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

June 6, 2018  - By
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My last column described the National Geodetic Survey’s (NGS) GPS on Bench Marks (BM) 2018 interactive web map, and provided an update and status report on stations observed in support of the 2018 GPS on BMs Program. It mentioned that all new data received by the cut-off date of Aug. 31 will be analyzed by NGS and, if appropriate, the results will be included in the next hybrid geoid model. This is a great opportunity to provide data that will help to improve the hybrid geoid model in your region. This column will provide an update and status report on stations observed in support of the 2018 GPS on BMs program and provide an example of how OPUS-shared results identified a station that may have moved since it was last leveled.

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

2018 Web Map

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

Map After Searching for PID KW0690

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

NGS Data Sheet for KW0690

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

OPUS Shared Solutions Web Page

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

Web Page After Clicking on PID Option

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

An Example of Selecting an OPUS Shared Solution for a PID

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

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

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

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

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

Excerpt from NGS Data Sheet for KW0690

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

Table of GPS on BMs Residuals for KW0690

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

Using NGS Data Sheet Point Radius Retrieval Option for KW0690

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

Output of NGS Data Sheet Point Radius Retrieval Option for KW0690

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

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

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

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

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

Using the xGeoid17 Web tool for Stations Nearby KW0690

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

Table of GPS on BMs Residuals for KW0690 Using xGeoid17b

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

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

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

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Count of Stations Completed by State
May 29, 2018

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

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