoid team would desire a valid GPS on BMs observation every 30 km. Therefore, some of the priority A stations are in areas void of any GPS on BMs stations. There may be many reasons for this but, most likely, it’s because it’s located in an unpopulated or mountainous region of the county. Either way, it may be difficult to obtain observations at these stations. The new hybrid geoid model will be created using whatever data are available. In these void areas, the geoid will be controlled by the nearest GPS on BMs stations. There is nothing wrong with this approach. The only issue will be that it will not be possible to evaluate the relation of the hybrid geoid model and NAVD 88 in these void areas. Figure 7 depicts the priority A stations and the population of cities in Northwestern Nevada and Northeastern California. The figure indicates that these priority A stations are located in an unpopulated region of Nevada. It’s obvious why there’s no GPS on BMs in this region since nobody lives there but the geoid doesn’t depend on population. In any event, if the user can obtain an observation in these regions it will really help in creating an accurate hybrid geoid model.

List of Priority “A” Stations by County in California

NGS’ process for determining which stations were outliers and which stations should be re-observed involved analyzing both GNSS and leveling data from NGS’ database. The GPS on BMs residuals were computed using the procedure described in the box titled “Procedure for Computing the GPS on BMs Residuals.”

Figure 7 – NGS 2018 GPS on BMs Program – Priority A Stations in California and Nevada. (Numbers are 2012 Population Values from Census – ESRI online)

Figure 8 depicts the location of the GPS on BMs stations in Illinois. The box titled “Summary of Statistics for GPS on BMs Residuals in Illinois” provides a summary of the GPS on BMs residuals for the State of Illinois. The results indicate that there are 804 GPS on BMs in Illinois and the residuals range between -14.1 cm to 31.2 cm. They have a mean of 6.0 cm with a standard deviation of 4.6 cm. The table titled “Statistics for GPS on BMs Residuals in Illinois With Rejections Removed” indicates that most residuals fall between 2 and 10 cm. The box titled “Summary of Positive and Negative Statistics for GPS on BMs Residuals in Illinois” provides a summary of the statistics for the positive and negative set of residuals.

Figure 8 – GPS on BMs Stations in the State of Illinois.

Figure 9 depicts the GPS on BMs residuals in the Springfield, Illinois, Region. During the detailed analysis of the latest GPS on BMs dataset, the analysts identified outliers that appeared to be large relative to their neighbors. Figure 9 depicts these outliers with a “X.” Stations designated with a “X” are stations that were designated as DO NOT USE in the creation of the hybrid geoid model. Figure 9 also indicates were the analyst recommended that a station should be observed before the creation of the next hybrid geoid model. These stations are labeled as Priority A stations on figure 9. Figure 10 is an enlargement of the same area that depicts a station that was recommended to be rejected in the hybrid geoid model (PID KB0702). The stations surrounding PID KB0702 all seem to be consistent with each other (residuals in smaller blue squares) so the analyst recommended that station KB0702 be rejected. At the same time, by rejecting this station, this creates a void area that needs to be filled. Therefore, the analyst also recommended that a new station be observed here; hence, the two priority A station plotted near the rejected station. Figure 11 is a plot of another rejected station (KB1018) in the same region but, in this case, the analyst did not recommend an additional observation in the area because there was another nearby station (station in red triangle) that was consistent with its neighbors (residuals in smaller blue squares).

Figure 9 – GPS on BMs Residuals Using xGeoid17b and Priority A Stations in Springfield, Illinois, Region (unit cm).

Figure 10 – GPS on BMs Residuals Using xGeoid17b – An Example of a Rejection (PID KB0702) Resulting with a Recommendation of a Priority A Station (units cm).

As previously mentioned, and provided in the box titled “Attributes Considered During Analysis,” several attributes were analyzed before making the recommendations but, typically, GPS on BMs residuals between +/- 5 cm were used to identify which stations needed to be investigated.

Attributes Considered During Analysis

➢ Total number of GPS observations
➢ Date of last GPS observation
➢ Whether or not the GPS station has repeat baselines
➢ Total number of times the mark has been leveled to
➢ Date of latest leveling
➢ Quality of leveling

Figure 11 – GPS on BMs Residuals Using xGeoid17b – An Example of a Rejection (PID KB1018) of an Outlier (units cm).

This analysis is 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 August 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. My next column will provide a status report on the 2018 GPS on BMs Program.

February 7, 2018

Discussing the new North American-Pacific Geopotential Datum of 2022 — Part 4

My last column focused on the National Geodetic Survey’s (NGS) current plans for estimating North American-Pacific Geopotential Datum of 2022 (NAPGD2022) GNSS-derived orthometric heights and incorporating geodetic leveling data into NAPGD2022 to establish orthometric heights consistent with GNSS-derived NAPGD2022 orthometric heights. It emphasized that after NAPGD2022 is established, the primary means for deriving orthometric heights on monuments will be using GNSS observations combined with the geoid model.

Recently, NGS published its second blueprint for the 2022 document titled “Blueprint for 2022, Part 2: Geopotential Coordinates.” The report addresses NAPGD2022 in detail. The intent of the document is to provide to the public the current status of plans by NGS to modernize the geopotential component of the National Spatial Reference System (NSRS) in 2022. This particular document covers the definition and determination of orthometric heights, geoid undulations, gravity, deflections of the vertical, dynamic heights, and any other quantity directly related to the geopotential field of the Earth. As mentioned my previous columns, NAPGD2022 will be replacing the North American Vertical Datum of 1988 (NAVD 88). The executive summary of report NGS 64 is provided in the box titled “Executive Summary, NOAA Technical Report NOS NGS 64, Blueprint for 2022, Part 2: Geopotential Coordinates.” Surveyors and mappers should obtain a basic understanding of the four interrelated products of NAPGD2022. They are GM2022, GEOID2022, DEFLEC2022, and GRAV2022. I’ve highlighted them in executive summary box below.

Executive Summary

NOAA Technical Report NOS NGS 64
Blueprint for 2022, Part 2: Geopotential CoordinatesIn 2022, the entire National Spatial Reference System (NSRS) will be modernized. This document addresses the geopotential aspects of the NSRS, including every vertical datum, the geoid, gravity, deflections of the vertical, and other quantities related to Earth’s gravity field. Every one of these related, yet semi-independent sources of information will be replaced with an internally consistent geopotential datum called the North American-Pacific Geopotential Datum of 2022 (NAPGD2022). Within NAPGD2022 four primary, interrelated time-dependent products will exist:

A global model of Earth’s geopotential field (GM2022)
Regional gridded geoid undulation models (GEOID2022)
Regional gridded deflection of the vertical models (DEFLEC2022)
Regional gridded surface gravity models (GRAV2022)

The three regions for the gridded models will be North America (covering CONUS, Alaska, Hawaii, the Caribbean, Canada, Mexico, Central America and Greenland), American Samoa and Guam/Commonwealth of Northern Mariana Islands (CNMI).

NAPGD2022 will be built upon the IGS frame, as only minor (entirely horizontal) differences will exist between the IGS frame and the four new terrestrial reference frames developed as part of the NSRS in 2022 (see NGS, 2017). Since these differences will be relatively small horizontal displacements (mainly due to Euler pole rotations), NAPGD2022 will operate equally well in any of four new frames.
Orthometric heights in NAPGD2022 will be defined through ellipsoid heights and GEOID2022. This means NAPGD2022 orthometric heights will primarily be accessed through Global Navigation Satellite System (GNSS) technology. GEOID2022 will be defined in a manner that best fits global mean sea level at the epoch of NAPGD2022. When global sea level changes by a threshold level of 20 centimeters, a new geoid model, and thus geopotential datum, will be released. Until then, updates to any component of NAPGD2022 will result in updating all components of NAPGD2022 using sequential version numbering.

Leveling in NAPGD2022 will retain its current role of providing high-accuracy local differential orthometric heights. The determination of absolute heights, however, which will provide the context of local differential heights, will reside in the GNSS domain (i.e., will be based on IGS ellipsoid heights).

Find this entire report here.

There is a lot of good information in the report and I would encourage everyone to download the report and read it. Some of the report is technical but most of it provides simple and easy to understand explanations of very technical terms. Pages 22 and 23 of NGS 64 provides a good summary of the different components of NAPGD2022 (see box tilted “Excerpt from Section 9 of NGS 64”).

Excerpt from Section 9 of NGS 64

9 The 2022 Geopotential Datum

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In 2022, the NSRS will contain one geopotential datum, capable of providing (at a minimum) the geoid undulation, acceleration of gravity, geopotential number, and deflection of the vertical at any given latitude, longitude, ellipsoid height, and time in a global ideal reference frame, such as the International Terrestrial Reference Frame (ITRF) or International GNSS Service (IGS) frames. The name of this datum will be the North American-Pacific Geopotential Datum of 2022 (NAPGD2022).

The foundational component of NAPGD2022 will be a spherical13 harmonic model of Earth’s external gravitational potential, called (for now) the Geopotential Model of 2022 (GM2022).

The GM2022 will be created for the entire Earth and will contain two components:

The first component will be time independent, fixed at some epoch (TBD14) to a at least degree and order of 2160,15 called (for now) the Static Geopotential Model 2022 (SGM2022).
Complementing SGM2022 will be a time-dependent model of Earth’s external gravitational potential, capable of capturing both secular and episodic changes of significance. This time-dependent model will be called (for now) the Dynamic Geopotential Model 2022 (DGM2022).

Three derivative products, based upon GM2022, but requiring additional information and providing higher-resolution regional information than is contained in GM2022 will be created:

A gridded geoid model GEOID2022,16 which will contain two components:

The first will be time independent, fixed at some epoch (TBD) called (for now) the Static Geoid model of 2022 (SGEOID2022).
Complementing this will be a time-dependent geoid undulation model, encompassing permanent geoid changes >= 1 millimeter per year, called the Dynamic Geoid model of 2022 (DGEOID2022).

A gridded deflection of the vertical, DoV, model (at the surface of the Earth) DEFLEC2022, which will contain two components:

The first will be time independent, fixed at some epoch (TBD) called (for now) the Static Deflection of the Vertical model of 2022 (SDEFLEC2022).
Complementing this will be a time-dependent DoV model, called the Dynamic Deflection of the Vertical model of 2022 (DDEFLEC2022).

A model for interpolating surface gravity GRAV2022, which will contain at least one, possibly two components:

The first will be time independent, fixed at some epoch (TBD) called (for now) the Static Gravity model of 2022 (SGRAV2022).
As a second, possible component, NGS will investigate the feasibility of a time-dependent surface gravity model.

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The three derivative-gridded products (GEOID2022, DEFLEC2022, and GRAV2022) will encompass three non-global areas. These three areas will be (latitude and longitude convention being positive north, positive east):

The boxes titled “Figure 9-1 From NOS NGS 64,” “9-2 from NOS NGS 64,” and “9-3 from NOS NGS 64” depict the regions that GEOID2022, DEFLEC2022 and GRAV2022 will cover.

Figure 9-1 From NOS NGS 64
The North American region for GEOID2022, DEFLEC2022 and GRAV2022

Figure 9-2 From NOS NGS 64
The American Samoa region for GEOID2022, DEFLEC2022 and GRAV2022

Figure 9-3 From NOS NGS 64
The Guam and CNMI region for GEOID2022, DEFLEC2022, and GRAV2022

So, what does this mean to the surveying and mapping community? First, as mentioned in my previous columns, there will be significant differences between NAPGD2022 and NAVD 88. Figure 1 depicts the approximate differences between NAPGD2022 and NAVD 88 in the conterminous United States.

Figure 1 – Approximate Change Between NAPGD2022 and NAVD 88 Using GPS on BMs Data (units = cm). [Figure 1 is from June 2017 Survey Scene column.]

For those still referring their products to NGVD 29, figure 2 depicts the approximate differences between NAPGD2022 and NGVD 29 in the conterminous United States.

Figure 2 – Approximate Change Between NAPGD2022 and NGVD 29 Using GPS on BMs Data (units = cm). [Figure 2 is from the June 2017 Survey Scene column].

My April 2017 Survey Scene column provided an estimate of the change between NAPGD2022 and NAVD 88 at bench marks with GNSS-derived ellipsoid heights in Alaska. Figure 3 is a plot of the GPS on BMs residuals computed using xGeoid16b geoid values, IGS08 ellipsoid heights, and NAVD 88 orthometric heights.

