This column discusses the results of the National Geodetic Survey (NGS) beta hybrid Geoid18 model and the differences between the beta model and the official hybrid geoid model, Geoid12B. It provides examples to explain the symbology of the Beta Geoid18 Web Map. GEOID18 will be the last hybrid geoid model that NGS will create before NAVD 88 is replaced by the North American-Pacific Geopotential Datum of 2022 (NAPGD2022). I encourage users to access, investigate and become familiar with the web map.
My last column included links to the NGS website that provides the beta coordinates and information about the latest Multi-Year CORS solution (MYCS 2). The column also noted that in late February 2019, NGS released a beta version of the latest hybrid geoid model. See Figure 1, “National Geodetic Survey’s Home Web Page.” This column discusses the Beta Geoid18 Web Map, the results of the hybrid Geoid18 model, and the differences between the beta model and the official hybrid model, Geoid12B.
Figure 1. National Geodetic Survey’s Home Web Page. (Screenshot: National Geodetic Survey)
The Geoid18 hybrid geoid model can be accessed here. See Figure 2, Excerpt from Beta Geoid18 Website. The site provides an opportunity for users to compute a Beta Geoid18 value for a particular station. I would encourage all users to obtain an understanding of the new hybrid model. Once again, it should be noted that this model is a beta model for users to test their workflows and should never be used for official or production work. This allows users to identifies potential issues and differences between Geoid12B and Geoid18, and then contact NGS if they have a question. NGS has done a tremendous job of explaining the Geoid18 process and results, and would appreciate users helping to evaluate the new hybrid model. Several of my previous columns have highlighted the NGS GPS on Bench Marks (GPS on BMs) program and how users have supported the development of the hybrid Geoid18 model: Part 5, Part 6, Part 7, Part 8 and Part 9.
The NGS Beta Geoid18 website provides access to GIS tools that allow users to identify changes between Geoid12B and Geoid18 in their area of interest. The site also states that the hybrid geoid model, Geoid18, will be the last hybrid geoid model that will be created before the new geopotential datum, NAPGD2022, is adopted as the official datum. This is the opportunity for users to be involved in the analysis of the Beta hybrid geoid model. NGS will consider changes to the Beta model until it becomes an official published product. This hybrid geoid model is slightly different from the previous hybrid geoid model, Geoid12B. Similar to Geoid12B, the majority of the design of the hybrid model comes from the relationship between the NGS’ GNSS-derived ellipsoid-derived heights and the leveling- derived orthometric NAVD 88 heights. In other words, the hybrid model is designed to fit to the NAVD 88 orthometric heights.
That said, since the creation of hybrid Geoid12b, there have been improvements in the underlying gravimetric geoid model used in Geoid18. These improvements include:
Better elevation data and improved digital elevation modelling techniques,
New gravity data from satellite gravity missions,
New airborne gravity data from the NGS GRAV-D program, and
Improved geoid modeling techniques.
My previous columns have focused on procedures and routines for establishing GNSS-derived orthometric heights. As I’ve mentioned in these columns, there are many ways to analyze and investigate GNSS data and adjustment results. I have provided basic concepts that I believe are important for users to understand. My October 2016 column focused on the NGS “GPS on BMS (GPSBM)” dataset that was used to create the last hybrid geoid model, Geoid12B.
As mentioned in my October 2015 column, the hybrid geoid model is designed to fit the published NAVD 88 leveling-derived orthometric heights. I highlighted that the GPS on BMs dataset can be used to identify potential issues in the NAVD 88 published orthometric heights. The October 2016 column provided tools and routines that can be used to identify potential issues in NAVD 88 heights and/or NAD83 (2011) published ellipsoid heights. In support of the Beta Geoid18, NGS performed a detailed analysis of the GPS on BMs stations that were used in the creation of Geoid18.
Figure 2. Excerpt from Beta Geoid18 Website. (Image: National Geodetic Survey)
If you click on the “Web Map button” on the Geoid18 web page (see arrow in Figure 2), you may see the statement highlighted in Figure 3. Clicking on the link will redirect you to the correct web site (see Figure 4.).