Figure 3 – Approximate Change Between NAPGD2022 and NAVD 88 Using GPS on BMs Data (units = cm). GPS on Bench Mark Residuals Using xGeoid16b in the State of Alaska – Referenced to IGS08 (units = cm) – Green Line Represents the Leveling Lines [Figure 3 is from the April 2017 Survey Scene column.

As outlined in NOS NGS 64 report and previously mentioned in this column, there are four interrelated products of NAPGD2022 – GM2022, GEOID2022, DEFLEC2022, and GRAV2022. What most surveyors will be using is GEOID2022 (SGEOID2022 and DGEOID2022). As explained in my last column, and part of NGS’ frequently asked questions about the new datums, users will access the NSRS using GNSS-derived ellipsoid heights and GEOID2022.

How will accessing the National Spatial Reference System (NSRS) change with the release of the new datums?The NSRS will be accessed using Global Positioning System (GPS) technology that references Continuously Operating Reference Stations (CORS) and relies on a time-dependent gravimetric geoid model. This method of accessing the NSRS is a paradigm shift from accessing NAD 83 and NAVD 88 through the use of geodetic survey marks.

It will not be necessary to connect to a geodetic monument, i.e., a bench mark, because the NATRF2022 ellipsoid height (hNATRF2022) is determined using the NGS CORS and the geoid model (NGEOID2022) is consistent with NATRF2022. In other words, GNSS ellipsoid heights (e.g., NATRF2022) combined with the geoid model (e.g., GEOID2022) will become the primary means for deriving orthometric heights on marks.

There will be a static geoid model of 2022, denoted as SGEOID2022, which will be fixed at a specific epoch. Since the geoid model changes due to various factors, such as changes in sea level, glacial rebound, and seismic activities, there will be a dynamic aspect of the 2022 geoid model, denoted as DGEOID2022. The permanent changes to the geoid model are small and will take several years to become significant to affect the typical survey and mapping product. Saying that, it is important to understand that there is a static and a dynamic aspect of the National geoid model. NGS will provide a single GEOID2022 value which will apply the appropriate static and dynamic components of the geoid model.

Even though, the primary access to NAPGD2022 will be using GNSS and a geoid model, users will still want to perform precise leveling observations and incorporate the results into NAPGD2022. My last column discussed incorporating leveling data into NAPGD2022. Differential leveling of high precision is used to observe elevation differences which are then used to establish precise heights of vertical control points (bench marks) above or below a reference surface, e.g., the North American Vertical Datum of 88 (NAVD 88) or North American-Pacific Geopotential Datum of 2022 (NAPGD2022). Differential leveling, conceptually a simple procedure, in practice lends itself to many types of small errors. To detect, reduce, and control these errors, specific procedures need to be adhered to and corrections must be applied. FGCS has documented the necessary procedures to be used in first-, second- and third-order geodetic leveling projects. Procedures do not always reduce error to tolerable values; therefore, additional corrections are applied by the office processing the data to remove known systematic errors.

The box titled “Excerpt from Special Report Results of the General Adjustment of the North American Vertical Datum of 1988” provides a summary of the corrections applied to the leveling data used in NAVD 88. As you can see, gravity (highlighted in the box) plays an important role in estimating accurate orthometric heights. This is where GRAV2022 is important, it is used during the process of converting observed leveling height differences into orthometric height differences.

Excerpt from Special Report – Results of the General Adjustment of the North American Vertical Datum of 1988
(https://www.ngs.noaa.gov/PUBS_LIB/NAVD88/navd88report.htm)
David B. Zilkoski, John H. Richards, and Gary M. Young
American Congress on Surveying and Mapping Surveying and Land Information Systems, Vol. 52, No. 3, 1992, pp.133-149Corrections Applied to Leveling Data
The leveling observations used in NAVD 88 were corrected for rod scale and temperature, level collimation, and astronomic, refraction, and magnetic effects (Balazs and Young 1982; Holdahl et al. 1986). All geopotential differences were generated and validated, using interpolated gravity values based on actual gravity data. Geopotential differences were used as observations in the least-squares adjustment, geopotential numbers were solved for as unknowns, and orthometric heights were computed using the well-known Helmert height reduction (Helmert 1890): H = C/(g + 0.0424H), where C is the estimated geopotential number in gpu, g is the gravity value at the benchmark in gals, and H is the orthometric height in kilometers. The weight of an observation was calculated as the inverse of the variance of the observation, where the variance of the observation is the square of the a priori standard error multiplied by the kilometers of leveling divided by the number of runnings.

This column highlighted two components of NAPGD2022 – the geoid undulation model of GEOID2022 and gravity model of GRAV2022. It expressed that these two models will be very important to future surveyors and mappers that are incorporating geodetic data into the North American-Pacific Vertical Datum of 2022 (NAPGD2022). As previously mentioned, I would encourage everyone to download and read NGS recently published second blueprint for 2022 document, titled “Blueprint for 2022, Part 2: Geopotential Coordinates.” This column also emphasized the significant differences between NAPGD2022 and the U.S. National Vertical Datums of NAVD 88 and NGVD 29. My next column will provide the latest details of NGS’ 2018 GPS on BMs campaign which will be used to develop transformation tools for converting products and services from NAVD 88 to NAPGD2022.

December 6, 2017

Discussing the new North American-Pacific Geopotential Datum of 2022 — Part 2

My last column highlighted some of the feedback provided by guest presenters at the NGS’ 2017 Geospatial Summit held on April 24-25 in Silver Spring, Maryland. That column also provided a discussion on the approximate differences between NAPGD2022 and NAVD 88 (and NGVD 29) at a national and local level. It was mentioned that to prepare for the new datums and develop implementation plans, users should obtain an understanding of the differences between NAPGD2022 and NAVD 88. The last column provided figures that depicted the approximate absolute and relative differences between the new vertical reference frame, North American-Pacific Geopotential Datum of 2022 (NAPGD2022) and NAVD 88. This column is the second in a new series of columns addressing topics associated with transitioning to the new North American-Pacific Geopotential Datum of 2022 (NAPGD2022).

The name of the National Geodetic Survey’s new vertical reference frame is the North American-Pacific Geopotential Datum of 2022 (NAPGD2022). So, what is a geopotential model? The following is the definition of a geopotential model from Wikipedia: “In geophysics, a geopotential model is the theoretical analysis of measuring and calculating the effects of Earth’s gravitational field.” [See the box titled “Definition of geopotential and geopotential model from Wikipedia.”]

Definition of geopotential and geopotential model from Wikipedia

In order for a height to a have physical meaning, the height system must have some relation to the Earth’s gravity field. Basically, for geodesists, a geopotential model is a way of measuring the effects of Earth’s gravitational field and the means to deriving a geoid model. So, what does the Earth’s gravity field look like? The box titled “Static Gravity Field – Anomalies” is a good image of the Earth’s gravity field created by the GRACE program.

Static Gravity Field – Anomalies
(Figure obtained from https://grace.jpl.nasa.gov/resources/28/)

It was mentioned in the last column that stakeholders across the federal, public and private sectors provided feedback and impacts of NGS New 2022 Datums on their products and services. All of these presentations are now available on NGS’ website. [See box titled “Website that contains the NGS 2017 Geospatial Summit Presentations.“] NGS did an excellent job of recording these presentations. The website allows the user to download the video and/or slides, as well as watch the presentations on their computer.

Website that contains the NGS 2017 Geospatial Summit Presentations
(https://www.ngs.noaa.gov/geospatial-summit/presentations.shtml)

Many surveyors and mappers will be providing services to Federal, state, and local agencies to assist them in their transitioning activities. I would encourage all users to watch the presentations by the partners to obtain an understanding of how these agencies’ products and services are going to be effected by a datum change. For example, the presentation by the Federal Emergency Management Agency (FEMA) can be found here.

This column will focus on two of the presentations by NGS employees – “Modernizing the Geopotential or Vertical Datum” and Monitoring Changes in the Geoid.” These two presentations are very important to obtaining an understanding of NAPGD2022. [See box title “NGS Presentation at the 2017 Geospatial Summit – “Modernizing the Geopotential or Vertical Datum.”]

NGS Presentation at the 2017 Geospatial Summit – “Modernizing the Geopotential or Vertical Datum”
(https://www.ngs.noaa.gov/geospatial-summit/presentations/modernizing-geopotential-vertical-datum.shtml)

Why is the Earth’s gravity field important to estimating GNSS-derived orthometric heights? Guidelines and procedures for estimating GNSS-derived heights were discussed in great detail in previous columns, such as Establishing Orthometric Heights Using GNSS — Part 1, Establishing Orthometric Heights Using GNSS — Part 2, Establishing Orthometric Heights Using GNSS — Part 3 and Establishing orthometric heights using GNSS — Part 4.

Slide 33 from the presentation titled “Modernizing the Geopotential or Vertical Datum” depicts the relationship between the ellipsoid, geoid, and orthometric heights. (See box titled “Slide 33 From “Modernizing the Geopotential or Vertical Datum.”)

Slide 33 From “Modernizing the Geopotential or Vertical Datum”
(https://www.ngs.noaa.gov/geospatial-summit/presentations/modernizing-geopotential-vertical-datum.shtml)

A previous column discussed how NGS developed their scientific and hybrid geoid models. The NAPGD2022 will begin with the best 3-dimension geopotential model available and derive the most accurate geoid model, e.g., GEOID2022, for establishing NAPGD2022 GNSS-derived orthometric heights. Just like NAVD 88 leveling derived heights need accurate gravity values to compute accurate orthometric heights and height differences, the geopotential model needs accurate, current gravity data to estimate local variations in the global model. The bottom line is that an accurate geopotential model is necessary for deriving an accurate geoid model that is necessary for establishing accurate GNSS-derived orthometric heights and height differences.

In the presentation “Modernizing the Geopotential or Vertical Datum,” Monica Youngman discussed the NGS project called “Gravity for the Redefinition of the American Vertical Datum (GRAV-D).” The goal of GRAV-D is to create a gravimetric geoid accurate to 1 cm where possible using airborne gravity data. The overall target is to enable users to obtain 2-cm accuracy orthometric heights from GNSS and a geoid model. View this website for more information on GRAV-D.

Once a geoid model is computed, e.g., GEOID2022, it will need to be validated to estimate the accuracy of the derived product. What does this mean to surveyors and mappers? In my opinion, the NAPGD2022 will help the surveying community maintain a vertical reference frame that’s reliable and traceable. Saying that, it is extremely important to know the relative accuracy of the geoid model used to establish GNSS-derived orthometric heights in NAPGD2022. As mentioned in my April column, NGS is performing geoid slope validation surveys (GSVS) to evaluate the current experimental geoid models being developed using GRAV-D data. In the presentation “Modernizing the Geopotential or Vertical Datum,” Derek Van Westrum discussed the GSVS projects. Evaluation of the experimental gravimetric geoid model is critical to the implementation of NAPGD2022 and should be part of a transition plan to the NAPGD2022. Performing a geoid slope validation project similar to NGS may be too expensive to be performed by most agencies. However, some agencies may be able to perform low budget geoid slope evaluation surveys. These surveys could include performing combined GNSS and leveling surveys to evaluate the relative accuracy of the gravimetric geoid model in areas that require accurate orthometric heights. Performing several of the gravimetric geoid evaluation surveys in major cities and/or areas that require accurate heights would help to facilitate the implementation of NAPGD2022.

These types of geoid evaluation surveys should be performed in areas of the country that are influenced by crustal movement. For example, in southern Louisiana and other parts of the Gulf Coast of the United States that are being influenced by subsidence (https://www.ngs.noaa.gov/heightmod/NOAANOSNGSTR50.pdf, https://www.ngs.noaa.gov/PUBS_LIB/Subsidence_at_Houston_Texas_TR_NOS131_NGS44.pdf). There is no doubt that NAPGD2022 will provide a more efficient and cost-effective way to maintain consistent and accurate orthometric heights; however, evaluating the relative accuracy of the geoid model is critical to a successful implementation of NAPGD2022.

The first phase of the GRAV-D project is the airborne gravity survey of entire country and its holdings; the second phase is the long-term monitoring of the change in the geoid. Not only is the NAVD 88 being replaced with a new datum but the geoid model, the underlying foundation of establishing GNSS-derived orthometric heights in NAPGD2022, will be constantly changing. The geoid will change but it will change very slowly. Saying that, it is still important for NGS to monitor changes in the geoid if users are going to establish and maintain GNSS-derived orthometric heights at the centimeter level. As part of the modernization of the vertical reference frame, NGS has outlined four components of a long-term monitoring plan. [See box titled “Components of a Long-Term Monitoring Plan.”]

Components of a Long-Term Monitoring Plan
(From presentation titled “Monitoring Changes in the Geoid” given by Dr. Theresa Damiani at the NGS 2017 Geospatial Summit)

What and Where to Monitor
How to Monitor in the Near-Term (next 1 to 3 decades)
Which Products Need to be Available
Long-Term Program Adaptation

The two most important components of the plan, in my opinion, are “What and Where to Monitor” and “How to Monitor in the Near-Term.” There are small changes in the geoid that occur over long periods of time. [See box titled “Slide 5 from presentation titled “Monitoring Changes in the Geoid.”]

Slide 5 from presentation titled “Monitoring Changes in the Geoid”
(From presentation titled “Monitoring Changes in the Geoid” given by Dr. Theresa Damiani at the NGS 2017 Geospatial Summit)

Dr. Damiani presented a slide that outlined NGS’ vision for vertical datum products as they are related to the geoid model. [See the box titled “NGS’ Vision for Vertical Datum Products, 2022 +.”] NGS will be publishing both static geoid models (S) and dynamic geoid models (D). The “S” static model will be a typical geoid model, aimed to capture the 1 cm-accurate model at a specific epoch, and the “D” dynamic model will capture the rate of change of the geoid at all places. Dr. Damiani mentioned in her presentation that NGS has initiated a program called “The Geoid Monitoring Service.” This service is a new project, initiated in January 2017, that is planned to be operational and produce NGS’ first “D” dynamic geoid by 2022.

NGS’ Vision for Vertical Datum Products, 2022 +
(From presentation titled “Monitoring Changes in the Geoid” given by Dr. Theresa Damiani at the NGS 2017 Geospatial Summit)

➢ In 2022, NGS will release “S” and “D” geoid models: static (S) and dynamic (D).

➢ The “S” static will be a typical geoid model, aimed to capture the 1 cm-accurate model at a TBD epoch.

➢ The “D” dynamic will capture the rate of change of the geoid at all places. In 2022, it will capture at least the continuous, permanent change signals such as Glacial Isostatic Adjustment (GIA).

➢ Both models will be integrated into OPUS, mostly invisible to users. Orthometric heights provided by OPUS will be time-sensitive, so that they are the combination of the static geoid model plus the geoid rate of change indicated by the dynamic model.

➢ NGS will provide separate tools to directly access both the “S” and “D” models.

This column discussed the basic foundation parameters of the North American-Pacific Geopotential Datum of 2022 (NAPGD2022); that is, a global geopotential model, the GRAV-D project, and the GEOID2022 geoid model. It emphasized that NAPGD2022 will provide a more efficient and cost-effective way to maintain consistent orthometric heights, but evaluating the relative accuracy of the geoid model is critical to a successful implementation of NAPGD2022. Performing GNSS/Leveling evaluation surveys will help in evaluating the relative accuracy of GEOID2022. NGS is developing geodetic routines and tools to assist users in transforming heights from NAVD 88 to NAPGD2022, and enabling the incorporation of geodetic leveling data into NAPGD2022 to establish NAPGD2022 orthometric heights. Future columns will address some of these tools and routines.

August 2, 2017

Discussing the new North American-Pacific Geopotential Datum of 2022 — Part 1

On April 24-25, 2017, the National Geodetic Survey (NGS) hosted the 2017 Geospatial Summit in Silver Spring, Maryland, to discuss its plans for replacing the North American Datum of 1983 (NAD 83) and the North American Vertical Datum of 1988 (NAVD 88) in 2022.

The summit was a day and a half long and provided an opportunity for NGS to share updates and discuss the progress of projects related to National Spatial Reference System (NSRS) Modernization. Stakeholders across the federal, public and private sectors also provided feedback and impacts of New Datums on their products and services.

The absolute differences between the new vertical reference frame, North American-Pacific Geopotential Datum of 2022 (NAPGD2022), and NAVD 88 are going to be large but, in most regions of the country, the relative differences over small areal extents will be small.

NGS is developing geodetic routines and tools to transform heights from NAVD 88 to NAPGD2022, and to facilitate the incorporation of geodetic leveling data into NAPGD2022 to establish NAPGD2022 heights. To prepare for the new datums and develop implementation plans, stakeholders should obtain an understanding of the differences between NAPGD2022 and NAVD 88.

My previous columns provided figures that demonstrated the approximate differences between NAPGD2022 and NAVD 88 heights at a national level. (See figure 1.) This column will provide feedback from stakeholders that participated in the Geospatial Summit and, using NGS’ GPS on BMs dataset, a discussion on the differences between NAPGD2022 and NAVD 88 (and NGVD 29) at a local level.

Figure 1 – Approximate Change Between NAPGD2022 and NAVD 88 Using GPS on BMs Data (units = cm). (Image: National Geodetic Survey)

Information about the summit and Summit Documents can be downloaded here.

Read an excerpt from website here.

If you check on the tab titled “Summit Documents” you can download the agenda and documents provided to participates. Read excerpts from the summit here.

The first day consisted of presentations by NGS leadership and personnel providing updates and discussing the progress of projects related to the NSRS modernization. The presentations by NGS employees can be downloaded from NGS’ presentations library at this web link. View an excerpt from NGS’ presentations library here.

The afternoon of day 2 were presentations by partners and stakeholders. (See box titled “Excerpt from NGS 2017 Geospatial Summit Agenda – Afternoon of Day 2.”)

Excerpt from NGS 2017 Geospatial Summit Agenda – Afternoon of Day 2
Day 2 Afternoon Agenda from NGS’ 2017 Geospatial Summit
Day 2: Tuesday, April 25, 20171:30 – 3:05 Impacts of New Datums on Programs and Partners (Part 1)
Coastal Mapping Program and VDatum: Mike Aslaksen and Stephen White, NOAA/NGS
Federal Emergency Management Agency (FEMA): Kimberly Pettit, FEMA
U.S. Geological Survey (USGS): Kari Craun, USGS
U.S. Army Corps of Engineers (USACE): Jim Garster, USACE
National Geospatial-Intelligence Agency (NGA): Stephen Malys, NGA
3:05 – 3:25 Break
3:25 – 4:55 Impacts of New Datums on Programs and Partners (Part 2)
Geospatial and Remote Sensing Customers: Amar Nayegandhi, Dewberry
Geographic Information System (GIS) Customers: Kevin Kelly, Esri
Global Navigation Satellite System (GNSS) Equipment Customers: Hamid Mahmoudabadi, Trimble Kyle Snow, Topcon
State Government Partners: Gary Thompson, N.C. Department of Public Safety
Local Government Partners: Vickie Anglin, Fairfax County Government, Virginia; Patrick Simon, Baltimore County Land Survey, Maryland
4:55 – 5:00 Wrap-up and closing

In order for consistency, NGS provided guidance and a set of template slides for guest presenters to use. Guest presenters were allotted 10 minutes to present and limited to four slides. The presentation by the guest presenters are not on NGS’ Presentations Library but I’ve been told that they will be available on the Summit website later this year. Gary Thompson, Chief of the North Carolina Geodetic Survey (NCGS), provided me a copy of his slides and gave me permission to include them in this column. (See box titled “Power point Slides Presented by Gary Thompson, Chief of NCGS, at the NGS 2017 Geospatial Summit.”) North Carolina has been very proactive in addressing the impacts of the new datums on NC products and services. North Carolina Geodetic Survey has established a North Carolina Geodetic Survey Advisory Committee that reviews NCGS products and services, and they have established the North Carolina 2022 Reference Frame Working Group to prepare for the new datums.

Slide: National Geodetic Survey

Powerpoint slides presented by Gary Thompson, chief of NCGS, at the NGS 2017 Geospatial Summit

All of the presentations by the invited guest speakers were interesting, and everyone followed NGS’ guidance which helped to focus the Summit on the main issues associated with a datum change. As expected, each stakeholder had their own set of issues and concerns about transitioning to a datum. The following are some common themes that I heard from the participants:

(1) There are a lot of products and services that will be effected by a datum change,
(2) An official transformation model between the old and new datum(s) published by NGS is critical for a successful transition to a new datum,
(3) Guidance documents that are “easily” understood by “non-geodesists” is required for a smooth implementation of a new datum, and
(4) More frequent geospatial summits and webinars are needed to provide updates on the status of the projects associated with NSRS modernization and to ensure user involvement in the process.

I contacted a couple of the guest presenters to discuss their feedback on the New Datums. As NAVD 88 Program Manager, I collaborated with many of them during the development and implementation of the NAVD 88. As in the transition from NGVD 29 to NAVD 88, it’s not the conversion of coordinates that’s a problem; a good transformation tool should meet that requirement. Saying that, it was stated that many users rely on commercial and open source software to convert their data, so they would like NGS to collaborate with others to ensure that these software suppliers are using the appropriate algorithms/information in their products. The integration with legacy data referenced to older datums may be complicated for some products and services; therefore, the process of transforming each product and service will need to be addressed individually. If all data are in digital form with the appropriate metadata, then the transformation should be relatively easy to accomplish and maps with new contour lines or new base flood elevations referenced to the new datum could be generated. However, how these new maps are integrated with old maps is a different issue. I will address some of these potential issues in future columns.

To prepare implementation plans, users must obtain a working knowledge of the differences between the old and new datums. As previous mentioned, the absolute differences between the new vertical reference frame, NAPGD2022, and NAVD 88 are going to be large but, in most regions of the country, the relative differences over small areal extents will be small. To evaluate the relative differences at the local level, the differences between NAPGD2022 and NAVD 88 (and NGVD 29) were computed for bench marks in the NGS’ GPS on BMs dataset. The NAD 83 (2011) latitude, longitude, and ellipsoid height of each station was transformed to the IGS08 reference frame using NGS’ HTDP web tool, and then the GNSS-derived orthometric height was computed using the following formula:

Approximate NAPGD2022 GNSS-Derived Orthometric Height
Equals
IGS08 Ellipsoid Height minus xGeoid16b Geoid Height (referenced to IGS08).

Figure 1 is a plot of the difference between the approximate NAPGD2022 height and the published NAVD 88 height for bench marks that are part of the GPS on BMs dataset and have the published attribute of “Adjusted.” It should be noted that these are only estimated changes because the final NAPGD2022 reference frame will not be exactly the same as the current IGS08 reference frame, but these estimates should serve the purpose of providing approximate changes for users to develop transition plans.

Since some users are still converting NGVD 29 heights to NAVD 88 heights, the approximate change between NAPGD2022 and NGVD 29 is provided in figure 2. VERTCON values were used to convert the NAVD 88 published heights to NGVD 29 heights, and then the difference between the approximate NAPGD2022 orthometric height and the NGVD 29 orthometric height was computed.

Figure 2 – Approximate Change Between NAPGD2022 and NGVD 29 Using GPS on BMs Data (units = cm). (Image: National Geodetic Survey)

As shown in figure 2, the absolute differences between the new vertical reference frame, NAPGD2022, and NGVD 29 are also going to be large but, once again, in most regions of the country, the relative differences over small areal extents will be small.

What does this look like in a local area? Figure 3 is a plot of the approximate change between NAPGD2022 and NAVD 88 in North Carolina and surrounding states, and figure 4 is plot of the approximate change between NAPGD2022 and NGVD 29 in North Carolina and surrounding states.

Figure 3 – Approximate Change Between NAPGD2022 and NAVD 88 in North Carolina and Surrounding States Using GPS on BMs Data (units = cm). (Image: National Geodetic Survey)

Figure 4 – Approximate Change Between NAPGD2022 and NGVD 29 in North Carolina and Surrounding States Using GPS on BMs Data (units = cm). (Image: National Geodetic Survey)

Figure 5 provides a more detailed depiction of the change between NAPGD2022 and NAVD 88 along the North Carolina Atlantic Coast. The differences appear to vary by several centimeters but some of these differences are due to errors in published heights (both ellipsoid and orthometric). These differences can be used to develop a transformation model but the user will need to know the accuracy of the model, globally and locally.

Figure 5 – Approximate Change Between NAPGD2022 and NAVD 88 along North Carolina Atlantic Coast Using GPS on BMs Data (units = cm). (Image: National Geodetic Survey)

Figure 6 is a detailed depiction of the change between NAPGD2022 and NGVD 29 in the same area as shown in figure 5. Comparing figures 5 and 6, the reader should notice that the differences between NAPGD2022 and NGVD 29 are about 30 cm larger (more negative) than the differences between NAPGD2022 and NAVD 88.