Figure 3. Result of Clicking on Web Map Button (Screenshot: National Geodetic Survey)
Figure 4. Web Map Option – Results after clicking https://arcg.is/vSn8K (Top Level of Beta Geoid18 Map) [Screenshot: National Geographic, Esri, Garmin, HERE, UNEP-WCMC, USGS, NASA, ESA, METI, NRCAN, GEBCO, NOAA, increment P Corp. | National Oceanic and Atmospheric Administration (NOAA), National Ocean Service (NOS), National Geodetic Survey (NGS)]This data layer provides the value of the post-modeled residuals for all of the GPS on Bench Marks that were part of the evaluation of the Beta GEOID18 model. This Feature Layer is used to populate several layers in the Beta GEOID18 Web Map including the layers called Residuals and GPSonBM. The data for this web map can be found here.The top level of the Beta Geoid18 Map depicts a high-level picture of the residuals. The residuals are in centimeters and represented by different colors. The larger green and yellow circles represent the number of features in the region. The individual GPS on BMs station information appear as the user zooms down. There is a lot of information provided on the Web Map site. The legend changes to provide more detailed information as the user zooms down on the map. I have highlighted four sections on the legend in Figure 5 and provided an explanation of the layers below:
This data layer provides the value of the post-modeled residuals for all of the GPS on Bench Marks that were part of the evaluation of the Beta GEOID18 model. This Feature Layer is used to populate several layers in the Beta GEOID18 Web Map including the layers called Residuals and GPSonBM. The data for this web map can be found here.
This data layer denotes whether the GPS on Bench Mark was used or rejected in the development of the Beta hybrid geoid GEOID18. The data for this web map can be found here.
This data layer denotes whether the GPS on Bench Mark was used or rejected in the development of the hybrid geoid GEOID12B. This has all of the same attributes as the spreadsheet provided on the NGS GEOID12B web page. More information can be found here.
This is a tile package that displays the difference between GEOID18 and GEOID12B in CONUS. It contains two overlayed raster files, one of which is the estimated error and the other is its hill shade. The data for this web map can be found here.
Figure 5. Legend of Beta Geoid18 Web Map (Screenshot: National Geodetic Survey)
Clicking on the “Content” link provides the data layers (see Figure 6). The user can turn these layers on and off depending on what they’re interested in analyzing.
Figure 6. Contents of Beta Geoid18 Web Map (Screenshot: National Geodetic Survey)
As previously stated, additional details are available as the user zooms into an area of interest (see Figure 7). Five stations have been highlighted in this figure to explain the symbology used on the Web Map site. See Figure 8 for these explanations.
Figure 7. Example of the details available in an area in Eastern North Carolina (Screenshot: National Geodetic Survey)
Figure 8. An Explanation of Stations Highlighted in box titled Example of the details available in an area in Eastern North Carolina (Screenshot: National Geodetic Survey)
When the user clicks on a station’s icon, another window appears that provides specific information about that station. See Figure 9. If the user clicks on the “More Info” button, the routine retrieves the NGS datasheet from the NGSIDB (see Figure 10). As the NGS datasheet states at the end of the description for station Y 247, the station has been obliterated by a mower, which is why it probably was not used in Geoid18.
Figure 9. Example of Information Available for Individual Stations (Screenshot: National Geodetic Survey)
Figure 10. NGS Datasheet for Station Y 247 (PID EX0083) (Screenshot: National Geodetic Survey)
Figure 11 provides all the information available for station Y 247. It should be noted that the station was used in Geoid12B and not used in Geoid18. This means that there will be differences between Geoid12B and Geoid18 in areas where a station was used in Geoid12B but not used in Geoid18. The amount of the difference will depend on the size of the post-modeled residual. In this example, the post-model residual is 7.39 cm.
Figure 11. Example of Geoid18 Information Available for Station Y 247 (Screenshot: National Geodetic Survey)
GPS on BMs data are usually based on different epochs of data; that is, the leveling data is usually observed at a different epoch than the GNSS data. This means, if the station has moved since the last time it was leveled, then the GNSS-derived ellipsoid height minus the leveling-derived orthometric height will not be equal to the geoid height. The procedure for computing GPS on BMs residuals was described in my February 2018 column. To determine if a bench mark had moved since it was last leveled, the analyst needs several nearby bench marks occupied by GNSS.Users have been very important to the development of Geoid18 by participating in NGS’ GPS on BMs program. These data have been used to improve the reliability of the hybrid geoid model. Users can now help by evaluating areas that have large changes between Geoid12B and Geoid18 (see box titled Figure 12). To help ensure that the appropriate stations were used to create the hybrid geoid model Geoid18, users could occupy nearby stations in the area to evaluate the reliability of the model. This will help NGS improve the reliability of the model in that region.
Figure 12. Example of a Large Difference Between Geoid12B and Geoid18 in Western North Carolina (Screenshot: National Geodetic Survey)
I described the NGS’ published height codes in my October 2016 column. In the case of Mitchell 2, there’s no leveling data in NGS’ database in the area surrounding Mitchell 2. There may be leveling projects that have been performed by other agencies such as the USGS but the leveling data have not been processed and loaded into NGS’ database. Users could help by performing GNSS observations on bench marks in the region that are in NGS’ database and/or by performing leveling observations between the GPS on BMs station and the nearest bench mark that has leveling data in NGS’ database.In the example of a large difference between Geoid12B and Geoid18 in Western North Carolina, station Mitchell 2 (PID FB2737) was used in Geoid12B but not used in Geoid18. It wasn’t used in Geoid18 because the NAVD 88 height was not based on an adjustment. According to the description, the leveling tie was performed by a field party that was performing a horizontal survey project (see Figure 13). The field party performed the appropriate leveling procedures but, in this case, the leveling data have not been placed in computer-readable form, so the orthometric height cannot be verified.
Figure 13. NGS Data Sheet for Station Michell 2 (PID FB2737) (Screenshot: National Geodetic Survey)
I encourage users to access the web map and investigate stations that have large post-modeled residuals and/or stations that were used in Geoid12B but were not used in Geoid18. The NGS analyst rejected stations based on pre- and post-modeled residuals but many times there wasn’t enough redundant information available to ensure the station should be rejected or used in the creation of the hybrid geoid model. Users should be commended for their participation in the GPS on BMs program. Hopefully, users will continue their support by evaluating the beta hybrid geoid model.
My last two columns (NGS 2018 GPS on BMs program in support of NAPGD2022 — Part 6 and NGS 2018 GPS on BMs program in support of NAPGD2022 — Part 7) described the National Geodetic Survey’s (NGS) GPS on BMs 2018 interactive web map, and provided an update and status report on stations observed in support of the 2018 GPS on BMs Program. This column will provide another update and status report on stations observed in support of the 2018 GPS on BMs program and provide an example of how the OPUS-shared results filled in a void area in West Virginia that will benefit the development of the hybrid geoid model GEOID18. The column will also provide an example of how OPUS Shared results identified a reset station that has an invalid NAVD 88 height, and the importance of having a least two OPUS Shared results to ensure the reliability of the OPUS solutions.
As mentioned in the last column, the GPS on BMs 2018 web page contains a link to a web map where users can determine which bench marks NGS would like users to occupy before the August 31, 2018, deadline. The box titled “2018 Web Map” depicts the map update as of July 27, 2018 (1738 priority marks completed). My last column reported that as of May 29, 2018, there were 1067 priority marks considered completed. During the past two months, 671 more priority stations have been reported completed. This is progress but this still only represents about 30 percent of the priority marks. Hopefully, this will increase dramatically during the month of August. Remember, the cut-off date for data to be included in the creation of the hybrid geoid model GEOID18 is August 31, 2018.
NGS periodically provides an update on the GPS on Bench Marks Program. On July 3, 2018, NGS sent an email to everyone that shared GPS data on NGS bench marks via OPUS or registered for NGS’ February 2018 webinar about GPS on Bench Marks. The email provided an update on the GPS on Bench Marks Program (see box titled “July 3, 2018, NGS Email on GPS on BMs Update”). The map provided in the update indicated that some of the new observations may generate changes between +/- 8 cm.