Figure 6 – Approximate Change Between NAPGD2022 and NAVD 29 along North Carolina Atlantic Coast Using GPS on BMs Data (units = cm). (Image: National Geodetic Survey)

Figure 7 is the difference between NAPGD2022 and NAVD 88 in western North Carolina. The local difference in the NC mountains is around -35 cm which is about 10 cm different from the NC Atlantic Coast. Questions that users need to address include: What is the accuracy of the transformation model? And What is the accuracy of the product or service being transformed? The transformation model will not replace the original survey results but may be useful for transforming some products and services.

Figure 7 – Approximate Change Between NAPGD2022 and NAVD 88 in the Western North Carolina Using GPS on BMs Data (units = cm). (Image: National Geodetic Survey)

Table 1 provides the average difference between NAPGD2022 and NAVD 88 (and NGVD 29) by State using the GPS on BMs dataset. This table shows that there are large differences between NAPGD2022 and both NGVD 29 and NAVD 88. No matter which datum the product or service is referenced to, it will probably need to be transformed to NAPGD2022.

Table 1 – Average Difference Between NAPGD2022 and NAVD 88 (and NGVD 29) by State Using GPS on BMs Dataset (units = cm). Click to enlarge. (Date: National Geodetic Survey)

Average Difference Between NAPGD2022 and NAVD 88 by State Using GPS on BMs Dataset (units = cm). Click to enlarge. (Date: National Geodetic Survey)

Table 2 provides the standard deviation of the average difference between NAPGD2022 and NAVD 88 by State. For example, North Carolina has a sample size of 1600 stations and its average difference is -28 cm with a standard deviation of 4.8 cm. Looking at figures 5 and 7, there appears to be a difference of 10 cm across the State. The States in the northwestern region of the United States have a larger difference between NAPGD2022 and NAVD 88 as well as a larger standard deviation. Oregon has a sample size of 195 stations and its average difference is -100.7 cm with a standard deviation of 13.0 cm, and Washington has a sample size of 266 stations and its average difference is -108.8 cm with a standard deviation of 9.0 cm. Figure 8 is a plot of the approximate change between NAPGD2022 and NAVD 88 in the northwest region of the United States.

As mentioned previously, these differences will vary from station to station because of a bias and trend between the two datums and due to remaining errors in published heights (both ellipsoid and orthometric). As I have noted in previous columns, many of the large relative differences between stations in a local area could be due to an invalid NAVD 88 published height because the bench mark moved since the last time the height of the bench mark was adjusted and published, and/or an undetected error in an ellipsoid height due to a weak GNSS project design. Either way, in my opinion, most of these stations with large relative differences don’t accurately represent the current NAVD 88. NGS’ modernization of the NSRS will provide a more accurate and consistent reference frame, and improve the user’s ability to obtain a current and accurate orthometric height.

Figure 8 – Approximate Change Between NAPGD2022 and NAVD 88 in the Northwest Region of the United States Using GPS on BMs Data (units = cm). (Image: National Geodetic Survey)

This column highlighted some of the feedback provided by guest presenters at the NGS’ 2017 Geospatial Summit held on April 24-25, 2017, in Silver Spring, Maryland. The column also provided a discussion on the approximate differences between NAPGD2022 and NAVD 88 (and NGVD 29) at a national and local level. To prepare for the new datums and develop implementation plans, users should obtain an understanding of the differences between NAPGD2022 and NAVD 88. This column is the first in a new series of columns addressing topics associated with transitioning to the new North American -Pacific Geopotential Datum of 2022 (NAPGD2022).

June 7, 2017
Surveyors’ coordinate systems for 2022 and beyond
Time.

Ask anyone what time means to them, and they will give you a different answer. Benjamin Franklin famously stated that “time is money.” Time for the surveyor can mean being out in the field retracing a boundary, drafting a plat or working with a client to help them see their goals achieved. Just like any other profession, time can be a friend or foe for the surveyor. We seem to run out of it more than we have an excess of it. Either way, time marches on as we go about our business.

Time, however, is changing the surveyor’s world and how we go about our methods of measurement. While it seems like a crazy concept, time is the major component requiring changes to geodetic procedural processes and how we will determine our locations in the future.

We will continue to see advances in hardware and software along with new interfaces and ways to collect and display survey data almost daily, and we will continue to deal with adaptation. However, surveyors must be ready for the next big challenge: a national horizontal and vertical adjustment of the National Spatial Reference System (NSRS) into a new standard. The North American Terrestrial Reference Frame of 2022 (NATRF2022.) is currently being developed by NGS and will replace NAD83 and NAVD88. Most surveyors will ask why we are getting ready for a historic change in datums. Easy — it’s all about time.

Expanded Variables

Just as early travelers thought the Earth was flat and learned it wasn’t through exploration and science, we are learning more everyday regarding how our world is changing. To get a better understanding of how our world is changing, NGS and the geodesy community have expanded the environmental variables of geographic location to areas including gravity, geoid undulations and geopotential data, plate tectonics and crustal evolution, and additional GNSS data analysis through satellites and continuously operating reference station (CORS) installations.

By introducing new attributes affecting coordinate data, including horizontal motions induced directly or indirectly by adjoining tectonic plates, horizontal motions induced by Global Isostatic Adjustment, other horizontal motions and all vertical motions in their entirety (per NGS NOAA Technical Report NOS NGS 62), data captured will be used to create an Intra-Frame Velocity Model (IFVM). Data following this format will be now be used to monitor the movement of survey positions from implementation forward. The key factor in which all the data is centralized is time.

My GPS World colleague David Zilkoski presented a thorough explanation (“NGS to Replace NAVD88 in 2022: What GNSS Users Need To Know) of the nuts and bolts behind the changes. Here are the basic reasons behind the new adjustment as provided by NGS:

NAD 83 and NAVD 88, although still the official horizontal and vertical datums of the National Spatial Reference System (NSRS), have been identified as having shortcomings that are best addressed through defining new horizontal and vertical datums.

Specifically, NAD 83 is non-geocentric by about 2.2 meters. Secondly, NAVD 88 is both biased (by about one-half meter) and tilted (about 1 meter coast to coast) relative to the best global geoid models available today. Both of these issues derive from the fact that both datums were defined primarily using terrestrial surveying techniques at passive geodetic survey marks. This network of survey marks deteriorate over time (both through unchecked physical movement and simple removal), and resources are not available to maintain them.

The new reference frames (geometric and geopotential) will rely primarily Global Navigation Satellite Systems (GNSS) such as the Global Positioning System (GPS) as well as an updated and time-tracked geoid model. This paradigm will be easier and more cost-effective to maintain.

Plate tectonics

These proposed changes to the NSRS, however, are based upon how much we have learned about our changing Earth using GNSS equipment and data collection. Time, as it turns out, is a big factor in how we measure and document locations. A point that is determined exactly here on this day at a specific moment will have moved due to plate tectonics and other variables to there over a period of time.

New Vertical Component

Another aspect of the datum change will be the definition of a new vertical component. Surveyors are familiar with the establishment of NGVD29 based upon mean sea level, and also NAVD88 being based upon the benchmark at Father Point/Rimouski, Quebec, Canada with reference to the International Great Lakes Datum of 1985. What science has taught us in the years beyond NAVD88 is that there is a greater force at play when it comes to the vertical piece of geolocation: gravity.

Yes, gravity keeps us on the ground and causes water to flow downhill, but the development of gravitational studies has led to incredible discoveries of how gravity affects elevation. It was always assumed that the gravitational pull on the earth was uniform worldwide, but with the development of instruments that can measure and map the variations in gravity, NGS will be redefining the vertical datum through a program called GRAV-D. NGS is currently flying in various portions of the U.S. and is scheduled to be completed by 2021 in order to roll out with the new horizontal program in 2022.

So, it turns out that time has been affecting not just our productivity but also our positions on the earth. Another famous quote by Paulo Coelho does hold true: “Time neither moves nor is stationary. Time changes.” Time has passed since this article began; did you feel the earth move?

What about our survey monuments and state plane coordinates?

For many surveyors, the main question is simple: why now? What is so bad with our existing NAD83 and NAVD88 datums?

Map courtesy of GISGeography, at http://gisgeography.com/state-plane-coordinate-system-spcs/

The reason is very simple; staying current with our favorite tool in the toolbox: GNSS. Surveyors have always been about “monuments” and perpetuation of data from established points located on the face of the Earth with published and/or known values. This concept has become even more important to the surveying community once the proliferation of geographic-based and state plane coordinate data was published for all to utilize. I touched on the surveyor’s use and data collection/perpetuation of location values in a past column (GPS World November 2016). As long as NGS updated the national database with more information and a simple adjustment every so often, life was good and simple.

But now we have worlds colliding; static monuments with published horizontal and vertical values in one corner, while in the other corner is the new paradigm of ever shifting crustal plates and changing positional values monitored by GNSS data through satellites and a network of CORS located worldwide.

This situation makes me harken back to one of my favorite “Ghostbusters” lines from Bill Murray’s character, Dr. Peter Venkman: “Human sacrifice, dogs and cats living together – mass hysteria…”

Okay, maybe it won’t be quite that bad but there will be many surveyors that will have trouble wrapping their minds around the new concept of “moving monuments.” Our reliance on state plane coordinate systems (SPCS) is at an all-time high with the sharing of data by various parties being more seamless than ever. The notion that a permanent monument’s positional values will be constantly changing is a head-spinner to most.

NGS has also stated that their new system and procedures will not maintain data values for SPCS (see NGS State Plane Flyer). There are currently 125 SPCS zones and 3235 county systems throughout the US and territories in place that rely on NGS data as the main framework, so having tools for reference and conversion in place will be crucial. Thus, it will be a herculean task to create a procedure/process to easily pass coordinate values between our many static systems worldwide and the new dynamic but very robust system underway from NGS.

Based upon information currently available about the NAD2022 system, it would be more efficient for all those who use geolocation data to modify their thinking to adapt to a dynamic coordinate system. However, this is a similar situation to early scientists and geographers throwing out all references to flat-earth maps and atlases. For surveyors in the twilight of their careers, these are radical items to consider and a far cry from the standardized chain and theodolite. (Maybe there will be mass hysteria…)

The good news is that we have very intelligent people in the surveying and geodetic community who are working on solutions for the masses. The beauty of newer technology is how quickly hardware and software can be adapted to fit these new data conditions. Getting the word out on these changes and educating our profession will be a key factor to its success.

Further Refinement of Coordinate Systems

While the use of GNSS has enabled the discovery of time as a significant variable in geolocation, it has also expanded out coverage area of coordinate systems to much larger areas. Distances that could not be computed prior to GNSS are now easily attained and large projects can be managed within a common coordinate system. County, regional and state agencies can now create large-scale GIS databases that utilize a single coordinate system as well.

However, there are two differing tracks being formed with the continued development of the new datum by NGS. While the new datum will become more precise and predictable, there are movements in opposing camps to make changes in user coordinate in the furthest possible ways: statewide single zone system versus county/regional low distortion projection (LDP) systems. They both have their strengths and weaknesses, and will depend on the application of the user to choose the appropriate system.
- Most states currently have two or more zones so there potential to combine all zones into one, but a major drawback will be the loss of accuracy away from the defining points. For GIS users, this accuracy will more than adequate and will allow the merging of data from across the state into one unified system.
- Surveyors, however, are an interesting bunch in that they accept only the most accurate AND precise measurements. The growing use of LDP is rapidly changing the implementation and management of coordinate system in smaller areas such as counties and regional DOT districts.
However, both systems have a place in our surveying and mapping world. NGS has stated that while they will help with transformation software and apps, it will leave the decision of legislative standards to each state. It will be paramount that each state study what makes the most sense for its users and pass the appropriate legislation.

“The days are long but the years are short”

As I look back and realize how much has changed with modern technology and overall knowledge of our profession, it is with much anticipation how much more will change with advancements we don’t even know about yet. The electronic distance meter (EDM) was revolutionary for many surveyors and I’ve waxed poetic about my feelings regarding RTK GNSS in past columns (GPS World May 2016). Once again, however, technology and information based upon its use has revolutionized our data system.

As a profession, surveyors have embraced GNSS use and data collection from the early implementation of the system. And while the advances of UAV use, laser scanners and LiDAR along with software improvements have revolutionized data collection, these proposed coordinate improvements by NGS will bring more potential quality information into the surveyor’s hands.