July 3, 2018, NGS Email on GPS on BMs Update
(Source: Email from National Ocean Service, NOAA; [email protected] to Dave Zilkoski)
Update: GPS on Bench Marks
Over 1,420 marks completed, and two months left to improve GEOID18 accuracy in your area!
Image: National Geodetic SurveyYour observations are making a difference! The color ramp in the map above reflects accuracy improvements in a hybrid geoid model from your recently submitted GPS observations. The improvements will be realized when NGS releases GEOID18.
In case you missed it
In early 2018, NGS released a list of priority bench marks where GPS data is needed to improve GEOID18, NGS’ last planned hybrid geoid model before The North American Vertical Datum of 1988 (NAVD 88) is replaced by the North American-Pacific Datum of 2022 (NAPGD2022). Data to support GEOID18 will be accepted until the end of August 2018. After that, GPS on Bench Marks (GPS on BM) efforts will expand to include other regions and will focus on data to improve future transformation tools.
How can I help?
Following the guidance provided on the NGS GPS on BM website, you can help by collecting static GPS data on adjusted NAVD 88 bench marks and submitting the data to NGS via OPUS Share. To improve efficiency and reduce unnecessary redundancy, we have created a GPS on Bench Marks 2018 web map to help contributors know where we have the data we need and where we still need GPS observations.
Thank you to our contributors
Over 1,700 observations have been submitted to date, completing the required observations for over 1,420 marks from our prioritized list. Each observation requires at least 4 hours of data collection with a survey grade GPS receiver, plus additional time for planning, travel, and data submission, so each one is a significant contribution. Visit the GPS on BM website for updates on our biggest data contributors and each state’s progress toward the goals.
Why are you receiving this email?
• You shared GPS data on NGS bench marks via OPUS, or
• You registered for our February 2018 webinar about GPS on Bench Marks.
We anticipate sending quarterly updates about these and related efforts. If you’d like to opt-out, click the “Manage Subscriptions” at the bottom of this email.
NOAA’s National Geodetic Survey
geodesy.noaa.gov
NGS is tentatively planning another webinar on the GPS on Bench Marks program for August 9, 2018 (2 pm to 3 pm eastern time). NGS will provide an update on the GPS on Bench Mark program and probably will highlight potential improvements between the current hybrid geoid model GEOID12B and the latest prototype version of the future hybrid geoid model GEOID18. I would encourage everyone to sign up for the NGS webinar series.
Source: Plot Generated Using ArcGIS
Users can subscribe to any or all of NGS four public subscription lists — news, webinar, training, and GPS on Bench Marks — by visiting the NGS subscription services web page and submitting their email address for the type(s) of notices they want to receive. (https://www.ngs.noaa.gov/INFO/subscribe.shtml)
As indicated in the figure provided in NGS’ July 3rd update on the GPS on Bench Marks program email, there are many areas of the country that have already benefitted from users participating in NGS’ GPS on BMs program. This column will highlight an area near Charleston, West Virginia, were users have been very active in providing OPUS Shared results. The box titled “GPS on Bench Marks near Charleston, West Virginia” depicts the marks that meet NGS’ criteria and will be involved in the development of the hybrid geoid model GEOID18. As you can see from the plot, there are several new stations that will be used in the development of the model which will help to improve the reliability of the product.
The box titled “An Example of OPUS Shared Stations in Charleston, West Virginia, Region” provides the stations’ PID and OPUS designation. The six OPUS Shared stations cover approximately a 50 km square area. Most of the stations are only 10 km apart. These stations will definitely help to improve the reliability of the hybrid GEOID18 model.
An Example of OPUS Shared Stations in Charleston, West Virginia, region
(Source: Plot Generated Using ArcGIS)
Source: Plot Generated Using ArcGIS
When using OPUS Shared results, users should always check to see if a station has been observed more than once. The box tilted “Differences in OPUS Shared Ellipsoid Heights in Charleston, WV, Region” lists the pairs of OPUS observations for the stations depicted in the previous plot. The column labeled “Difference in Ellipsoid Heights” provides the differences in ellipsoid heights based on the two different OPUS Shared results. All differences are less than 1.5 cm and most are less than 1.0 cm. This is indicating good repeatability to the cm level but this may not be indicating accuracy. These stations were observed one day apart but observed at about the same time of the day. They could have the same systematic errors effecting the results such as multipathing and satellite geometry. When performing the second OPUS Shared observation, users should select a different time of day to improve the chances of detecting, reducing, and/or eliminating the effects of remaining systematic errors.
Differences in OPUS Shared Ellipsoid Heights in Charleston, West Virginia, region
Source: National Geodetic Survey
The box titled “Differences in OPUS-Shared GNSS-Derived Orthometric Heights Using GEOID12B and Published NAVD 88 Heights” provides the differences between the GNSS-derived orthometric heights using GEOID12B and the published NAVD 88 values. This table indicates that there is a large difference (23.4 cm) for station HX2382 (L105 Reset 1962). Since the two ellipsoid heights only differ by 1.0 cm, this is an indication that the station probably moved since it was Reset or the reset observations were performed incorrectly. Either way, this station should not be used in the development of the hybrid model or used by anyone for geodetic control.
Differences in OPUS-Shared GNSS-Derived Orthometric Heights using GEOID12B and Published NAVD 88 Heights
Source: National Geodetic Survey
Since GEOID12B is a hybrid geoid model that was designed to be consistent with NAVD 88 values, I always compute differences between GNSS-derived orthometric heights using the experimental geoid model and published NAVD 88 height values. I described this process in my October 2015 column (http://stage.globalpositioningnews.com/establishing-orthometric-heights-using-gnss-part-3/). The box titled “Differences in OPUS-Shared GNSS-Derived Orthometric Heights Using xGeoid17b and Published NAVD 88 Heights” provides the differences between the GNSS-derived orthometric heights estimated using IGS08 (2005) ellipsoid heights with the xGeoid17b geoid model and published NAVD 88 heights. The values in the column labeled “GNSS-Derived Orthometric Height minus Published NAVD 88” represent an approximate difference between NAPGD2022 and NAVD 88. The box titled “OPUS-Shared GNSS-Derived Orthometric Heights Using xGeoid17b minus Published NAVD 88 Heights” provides a plot that depicts these differences.
Differences in OPUS-Shared GNSS-Derived Orthometric Heights Using xGeoid17b and Published NAVD 88 Heights
Source: National Geodetic Survey
OPUS-Shared GNSS-Derived Orthometric Heights Using xGeoid17b minus Published NAVD 88 Heights
(Source: Plot Generated Using ArcGIS)
Source: Plot Generated Using ArcGIS
Once again, it should be noted that PID HX2382 value is much different from the other values. To look for outliers, a mean difference was removed from the results. The box titled “OPUS-Shared GNSS-Derived Orthometric Heights Using xGeoid17b minus Published NAVD 88 Heights with a Mean Value Removed” makes it easier to see that station HX2382 is an outlier. The station is approximately 25 cm different from its neighboring stations that are only 10 km away. As previously mentioned, this station apparently moved since being Reset in 1962 or the reset observations were performed incorrectly. Identifying stations that have moved since the last time they have been leveled is one of the benefits of participating in the GPS on BMS program.
OPUS-Shared GNSS-Derived Orthometric Heights Using xGeoid17b minus Published NAVD 88 Heights with a Mean Value Removed
(Source: Plot Generated Using ArcGIS)
Source: Plot Generated Using ArcGIS
For completeness, both a bias and trend were removed from the differences since IGS08 (2005) GNSS-derived orthometric heights and NAVD 88 heights indicate that there’s an apparent long-wavelength trend between the two sets of values. The box titled “OPUS-Shared GNSS-Derived Orthometric Heights Using xGeoid17b minus Published NAVD 88 Heights with Bias and Trend Removed” depict the differences with a bias and trend removed. As in the other figures, PID HX2382 clearly indicates that it is an outlier relative to its neighbors. This station would be rejected by the geoid team when creating the next hybrid geoid model.