And while time is money as Mr. Franklin famously stated, 2022 is just around the corner. A good friend of mine is famous for saying: “Good coordination begins with good coordinates.” The work performed by NGS is helping us do just that. The entire surveying, mapping and geodetic community has lots to accomplish to be ready for the changes from NGS. Let’s get to work.
May 3, 2017

Analyzing NGS’ GPS on benchmark dataset used to make GEOID12B — Part 9

These columns have focused on procedures and routines for establishing GNSS-derived orthometric heights. There are many ways to analyze and investigate GNSS data and adjustment results. I have provided some basic concepts that I believe are important for users to understand.

The selection of constraints is a very important part of establishing accurate and consistent NAVD 88 GNSS-derived orthometric heights. All of the analysis and recommendations have been based on using the National Geodetic Survey‘s latest scientific geoid model.

I recommend first performing the analysis using the scientific geoid model because the hybrid geoid model has been warped to be consistent with the published NAVD 88 values. However, as mentioned in Part 7 (June 2016), in practice, GNSS-derived orthometric heights are incorporated into the NAVD 88 using the latest hybrid geoid model GEOID12B. This column will focus on the NGS “GPS on BMS (GPSBM)” dataset that was used to create the hybrid geoid model.

As mentioned in Part 3 (October 2015), the hybrid geoid model is designed to fit the published NAVD 88 leveling-derived orthometric heights. Saying that, the GPSBM dataset can be used to identify potential issues in the NAVD 88 published orthometric heights. GNSS users should be familiar with this dataset and how it can be used in their analysis. This column will provide tools and routines that can be used to identify potential issues in NAVD 88 heights and/or NAD83 (2011) published ellipsoid heights.

The National Geodetic Survey provides information on the bench marks occupied by GPS that were used to make GEOID12B.

The write up from the NGA website is given below. I have highlighted a few sentences that I’ll address in this column.

Write up from: GPS On Bench Marks (GPSBM) Used To Make GEOID12B

Each of the below regions uses variants of the NAD 83 reference frame and a local vertical datum. Several versions of NAD 83 exist conforming to significant plates: Pacific, Mariana, and North America. Likewise, each region has its own vertical datum. It is not possible to level across water, so islands will have selected a tide gauge to serve as the local datum point and all leveling is tied to that site. The only exception to this is Hawaii. No tide gauge was selected in the Hawaiian Islands and no vertical datum has been established as of yet. Hence, GEOID12B in Hawaii transforms between NAD 83 (PA11) and the same geopotential (geoid) surface as the USGG2012 model ( W0 = 62636856.00 m**2/s**2).

Items that are listed in the below table include the final GPSBM files for each region as both Excel spreadsheets and text files as well as thumbnail images linked to larger images showing the distribution of the GPSBM’s. Alaska and the island regions are more consistent, so not many points were dropped and each is provided in its own spreadsheet/text file and identified with the appropriate ellipsoidal reference frame and level datum (see below).

The most significant work occurred in the COnterminous United States (CONUS). For CONUS, there were 24,782 points with 911 rejected leaving 23,961. These were supplemented from the OPUS-database with 737 points of which 238 were rejected leaving 499. There were also 579 points in Canada with 5 rejected leaving 574. In Mexico, there 744 of which 497 were clipped since they were too far south and another 70 were rejected leaving 177. This brings a total of 26,932 points of which 1,721 were rejected or clipped and 25,211 retained for modeling GEOID12B. The data in Canada and Mexico provide continuity up to and across the U.S. borders but do not make the GEOID12B model valid in those countries.

Points were rejected either because the State Advisor recommended it be dropped (e.g., known subsidence region), the residual ellipsoid height errors (from the NA2011 project) indicated a point was too noisy in comparison to other points in a state/region, the orthometric height was suspect, or the residual errors during geoid modeling were too high. The corresponding error flags are ‘S’, ‘h’, ‘H’, and ‘N’ as seen on the spreadsheet and text files. These points then represent the control data that were used to define the transformation between NAD 83 and NAVD 88 for CONUS.

The control data were much simpler in other regions due to the lack of quantity (more than two orders of magnitude less). Data in these regions follows a similar pattern where some data are rejected based on the codes given above for CONUS. The columns on the right side give the respective datums realized by GEOID12B for each region.

REGION	Excel Spreadsheets	GeoPDF maps	Ellipsoidal Reference Frame	Vertical Datum
CONUS	(xlsx) , (xls)	CONUS	NAD83 (2011)	NAVD88
Alaska	(xlsx) , (xls)	AK	NAD83 (2011)	NAVD88
Puerto Rico	(xlsx) , (xls)	PR	NAD83 (2011)	PRVD02
U.S. Virgin Islands	(xlsx) , (xls)	USVI	NAD83 (2011)	VIVD09
Am. Samoa	(xlsx) , (xls)	AS	NAD83 (PA11)	ASVD02
Guam	(xlsx) , (xls)	Guam	NAD83 (MA11)	GUVD04
CNMI	(xlsx) , (xls)	CNMI	NAD83 (MA11)	NMVD03

Table 1 is an excerpt of the excel spreadsheet for the GPSBM dataset and provides a sample of the contents. The headings of the columns are fairly self-explanatory. What’s important here is that the excel spreadsheet provides the name, latitude, longitude, NGS’ PID, the ellipsoid height and orthometric height of the stations used in making GEOID12B.

Table 1
Excerpt of the Excel spreadsheet for GPS on benchmarks (GPSBM) used to make GEOID12B.

The “GPS On Bench Marks (GPSBM) Used To Make GEOID12B” write up states that 1,721 stations were rejected and were not used in developing the hybrid geoid model. It also states that for the conterminous United States (CONUS), there were 24,782 stations with 911 rejected leaving 23,961. This column is going to focus on CONUS but the analysis can be performed everywhere.

As the write up states, stations were rejected for four different reasons:

Code S – The State Advisor (now called Regional Geodetic Advisors) recommended it be dropped,
Code h – The residual ellipsoid height errors from the NAD 83 (2011) project indicated that the point was too noisy,
Code H – The orthometric height was suspect,
Code N – The residual errors during geoid modeling were too high.

These rejected stations were not used to make the hybrid geoid model but since the hybrid geoid model is distorted to fit the NAVD 88, these rejected stations as well as stations nearby the rejected stations should be re-evaluated using the latest scientific geoid model, e.g. xGeoid16b.

So, what should the user do with the GPSBM table? I recommend that users perform the following steps when analyzing the stations in the GPSBM table.

Step 1: Compare the modeled GEOID12B (N12B) value to the computed GPS/Leveling (h minus H) value using the following formula: Published N12B from the NGS data sheet minus (ellipsoid height from the GPSBM table minus orthometric height from the GPSBM table). We discussed this procedure a year ago in Part 3 (October 2015). It should be noted that the orthometric height in the GPSBM table may be different than the published NAVD 88 height on the NGS data sheet if the station has been readjusted since the GPSBM table was created.

Step 2: Repeat the procedure in Step 1 using the latest NGS experimental geoid model, e.g. xGeoid16b. At this time, NGS only provides the experimental geoid models referenced to IGS08 so the user will have to use NGS’ xGeoid16 web tool to obtain the station’s IGS08 ellipsoid height and xGeoid16b value. The input to the tool is the station’s NAD 83 (2011) coordinates (latitude, Longitude, and ellipsoid height). [An example of using the xGeoid16 web tool is provided in the box titled “Example of Using NGS xGeoid16 Web Tool.”] As discussed in Part 3 (October 2015), the user will have to remove a bias and trend based on the differences in the region.

The user could also transform xGeoid16b/IGS08 geoid values to xGeoid16b/NAD 83 (2011) geoid values using their own tools, and then remove a bias and trend based on the differences. Michael Dennis, a PhD candidate at Oregon State University, created an ArcGIS raster of the xGeoid16b model, where his model has been referenced to NAD 83 (Michael L. Dennis, RLS, PE, MS Civil Eng., Geodetic Analysis, LLC, 55 Creek Rock Road, Sedona, AZ 86351). He removed a trend using the GPS/Leveling data set as input; therefore, this raster file is a form of a hybrid geoid model distorted only to remove the tilt assumed to be in the NAVD 88. I will refer to this model as Geoid16B_NAD83 to avoid confusion with NGS’ xGeoid16b model.

Example of Using NGS xGeoid16 Web Tool
Your input in NAD83 (2011)/GRS80 Ellipsoid:
Latitude	Longitude	Ellipsoid Height	Station
38 43 54.95105	79 58 19.75931	599.253	L 275
Your Result in IGS08/GRS80 Ellipsoid:
Latitude	Longitude	Ellipsoid Height
38 43 54.98136	79 58 19.78679	597.984

Geoid Model	Geoid Height(m)	Ortho Height(m)	Change in Ortho Height(m)*
GEOID12B	-32.086	630.07	-0.493
USGG2012	-31.592	629.576	0.001
xGEOID16A	-31.594	629.578	-0.001
xGEOID16B	-31.593	629.577	0
*Orthometric height difference between xGEOID16B to model shown

Step 3: Use the station’s data sheet to identify how the station’s orthometric height was determined; for example, was it rigorously adjusted into the NAVD 88 (published height attribute – Adjusted). We discussed the attributes of the NGS data sheet in Part 5 (February 2016). A summary of the attributes from the NGS data sheet DSDATA.TXT file is provided in the box titled “Extracted from NGS’ DSDATA.TXT.” I have highlighted the most common attributes of the stations involved in making GEOID12B.

Extracted from NGS’ DSDATA.TXT
***************************************************************************
* dsdata.txt *
***************************************************************************
There are various Vertical Control sources, as specified below:ADJUSTED = Direct Digital Output from Least Squares Adjustment of Precise Leveling.
(Rounded to 3 decimal places.)ADJ UNCH = Manually Entered (and NOT verified) Output of Least Squares Adjustment of Precise Leveling.
(Rounded to 3 decimal places.)

POSTED = Pre-1991 Precise Leveling Adjusted to the NAVD 88 Network After Completion of the NAVD 88 General Adjustment of 1991.
(Rounded to 3 decimal places.)

READJUST = Precise Leveling Readjusted as Required by Crustal Motion or Other Cause.
(Rounded to 2 decimal places.)

N HEIGHT = Computed from Precise Leveling Connected at Only One Published Bench Mark.
(Rounded to 2 decimal places.)

RESET = Reset Computation of Precise Leveling.
(Rounded to 2 decimal places.)

COMPUTED = Computed from Precise Leveling Using Non-rigorous Adjustment Technique.
(Rounded to 2 decimal places.)

GPSCONLV = Leveled Orthometric Height tied to GPS HT_MOD Orthometric Height.
(Rounded to 2 decimal places.)

LEVELING = Precise Leveling Performed by Horizontal Field Party.
(Rounded to 2 decimal places.)

H LEVEL = Level between control points not connected to bench mark.
(Rounded to 1 decimal places.)

GPS OBS = Computed from GPS Observations.
(Rounded to 1 decimal places.)

VERT ANG = Computed from Vertical Angle Observations.
(Rounded to 1 decimal place; If No Check, to 0 decimal places.)

SCALED = Scaled from a Topographic Map.
(Rounded to 0 decimal places.)

U HEIGHT = Unvalidated height from precise leveling connected at only one NSRS point.
(Rounded to 2 decimal places.)

VERTCON = The NAVD 88 height was computed by applying the VERTCON shift value to the NGVD 29 height.
(Rounded to 0 decimal places.)

Step 4: Use the station’s NGS data sheet to determine the adjustment date of the station’s published NAVD 88 orthometric height. We discussed this in Part 7 (June 2016). As mentioned in Part 7, if the station has a different adjustment date than other stations nearby, there could be inconsistencies due to adjustment distribution corrections and/or movement.

Step 1 was demonstrated in Part 3 (October 2015) so we don’t need to describe the process in this column. Comparing published GEOID12B values with computed values is the first step; the difference is an indication of how well the data fit the model and can be useful for identifying large outliers. It can be helpful in prioritizing where additional observation should be obtained when there are limited resources. Provided below is an example of where to obtain the information for comparing the modeled GEOID12B (N12B) value to the computed GPS/Leveling (h minus H) value using the following formula: Published N12B from the NGS data sheet minus (ellipsoid height from the GPSBM table minus orthometric height from the GPSBM table). The user can obtain the GEOID12B value from the NGS data sheet [see box titled “Excerpt from NGS Data Sheet For Station L 275 (HW2088)”]; for this example, the GEOID12B value for station L 275 is -30.813 m. Table 2 is an excerpt from the GPSBM file that contains the ellipsoid height (599.253 m) and the orthometric height (630.016 m) for station L 275. It should be noted that the ellipsoid and orthometric heights in the GPSBM table are given in millimeters. The first row of table 3 provides the results of the computation: [-30814 mm – (599253 mm – 630016m m) = 51 mm], or 5.1 cm.

Table 2
Excerpt of the Excel spreadsheet for GPS on benchmarks (GPSBM) used to make GEOID12B – Stations on plots in this column.

Excerpt from NGS Data Sheet For Station L 275 (HW2088)
PROGRAM = datasheet95, VERSION = 8.9.1
1 National Geodetic Survey, Retrieval Date = OCTOBER 1, 2016
HW2088 ***********************************************************************
HW2088 CBN – This is a Cooperative Base Network Control Station.
HW2088 DESIGNATION – L 275
HW2088 PID – HW2088
HW2088 STATE/COUNTY- WV/RANDOLPH
HW2088 COUNTRY – US
HW2088 USGS QUAD – MILL CREEK (1995)
HW2088
HW2088 *CURRENT SURVEY CONTROL
HW2088 ______________________________________________________________________
HW2088* NAD 83(2011) POSITION- 38 43 54.95105(N) 079 58 19.75931(W) ADJUSTED
HW2088* NAD 83(2011) ELLIP HT- 599.253 (meters) (06/27/12) ADJUSTED
HW2088* NAD 83(2011) EPOCH – 2010.00
HW2088* NAVD 88 ORTHO HEIGHT – 630.016 (meters) 2066.98 (feet) ADJUSTED
HW2088 ______________________________________________________________________
HW2088 NAD 83(2011) X – 867,581.099 (meters) COMP
HW2088 NAD 83(2011) Y – -4,906,352.726 (meters) COMP
HW2088 NAD 83(2011) Z – 3,969,521.039 (meters) COMP
HW2088 LAPLACE CORR – 0.13 (seconds) DEFLEC12B
HW2088 GEOID HEIGHT – -30.814 (meters) GEOID12B
HW2088 DYNAMIC HEIGHT – 629.553 (meters) 2065.46 (feet) COMP
HW2088 MODELED GRAVITY – 979,873.5 (mgal) NAVD 88
HW2088
HW2088 VERT ORDER – FIRST CLASS II
HW2088
HW2088 Network accuracy estimates per FGDC Geospatial Positioning Accuracy
HW2088 Standards:
HW2088 FGDC (95% conf, cm) Standard deviation (cm) CorrNE
HW2088 Horiz Ellip SD_N SD_E SD_h (unitless)
HW2088 ——————————————————————-
HW2088 NETWORK 1.00 1.94 0.45 0.36 0.99 -0.05669181

Table 3 contains the comparisons between modeled geoid values and their computed geoid values for five station pairs that have large relative differences. Looking at table 3 one can see that there are several large relative differences between the published GEOID12B model and computed geoid model (see column titled “N12B minus (h-H)” in table 3). This doesn’t mean that the model is incorrect, it only means that there were large relative differences that the model had to account for. As previously mentioned, GEOID12B was created to be consistent with the NAVD 88.

Since the experimental geoid model xGeoid16b_NAD is not distorted to conform to the NAVD 88 everywhere, it should provide better information for identifying outliers and determining which stations appear to be inconsistent with its neighbors.

Figure 1 - All GPS on BMS Residuals Using Geoid16b_NAD model (note: rejections by geoid team have been removed). — Figure 1 – All GPS on BMS Residuals Using Geoid16b_NAD model (note: rejections by geoid team have been removed).

Table 3
Table of selected stations involving large relative differences depicted in plots in this column.
(Results are provided for GEOID12B and Geoid16B_NAD Models*)
*Michael Dennis, a Ph.D. candidate at Oregon State University, created the xGEOID16B ArcGIS raster, where the model has been referenced to NAD 83 with a trend and bias added to account for the apparent tilt in the NAVD 88. This model is denoted as Geoid16B_NAD (N16b) in this column.

Figure 1 is a plot of all of the GPSBM residuals using the Geoid16B_NAD83 model. This plot indicates that there are a lot of large residuals. First, let’s define what I’m calling residuals. The residuals on my plots are the differences between the modeled geoid height value and the computed geoid height value using the ellipsoid height (h) and orthometric height (H) from the GPSBM data set; that is, residual = modeled gravity value – (h minus H). The largest negative residual is -37.3 cm and the largest positive residual is 33.8 cm.

Figure 2 – Positive GPS on BMS Residuals Using Geoid16b_NAD model (note: rejections by geoid team have been removed).

Figure 2 is a plot of the positive GPS on BMS residuals using Geoid16b_NAD geoid model. There are 5957 residuals greater than 5 cm (not including the stations rejected by the NGS geoid team). As you can see, it appears that most of the positive residuals are on the eastern half of the United States.

Figure 3 - Negative GPS on BMS Residuals Using Geoid16b_NAD model (note: rejections by geoid team have been removed). — Figure 3 – Negative GPS on BMS Residuals Using Geoid16b_NAD model (note: rejections by geoid team have been removed).

Figure 3 is a plot of the negative GPS on BMS residuals using Geoid16b_NAD geoid model. There are 4113 residuals less than -5 cm (not including the stations rejected by the NGS geoid team). As you can see from the plot, the negative residuals appear to be more evenly distributed across the United States than the positive residuals. It does, however, appear that there are more negative residuals greater than -5 cm along the Gulf Coast, Atlantic Coast, and the Great Lakes than there are positive residuals greater than 5 cm. In addition, there appears to be a lot of negative residuals in the northeastern United States.

Figure 4 – GPS on BMS Residuals Using Geoid16b_NAD model in North Carolina and South Carolina (note: rejections by geoid team have been removed).

Figure 4 is a plot of the GPS on BMS residuals using the Geoid16b_NAD geoid model in the North Carolina and South Carolina border region. What’s interesting about this plot is that South Carolina doesn’t seem to have many negative residuals where North Carolina has both negative and positive residuals. We will look at this in more detail later in this column.

Figure 5 – GPS on BMS Residuals Using Geoid16b_NAD model in Washington and Oregon Region (note: rejections by geoid team have been removed).

Figure 5 is a plot of the GPS on BMS residuals using Geoid16b_NAD model in the Washington and Oregon Region. This graphic shows some large grouping of negative and positive residuals, especially along the Pacific Coast in Northwestern Washington State.

Now, let’s look at some large relative differences in residuals between stations that are spatially close together. Figure 6 is a plot of large relative differences between groups of GPS on BMS residuals (using Geoid16b_NAD model) at the North Carolina/South Carolina border. In figure 6, two stations (FA1337 and FA1560) are about 20 km apart and the difference in residuals is -18.6 cm (-12.4 cm minus 6.2 cm). This is a large difference for only 20 km. What is even more significant is that the group of stations near FA1337 are all negative residuals (around -10 cm) and the group of stations near FA1560 are all positive residuals (around 6 cm), this could be an indication of a large distribution correction due to the NAVD 88 design. We discussed the distribution correction in Part 7 (June 2016). These stations definitely needs to be investigated.

The next step in my process is to look at the NGS data sheets for these stations to determine how the stations were adjusted.

Step 3: Look at the station’s data sheet to identify how the station’s orthometric height was determined; for example, was it rigorously adjusted into the NAVD 88 (published height attribute is “Adjusted”) or was it determined by precise leveling performed by horizontal field party (published height attribute is “Leveling”).

The data sheet for station FA1337 states that the NAVD 88 attribute code is “GPS OBS.” [See box titled “Excerpt from NGS Data Sheet for PID FA1337.”] The data sheet for FA1560 states that the NAVD 88 attribute code is “Adjusted.” The orthometric height on the GPSBM file is different than the current published NAVD 88 orthometric height for station FA1337 (See table 3). This station’s leveling-derived orthometric height was superseded by a GNSS-derived orthometric height. Saying that, the GPSBM file only uses leveling-derived orthometric heights; therefore, stations that have been superseded by GNSS surveys are still included in the GPSBM file but their original published leveling-derived height is used for the analysis. Table 3 provides the orthometric height for FA1337 that was used in making GEOID12B. As previously mentioned, stations may be rejected by the geoid team based on the criteria outlined in the beginning of this column. Saying that, neither of the two stations were rejected by the NGS geoid team. This implies that the stations were consistent with their neighbors as far as the geoid model was concerned. Figure 6 confirms that all the stations around FA1337 and FA1560 are consistent with each other based on the Geoid16b_NAD geoid model. The fact that the two groups differ by 18 6 cm needs to be investigated.

Excerpt from NGS Data Sheet for PID FA1337
PROGRAM = datasheet95, VERSION = 8.9.1
1 National Geodetic Survey, Retrieval Date = OCTOBER 3, 2016
FA1337 ***********************************************************************
FA1337 HT_MOD – This is a Height Modernization Survey Station.
FA1337 DESIGNATION – RU 36
FA1337 PID – FA1337
FA1337 STATE/COUNTY- NC/RUTHERFORD
FA1337 COUNTRY – US
FA1337 USGS QUAD – FOREST CITY (1993)
FA1337
FA1337 *CURRENT SURVEY CONTROL
FA1337 ______________________________________________________________________
FA1337* NAD 83(2011) POSITION- 35 18 08.14237(N) 081 51 17.93516(W) ADJUSTED
FA1337* NAD 83(2011) ELLIP HT- 249.869 (meters) (06/27/12) ADJUSTED
FA1337* NAD 83(2011) EPOCH – 2010.00
FA1337* NAVD 88 ORTHO HEIGHT – 281.79 (meters) 924.5 (feet) GPS OBS
FA1337 ______________________________________________________________________

Figure 6 - GPS on BMS Residuals: Large Relative Differences Between a Group of Stations at the North Carolina/South Carolina Border (note: rejections by geoid team have been removed) — Figure 6 – GPS on BMS Residuals: Large Relative Differences Between a Group of Stations at the North Carolina/South Carolina Border (note: rejections by geoid team have been removed)

Figure 7 is a plot of the GPS on BMS residuals using Geoid16b_NAD that depicts a large difference between two stations only 20 km apart near the Maryland/West Virginia border. I will use this station pair to demonstrate the next step in my process.

Step 4 is to use the station’s NGS data sheet to determine the adjustment date the of station’s published NAVD 88 orthometric height.

The NAVD 88 attribute on the NGS data sheet states that both of these stations are coded as “Adjusted” but station JW0639 adjustment date is April 1995 (see box titled “excerpt from NGS Data Sheet for PID JW0639”) and JW1296 adjustment date was in June 1991 (the General Adjustment of NAVD 88). These large relative differences could be due to inconsistencies between adjusted heights due to the adjustment distribution corrections and/or constraints imposed in the April 1995 adjustment. Bench marks near the stations should be observed to determine if the same large relative difference exists, and the 1995 NAVD 88 adjustment project report should be reviewed to determine if a large distribution correction was applied.

Excerpt from NGS Data Sheet for PID JW0639
1 National Geodetic Survey, Retrieval Date = OCTOBER 3, 2016
JW0639 ***********************************************************************
JW0639 CBN – This is a Cooperative Base Network Control Station.
JW0639 DESIGNATION – J 17 RESET
JW0639 PID – JW0639
JW0639 STATE/COUNTY- MD/GARRETT
JW0639 COUNTRY – US
JW0639 USGS QUAD – ACCIDENT (1994)
JW0639
JW0639 *CURRENT SURVEY CONTROL
JW0639 ______________________________________________________________________
JW0639* NAD 83(2011) POSITION- 39 37 53.59739(N) 079 18 57.44776(W) ADJUSTED
JW0639* NAD 83(2011) ELLIP HT- 701.266 (meters) (06/27/12) ADJUSTED
JW0639* NAD 83(2011) EPOCH – 2010.00
JW0639* NAVD 88 ORTHO HEIGHT – 732.713 (meters) 2403.91 (feet) ADJUSTED
JW0639 ______________________________________________________________________
*
*
*
JW0639
JW0639.The orthometric height was determined by differential leveling and
JW0639.adjusted by the NATIONAL GEODETIC SURVEY
JW0639.in April 1995.
JW0639

Figure 7 – GPS on BMS Residuals Using Geoid16b_NAD: Large Relative Difference Between Stations About 20 km Apart Along MD/WV Border (note: rejections by geoid team have been removed).