It should be noted that except for the Reset station, all of the differences are less than 2 cm. Although, some relative differences between closely-spaced stations approach 4 cm. For example, the differences between stations HX1746 and HX2496 is -3.7 cm (-1.8 cm – 1.9 cm). The differences in ellipsoid heights from the OPUS Shared solutions are all less than 1.5 cm, even the differences between ellipsoid heights for station HX2382 is only 1 cm. This is an indication that the reset station, HX2382, does not have a valid NAVD 88 published height and should not be used as control. Surveyors that adhere to the FGCS specifications and procedures would realize that this station did not have a valid NAVD 88 height and would not use the published NAVD 88 as control in their project. For example, surveyors performing a leveling project would perform a 2- or 3- mark leveling tie and the results would indicate that the station had moved since it was last leveled.
OPUS-Shared GNSS-Derived Orthometric Heights Using xGeoid17b minus Published NAVD 88 Heights with Bias and Trend Removed
(Source: Plot Generated Using ArcGIS)
Source: Plot Generated Using ArcGIS
This type of validation procedure should also apply for OPUS users. If a user obtains one OPUS solution and proceeds to perform a survey from that station, the user does not know whether the OPUS height value is reliable or accurate. One solution does not provide any indication of reliability.
The OPUS Shared station PID SV0942 (A 25) is an example of two OPUS Shared results generating ellipsoid height values that differ by 10 cm. (See yellow highlighted section in the box titled “Differences in OPUS Shared Ellipsoid Heights for PID SV0942.”) This large difference is significant when you performing a survey where you need heights to better than 3 cm (0.1 foot). This is one reason that NGS requires two OPUS Shared solution for every mark used in the development of the hybrid geoid model.
Differences in OPUS Shared Ellipsoid Heights for PID SV0942
Source: National Geodetic Survey
In the OPUS Shared solutions of PID SV0942, the latest OPUS Shared GNSS-derived orthometric heights (2018-07-14) agrees to about a cm with the published NAVD 88 height, while the 2014 Opus Shared GNSS-derived orthometric height is -11.4 cm different from the published NAVD 88 value. (See yellow highlighted section in box titled “Differences in OPUS-Shared GNSS-Derived Orthometric Heights Using GEOID12B and Published NAVD 88 Heights for PID SV0942.”)
Differences in OPUS-Shared GNSS-Derived Orthometric Heights Using GEOID12B and Published NAVD 88 Heights for PID SV0942
Source: National Geodetic Survey
It should be noted that the error estimates provided in the Opus Shared output indicate the ellipsoid heights are good to about +/- 1 cm. (See highlighted section in box titled “Two OPUS Shared Solution for PID SV0942.”) Saying that, the two NAD 83 (2011) ellipsoid heights disagree with each other by 10.2 cm. I like a quote that is attributed to Mark Twain – “It ain’t what you don’t know that gets you into trouble. It’s what you know for sure that just ain’t so.” (Obtained from http://lukefostvedt.com/famous-quotes-about-statistics/). I’m not suggesting that Opus Shared solutions results are incorrect. I’m attempting to provide an example of why users need to repeat all observations and to demonstrate how error estimates can be misleading.
“It ain’t what you don’t know that gets you into trouble.It’s what you know for sure that just ain’t so.”
The number of GPS on Bench Mark stations completed as of July 27, 2018, represents about 30 percent of the total number of stations that need to be observed. As I have explained in previous columns, there are many invalid GPS on BMs stations that may be used in the next hybrid geoid model unless more bench marks with valid NAVD 88 heights are observed with GNSS. NGS will accept data for inclusion in the next hybrid geoid model, GEOID18, until the end of August 2018. After that, NGS’ GPS-on-Bench-Mark Program will expand to include other regions and will focus on data to improve NGS datum transformation tools. This column provided an update and status report on stations observed in support of the 2018 GPS on BMs program, provided an example of how the OPUS Shared results can be used to identify a station that may have moved since it was last leveled, and the importance of repeating OPUS observations. I would encourage users to register for NGS’ next webinar on the GPS on Bench Mark Program scheduled for Thursday, Aug. 9th to hear the latest status of the program.
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
(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.)
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
(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.)
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.