Figure 8 – GPS on BMS Residuals Using Geoid16b_NAD: Large relative Difference Between Stations 15 km Apart in Randolph County, West Virginia (note: rejections by geoid team have been removed).

Figure 8 is a plot of GPS on BMS residuals using Geoid16b_NAD that depicts a large relative difference between stations 15 km apart in Randolph County, West Virginia. This plot involves station HW3677 which has a published NAVD 88 attribute of “Leveling.” (See box titled “Excerpt from NGS Data Sheet for PID HW3677.”) The excerpt from the data sheet has the following statement: “The orthometric height was determined by differential leveling. The vertical network tie was performed by a horz. field party for horz. obs reductions. Reset procedures were used to establish the elevation.”

It would be useful if stations near this station were observed by GNSS surveys to determine what is occurring in this region.

Excerpt from NGS Data Sheet for PID HW3677
1 National Geodetic Survey, Retrieval Date = OCTOBER 2, 2016
HW3677 ***********************************************************************
HW3677 DESIGNATION – GPS 1
HW3677 PID – HW3677
HW3677 STATE/COUNTY- WV/RANDOLPH
HW3677 COUNTRY – US
HW3677 USGS QUAD – MILL CREEK (1995)
HW3677
HW3677 *CURRENT SURVEY CONTROL
HW3677 ______________________________________________________________________
HW3677* NAD 83(2011) POSITION- 38 37 50.21531(N) 079 55 29.64175(W) ADJUSTED
HW3677* NAD 83(2011) ELLIP HT- 1129.355 (meters) (06/27/12) ADJUSTED
HW3677* NAD 83(2011) EPOCH – 2010.00
HW3677* NAVD 88 ORTHO HEIGHT – 1159.91 (meters) 3805.5 (feet) LEVELING
HW3677 ______________________________________________________________________
*
*
*
*
HW3677
HW3677.The orthometric height was determined by differential leveling.
HW3677.The vertical network tie was performed by a horz. field party for horz.
HW3677.obs reductions. Reset procedures were used to establish the elevation.
HW3677

Figure 9 is a GPS on BMS residual plot of large relative stations about 30 km apart in Wasco County, Oregon. This plot has two stations with large differences and both stations have the NAVD 88 attribute of “Adjusted.” Their NGS data sheet states that they were both established in the general adjustment of NAVD 88 in June 1991. In this particular case, the leveling in this region is very old. As described in Part 7 (June 2016), you can retrieve all project identifiers for those projects with observations to or from a station using the station’s PID. The output from the NGS Data Sheet Mark Source Routine for PID RC1228 is shown in the box titled “Output from NGS Data Sheet Mark Source Routine.”

Output from NGS Data Sheet Mark Source Routine
Program: mark_sources Version: 3.0 Date: May 1, 2013RC1228OR/065 J 108
———————————————————-
GPS_OBS
———–
GPS_OBS FORE_POINT in GPS1655
DIR_OBS
———–
DIST_OBS
———–
VERT_OBS
———–
LEV_OBS
———–
LEVEL_OBS
———–
LEVEL_OBS STAND_POINT in L3410
LEVEL_OBS FORE_POINT in L3410***********************************************************

Figure 9 – GPS on BMS Residuals Using Geoid16b_NAD: Large relative stations about 30 km apart in Wasco County, Oregon (note: rejections by geoid team have been removed).

Excerpt from NGS Data Sheet for PID RC1228

PROGRAM = datasheet95, VERSION = 8.9.1
1 National Geodetic Survey, Retrieval Date = OCTOBER 2, 2016
RC1228 ***********************************************************************
RC1228 DESIGNATION – J 108
RC1228 PID – RC1228
RC1228 STATE/COUNTY- OR/WASCO
RC1228 COUNTRY – US
RC1228 USGS QUAD – WAPINITIA (1996)
RC1228
RC1228 *CURRENT SURVEY CONTROL
RC1228 ______________________________________________________________________
RC1228* NAD 83(2011) POSITION- 45 06 49.69715(N) 121 19 19.81396(W) ADJUSTED
RC1228* NAD 83(2011) ELLIP HT- 624.596 (meters) (06/27/12) ADJUSTED
RC1228* NAD 83(2011) EPOCH – 2010.00
RC1228* NAVD 88 ORTHO HEIGHT – 646.140 (meters) 2119.88 (feet) ADJUSTED
RC1228 ______________________________________________________________________
*
*
*
RC1228
RC1228 HISTORY – Date Condition Report By
RC1228 HISTORY – 1934 MONUMENTED CGS
RC1228 HISTORY – 1985 MARK NOT FOUND USPSQD
RC1228 HISTORY – 1985 MARK NOT FOUND USPSQD
RC1228 HISTORY – 20001010 GOOD OR-065

Figure 10 – GPS on BMS Residuals Using Geoid16b_NAD: Large relative Differences between Stations along the Oregon/Washington Border (note: rejections by geoid team have been removed).

Figure 10 is a plot of GPS on BMS residuals using Geoid16b_NAD depicting large relative differences between stations along the Oregon/Washington State border. It is the near Puget Island along the Columbia River. Station SC0330 and SC1086 are only 7 km apart and the relative difference is -20 cm (-11.4 cm minus 8.6 cm). This could be an issue with the NAVD 88 network design because there doesn’t appear to be many river crossing along the river between border stations. The fact that the residuals on the Washington State side are negative and the Oregon State side are positive is an indication that the stations need to be investigated.

Figure 11 – GPS on BMS Residuals Using Geoid16b_NAD: Large Negative Residuals North of Border between Oregon and Washington and Positive (or Small Negative) Residuals South of Border (note: rejections by geoid team have been removed).

The last figure, figure 11, is a plot of the GPS on BMS residuals using Geoid16b_NAD model that depicts large negative residuals north of the border between Oregon and Washington and positive (or small negative) residuals south of the border. This plot shows that the northern side of the river has large negative residuals all the way to the Pacific Coast. Once again, this is an indication that this portion of the NAVD 88 network should be investigated.

This column has focused on analyzing NGS’ GPS on BM data set that is used to make NGS’ hybrid geoid models. It provided procedures that users could employ when analyzing the differences between the modeled geoid values and the computed geoid values using GPS/Leveling data. This GPSBM data set or one similar will be used to make the next hybrid geoid model, as well as provide input to the transformation model between NAVD 88 and the new 2022 Vertical Reference System. All geospatial users should help develop this GPS on BMS data set to help improve the National Spatial Reference System and future hybrid geoid models. This column provided several examples of large relative differences in residuals between neighboring stations. Each example represents stations that should investigated based on different reasons, such as a weak NAVD 88 leveling network design in the region, the station’s published height attribute code implies that the station was not rigorously adjusted into the NAVD 88, and station pairs have different adjustment dates indicating a possible adjustment distribution correction issue or movement.

NGS has a program called “GPS on Bench Mark” to support users that occupy bench marks with GNSS equipment. This web site contains a lot of good information and provides the users with methods to recover, observe, and report information about stations in NGS’ database. The write up from the webpage is given below. I have highlighted a few sentences that the reader may find useful.

Write up from: GPS on Bench Marks?

What is GPS on Bench Marks?

Improve the National Spatial Reference System (NSRS):

Recover: Look up the description of an existing bench mark and visit the bench mark of your choice.
Observe: Record field notes, take digital photos, and collect GPS observations or coordinates for the bench mark you visit.
Report: Use online tools to send the information to NGS.

Where?

Currently there are over 400,000 bench marks across the Conterminous United States (CONUS), Alaska, Hawaii and all U.S. territories. Tidal marks and bench marks are used for determining heights. Use the maps to prioritize which bench marks to observe.

Who can participate?

Anyone with Global Positioning System (GPS) enabled phones, hand held devices or survey-grade GPS receivers can participate. Recommended procedures vary depending on the type of equipment used.

When should I start?

You can collect and share information any time. Join volunteer efforts across the United States in celebration of National Surveyors Week beginning March 20, 2016. Contact the local National Society of Professional Surveyors chapter or your NGS geodetic advisor to learn about projects being planned in your local area.

How?

For specific information on how to help please visit the Recover, Observe, and Report web pages that have instructions. Other resources include “Hunting for Marks!” and Geocaching Benchmark Hunting.

Why does this matter?

By providing GPS on benchmarks today you can help NGS improve the next hybrid geoid model, increasing access to NAVD 88, and enabling conversions to the new vertical datum in 2022.

You can also help the local surveying community know about nearby marks by improving scaled horizontal positions and updating the mark condition or description by submitting a mark recovery.

What happens next?

NGS will use your data to update its databases and improve future models and tools. If you still have questions, contact the GPS on BM Team.

In addition to participating in the NGS’ GPS on Bench Mark program, all geospatial users should participate in NGS’ 2017 geospatial summit, which will be held in April in Silver Spring, Maryland.

This summit is an opportunity for all users of the National Spatial Reference System (NSRS) to obtain a better understanding of NGS’ plans to modernize the NSRS. Users will be able to provide feedback directly to NGS leadership. My next column will address NGS plans to replace the North American Vertical Datum of 1988 in 2022.

October 5, 2016

Highest Peak in North America to be Surveyed

At 20, 320 feet, Mount McKinley is North America’s highest peak. (Photo courtesy of Todd Paris, UAF).

A new GPS survey of Mount McKinley, the highest point in North America, will update the commonly accepted elevation of McKinley’s peak, 20,320 feet. The last survey was completed in 1953.

The USGS, along with NOAA’s National Geodetic Survey (NGS) and the University of Alaska Fairbanks (UAF), are supporting a GPS survey of the Mount McKinley apex. Surveying technology and processes have improved greatly since the last survey and the ability to establish a much more accurate height now exists, the USGS said.

The Mount McKinley survey team, and their equipment, are expected to face temperatures well below zero, high winds and frequent snow. Current forecast, courtesy of NOAA. (Photo courtesy of Todd Paris, UAF).

An experienced team of four climbers, one from UAF and three from CompassData, will start the precarious trek to the summit with the needed scientific instruments in tow, in the middle of June. They plan to return on or before July 7 and begin work with the University of Alaska Fairbanks and NGS processing the data to arrive at the new summit elevation.

With the acquisition of new elevation (ifsar) data in Alaska as part of the 3D Elevation Program, there have been inquiries about the height of the summit. The survey party is being led by CompassData, a subcontractor for Dewberry on a task awarded under the USGS’ Geospatial Products and Services Contract (GPSC).

Using modern GPS survey equipment and techniques, along with better gravity data to improve the geoid model in Alaska, the partners will be able to report the summit elevation with a much higher level of confidence than has been possible in the past.

According to CompassData, the survey equipment includes two Trimble R10 antennas and one Trimble Net-R9 with a Zephyr-2 antenna. Also being taken up the mountain is a 10-meter specialized glacier avalanche probe and a Ground Penetrating Radar (GPR), courtesy of the University of Alaska Fairbanks.

It is anticipated the newly surveyed elevation will be published by the National Geodetic Survey in late August.

Climbing Mount McKinley, North America’s highest peak, is a daunting task for even the most experienced mountaineers at Denali National Park in Alaska. (Photo courtesy of National Geographic).

June 16, 2015
NGS Seeks Geodesist
The National Geodetic Survey is seeking applicants for a geodesist (real-time kinematic network) with the Spatial Reference System Division. Applications are being accepted through December 4.

The individual selected for this position will:
- Serve as subject matter expert and liaison to the Real-Time Kinematic Network (RTN) community.
- Assist in defining policy and guidance related to RTN for geodetic products and services.
- Serve as customer service representative responding to inquiries from various users regarding the geodetic products and services related to OPUS and RTN’s.
- Perform software maintenance, development, testing, debugging, and refining NGS software to improve NGS Online Positioning User Service (OPUS) Products and Services for use in validating RTN base stations.
Application links are:

Current U.S. government employees

All U.S. citizens
November 24, 2014
Seven Free Alternatives to OPUS GPS Post-Processing During U.S. Federal Government Shutdown
On October 1, 2013, the U.S. federal government shut down and furloughed 800,000 non-essential workers. While services considered essential remained active, those considered non-essential services, like the National Geodetic Survey’s Online Positioning User Service (OPUS), were shutdown. OPUS is a free, online GPS post-processing service. If you try to access www.ngs.noaa.gov, the following screen will be displayed:

Photo: NOAA

For those of you who rely on OPUS for GPS post-processing, now is a great time to try one of the other seven online post-processing services available and not subject to the U.S. federal government. Yes! I wrote seven, and the results from those seven are comparable to OPUS. The other seven, free online GPS post-processing services are:

CSRS-PPP: Canadian Spatial Reference System, Natural Resources Canada

AUSPOS: Geoscience Australia

GAPS: University of New Brunswick

APPS: Jet Propulsion Laboratory

SCOUT: Scripps Orbit and Permanent Array Center (SOPAC), University of California, San Diego

magicGNSS: GMV

CenterPoint RTX: Trimble Navigation

My colleague Mark Silver, creator of the X90-OPUS receiver I wrote about a few months ago, embarked on an effort to run test data through each of the online post-processing services to demonstrate that there are free, online GPS post-processing services available worldwide that produce results comparable to OPUS. The following report is the result of his efforts:

A Comparison of Free GPS Online Post-Processing Services

By Mark Silver

You are probably familiar with the National Geodetic Survey’s OPUS suite of online post processing tools (OPUS-Static, OPUS-Rapid Static and OPUS-Projects.) These services are capable of producing centimeter-level positioning from static GPS observations. What you may not realize is there are at least six viable alternatives to OPUS.

All are free, easy to use, provide world-wide coverage, and generate surprisingly similar results.

Since each uses a unique baseline tool and processing strategies they form an excellent reality check against each other.

IGS orbits and the IGS permanent CORS arrays are used by many of the services, however some use proprietary equipment arrays and orbit products that provide additional redundancy.

How comparable are these services? Which one is the best?

Criteria for Comparing

Comparing results is a difficult proposition:
- The true/correct answer for any site is unknown.
- What grading scale should be used? Should elevation differences be weighted differently than horizontal differences?
- Should the peak-to-peak range or the standard-deviation be prized?
- Should comparisons be made on long 24-hour data sets or short 2-hour occupations?
- Is a single data set sufficient for a meaningful comparison or are multiple data sets preferable?
- Should a service be ‘thrown out’ of consideration because the solutions are substantially different from the mean?
The answer to all of these questions is “it depends.” Your evaluation will depend on your specific application.

For this evaluation, the following rules governed the data set selection:
- Choose a site known to be stable with a clean EMI environment.
- Use 24-hour observation sets to enable ‘best case’ processing.
- Use a sufficiently large data set, 32-consecutive days, to expose trends.
- Choose a time period, 90-days in the past, so precise orbits are available to reduce ephemeris effects.
- Only consider GPS data.
- Use default settings for every option on each processing service.
Scoring

This would not be as interesting without a little competition.

To keep the evaluation simple, the sum of the X, Y and Height range will be the score and the services will be ranked from lowest score to highest score, with the low score being the ‘best.’

Range was chosen as an indicator of the expected maximum error that might be encountered if only a single 24-hour file was observed.

The combined range rewards a processing scheme that best estimates delays, interference, clock errors and other sources of change that occurred during the 32-day trial.

Remember that the every aspect of this ‘competition’ is arbitrary: from the selection of observation sets, to the final scoring system.

The real take-away from this evaluation is not that one service is better, but how close all of the services are to each other.

Two services (JPS’s APPS, magicGNSS) won’t be acceptable to the average user and a third (RTX Centerpoint) may not work for some users based on receiver and antenna support. Details of these problems are presented with the service descriptions below.

The Test Data

SGU1 in St. George, UT USA was chosen as the observation base. The observations consist of 32 consecutive days (May 3, 2013 through June 3, 2013), 24-hour observation files, 30-second interval, GPS only data. The data files were downloaded from the NGS CORS archive.

Each of the 32 files were submitted to each of the processing services and the results have been tabulated for X, Y and Ellipsoid Height. All data is presented in IGS08 current epoch framed coordinates. All data has been projected to UTM Meters for these comparisons.

The Average Values

Remember, the real story is how close each of these services produce results to one another. Let’s look at the average positions from each service and the difference from OPUS:

Fig 1: Average Solution Difference from OPUS

As you can see in Figure 1 above, the services were generally within 5mm of OPUS in X, Y and Height.

Position Tracking vs. Time

Fig 2: Service Results X vs. Time

Fig 3: Service Results X Range, Average

Fig 4: Service Results vs. Time

Fig 5: Service Results Y Range, Average

Fig 6: Service Results Z vs. Time

Fig 7: Service Results Z vs. Time

And the Winner Is…

Following are the scores, based on the combination of X, Y and Height range:

Fig 8: The Scores

Score ranking (remember this is just for fun as the services provided remarkably similar results):
1. AUSPOS
2. CenterPointRTX
3. GAPS
4. APPS
5. OPUS
6. CSRS-PPP
7. magicGNSS
There is a significant issue in the JPL APPS’s reported output positions, which will keep it from being of any use to most users. magicGNSS’s results are significantly different than the other services. User’s should independently evaluate magicGNSS’s suitability for their purpose. SOPAC’s SCOUT could not be evaluated because it patently does not support either the receiver or antenna that was used at the test site.

AUSPOS: Geoscience Australia

Score: 0.023

Submittal Page: http://www.ga.gov.au/bin/gps.pl

AUSPOS is a free service from Geoscience Australia. Access is via a simple web interface, the antenna height and type are entered along with a email address for the returned report set. File submission is via FTP or directly from the web interface.

The returned PDF report is the best looking of the reviewed services and includes a Processing Summary showing a map of the CORS sites that were used in the solution. SINEX files are also available.

AUSPOS uses the Bernese GNSS Software for processing baselines, IGS orbits and IGS network stations. Solutions are available for anywhere on the earth.

RINEX files need to be at least 1-hour in length, 6-hour files are recommended. Compact RINEX files are also accepted. Files may be compressed with UNIX, Hatanaka, ZIP, gzip or bzip compression.

Centerpoint RTX Post Processing: Trimble Navigation Limited

Score: 0.030

Submittal Page: http://www.trimblertx.com/UploadForm.aspx

CenterPoint RTX Post Processing is a free service offered by Trimble.
It works anywhere in the world and is based on a proprietary Trimble 100+ worldwide CORS network. Accuracy is 2 cm with 1-hour of observation data; 1 cm with 24-hours. Files longer than 24-hours are not accepted.

RTX uses GPS, GLONASS and QZSS tracked SV’s.

The reported output frames include ITRF2008 at current epoch and a user selectable frame like NAD83/2011 2010.0. RTX is one of the few services that will directly export NAD83 framed results.
A single page PDF and a XML result file are returned by RTX. Unfortunately, it is not possible to copy numerical results from the read-only PDF result file to the clipboard.

RTX supports a limited number of receivers (Trimble, Ashtech, Javad, some Leica, some Topcon) and a relatively small subset of IGS modeled antennas. For this test, TEQC was used to stuff the RINEX headers with a comparable Trimble receiver to the actual Ashtech ProFlex 500 receiver that is in use at SGU1. This was all that was required to spoof an accepted device. If the antenna had not been listed, it would have been necessary to spoof the antenna and adjust the height to reflect the difference in L1 phase center offset.

GAPS: University of New Brunswick

Score: 0.032

Submittal Page: http://gaps.gge.unb.ca/indexv520a.php

GAPS is an ongoing project at the University of New Brunswick and was developed by the Department of Geodesy and Geomatics Engineering.

File submission is by a web page and GAPS provides a large number of user inputs and potentially allows the highest level of customization of any of the reviewed services:
- You may enter a priori coordinates, and a priori constraints
- GAPS accepts static or kinematic files
- You can set the elevation mask
- The Neutral Atmosphere Delay model is selectable
- Earth Body Tides and Ocean Tidal Loading can be applied or disabled
GAPS only processes GPS data (no GLONASS.)

Submitted filenames must adhere to the SSSSDDDh.YYt file format. GAPS accepts RINEX and compact RINEX files, they may optionally be gzip, unix compressed or ZIP compressed.

APPS: Jet Propulsion Laboratory

Score: 0.033

Submittal Page: http://apps.gdgps.net/apps_file_upload.php

WARNING! APPS only reports the derived position to the nearest decimeter-meter in geographic (lat/lon) coordinates, while reporting ECEF coordinates to a fraction of a millimeter. If you choose to use APPS, you will need to manually convert the ECEF XYZ to geographic coordinates.

JPL’s APPS is based on GIPSY-OASIS (currently version 5). APPS uses NASA’s 70+ Global GPS Network plus densification from other systems (100+ total receivers distributed globally.) Solutions are typically available with 5 seconds delay from observation.

APPS is easy to use, you just specify a file to upload and then click on ‘Upload’ it takes only 15 seconds to get a result after the file upload is complete. You can optionally register for a free account and use email or FTP for bulk uploads.

APPS also has receiver Live Performance Monitoring: (http://www.gdgps.net/monitoring/index.html) which generates a real time graph of three receivers spread through the world.

OPUS: U.S. National Geodetic Survey

Score: 0.035

Submittal Page: http://www.ngs.noaa.gov/OPUS/

OPUS solutions are the most common PPP Post-Processed solution in the United States. Two flavors of OPUS are available for single points:
1. OPUS-Static: Available worldwide, requires 2-hours of data
2. OPUS-Rapid Static: Available with sufficient nearby CORS stations, requires 15-minutes of data
Long occupations (6+ hours) result in excellent horizontal and GPS-derived ellipsoid heights.

The new OPUS-Projects service processes multiple receivers through multiple sessions to a final processed network adjustment.

CSRS-PPP: Natural Resources Canada

Score: 0.039

Submittal Page: http://webapp.geod.nrcan.gc.ca/geod/tools-outils/ppp.php

Before using CSRS-PPP, you will need to register for a free user account.

CSRS has a fantastic desktop application named PPP-Direct that you can just drag and drop files onto. PPP-Direct automatically submits the file and saves all typing, greatly reducing the chance of error.

CSRS-PPP uses both GPS and GLONASS (if available) observables. Ocean Title Loading corrections can be overridden.

CSRS-PPP will accept single frequency files for processing. CSRS will accept RINEX and Compact RINEX, and will decode ZIP, GZIP and unix compression formats.

CSRS-PPP has a fantastic PDF report, a .csv file detailing results epoch by epoch and a great machine readable summary file.

The desktop submission tool, coupled with the great output reports made CSRS-PPP my favorite tool.

magicGNSS: GMV

Score: 0.081

Submittal Instructions: http://magicgnss.gmv.com/ppp/

magicGNSS Blog: http://magicgnss.gmv.com/wordpress/

magicGNSS accepts emailed files and returns solutions by email. Turnaround time is fast and features a nice PDF report plus SINEX, receiver clock bias files, tropospheric delay, KML trajectory and RINEX CLK clock bias files.

Static and kinematic files with observations from GPS, GLONASS are processed by magicGNSS and the service reportedly Galileo-ready.

magicGNSS uses a subset of IGS stations to provide core coverage.

SCOUT: Scripps Orbit and Permanent Array Center (SOPAC). University of California, San Diego

Scout accepts RINEX and compact RINEX files, compressed (Z, gz, ZIP) submitted from an FTP site or pushed onto a provided FTP server.

Files must be generated on a limited subset of receivers and antennas. While the IGS antenna and receiver files are the basis for acceptable devices, not all IGS-listed devices are on the allowable device list. SCOUT documentation specifically warns against spoofing devices and antennas.

SCOUT uses the GAMIT processing engine.

Because the test data for this article is from a unsupported receiver and the submittal process requires a FTP host server with anonymous access which most users will not bother with, the output from SCOUT was not evaluated.

Conclusion

The similarity of results between all of the services I processed is amazing. That they differ only by millimeters demonstrates the robustness of the algorithms and processes they use.

The difference between AUSPOS, RTX, GAPS, OPUS and CSRS-PPP solutions are negligible. For important positioning projects, it undoubtedly makes sense to use them all.

For locations in the United States, OPUS and RTX return NAD83-2011 framed results. Only OPUS returns derived orthometric heights using GEOID12A. While OPUS has more provenance than the other services, it is easy enough to submit important observations to multiple services as a reality check for important positions.

###

As you read from Mark’s report above, even though OPUS is shut down until the U.S. Congress can resolve its differences, don’t let that stop you from processing your GPS static sessions. However, some level of due diligence on your part is needed as requirements vary for each service. For example, static sessions for the OPUS-RS service can be as short as 15 minutes while other services require two hour GPS static sessions. Furthermore, some services process GPS L1 data while others require both GPS L1 and GPS L2 observations.

See you next month.

Follow me on Twitter at https://twitter.com/GPSGIS_Eric
October 2, 2013