Establishing orthometric heights using GNSS — Part 11

Strategically Occupying Stations to Support NGS’ GPS on Bench Marks Program

This is the 11th segment in my series on “Establishing Orthometric Height Using GNSS.” Each column has focused on a specific topic and provided procedures and tools for analyzing that topic. The columns are meant to build on each other. When addressing a topic that has been discussed in a previous column, web links are provided so the reader can review the previous columns.

The last column, December 2016, highlighted NGS plans for the 2022 Vertical Reference Datum and provided approximate height differences that users can expect to see. It also provided a little history behind the differences between the NGVD 29 and NAVD 88, and how each replacement of the United States’ National vertical reference datum is improving the user’s ability to obtain the most accurate orthometric height. The 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. Each example represented stations that should be 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.

The following questions still need to be addressed: (1) Is the large difference due to an issue with the NAVD 88 orthometric height or the NAD 83 (2011) ellipsoid height? and (2) Should the station be included in the development of NGS’ hybrid geoid models? This column will provide 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. This information will be useful to NGS when developing hybrid geoid models and the 2022 Vertical Transformation model.

At this moment, the user is limited to what they can do to assist in identifying the problem. There are basically two options: (1) perform precise leveling observations between two or more stations and/or (2) perform accurate GNSS observations between two or more stations. Performing geodetic leveling between two stations is the desired option but is very expensive and time consuming; however, performing accurate GNSS observations between the two stations is relatively inexpensive and, if NGS’ OPUS-Projects is used to process the data then it is relatively simple to determine accurate NAD 83 (2011) ellipsoid heights and height differences. Even if the project is not submitted to NGS for inclusion into NAD 83 (2011), OPUS-Projects provides a easy and traceable mechanism for others to analyze the results and make their own decision.

First, let’s look at what NGS provides the user on their GPS on Bench Mark Program. The October 2016 column discussed the GPS on Bench Mark dataset used to create GEOID12B. It provided basic information about the program and provided links to websites that address the program. This column will provide additional information that will be useful for those individuals that desire to participate in the GPS on Bench Mark program. The website provides information on bench mark reconnaissance and recovery. NGS outlines to the user how to use their data files to perform a desktop reconnaissance. They provide eight steps that they believe will be helpful to the user when supporting the GPS on Bench Mark program. (See box titled “NGS’ Suggested Eight Steps for Users to Follow When Participating in the GPS on Bench Mark Program.”)

NGS’ Suggested Eight Steps for Users to Follow When Participating in the GPS on Bench Mark Program North American Vertical Datum of 1988 (NAVD 88) consists of a leveling network on the North American Continent, ranging from Alaska, through Canada, across the United States, affixed to a single origin point on the continent: Desktop reconnaissance Reconnaissance materials Reconnaissance equipment Bench Mark Hunting Photos Observe and record Plan for Survey Observation Add your Planned Observation to the ArcGIS Online Map

Each step has a short narrative that provides helpful information for users that want to participate in the program. This column will focus on the first step titled Desktop reconnaissance. (See box titled “Excerpt from the National Geodetic Survey on Bench Mark Reconnaissance and Recovery.”)

Excerpt from the National Geodetic Survey on Bench Mark Reconnaissance and Recovery North American Vertical Datum of 1988 (NAVD 88) consists of a leveling network on the North American Continent, ranging from Alaska, through Canada, across the United States, affixed to a single origin point on the continent: 1. Desktop reconnaissance Bench marks of First and Second order leveling are targeted for GPS observations. Identify where you are looking for survey control. Generally surveyors try to tie into the NSRS without traveling too far from their project areas. Once you have determined your area of interest, use mapping applications to find marks that meet your criteria. The two recommended mapping applications are the NGS Data Explorer and DSWorld. The NGS database does not always get updated when geocachers recover marks on their web site, but DSWorld does provide information from their web site by showing a when it has been recovered. To help assist surveyors and geocachers we have also created an ArcGIS Map Package , a zip file for non ArcGIS users and an ArcGIS Online (AGOL) Web Map available using the links below. The Web Map Application is available using any browser and the Map Package and zip file is for users interested in performing their own analysis. GPS on Bench Marks AGOL Map NGS GPS on Bench Marks Esri Map Package (~178 MB) NGS GPS on Bench Marks Shapes/rasters (~88 MB)

These datasets provide the bench marks that were used in the creation of Geoid12B as well as the new GPS on bench marks that have been incorporated into NAD 83 (2011) since the creation of Geoid12B. This is useful information for those that want to occupy different bench marks than those previously observed with GNSS, and it is especially useful for identifying areas of the country that do not have enough bench marks occupied by GNSS. However, as I mentioned in my October 2016 column, the GPS on Bench Mark dataset can also be useful for identifying issues with NAVD 88 orthometric heights and NAD 83 (2011) ellipsoid heights. In the October 2016 column, I recommended that users perform an analysis of the differences between the published Geoid12B values and computed values from the NGS datasheet. (See box titled “Excerpt from October 2016 column – Analyzing Stations in the GPSBM Table.”)

Excerpt from October 2016 column – Analyzing Stations in the GPSBM Table. 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. 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 column 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. 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 column 3 (October 2015), the user will have to remove a bias and trend based on the differences in the region. 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 column 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. 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 column 7 (June 2016). As mentioned in column 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.

If you download the Zip file or the Esri Map Package, you should have a layer titled “NGS_Bench_Marks.” This layer contains all the bench marks from the NGS database that have NAVD 88 orthometric heights with the attribute “ADJUSTED.” It should be noted that this is not the complete list of stations used to create the hybrid geoid model GEOID12B. This file only contains bench marks that were established using precise geodetic leveling procedures and incorporated into NAVD 88 using NGS’ leveling adjustment program. The list of attributes and their meaning was provided in my February 2016 column. The ArcGIS NGS_Bench_Marks layer contains a NAVD 88 orthometric height, a Geoid12B value, and an ellipsoid height if the station was occupied in a GNSS project. The ArcGIS user can select all bench marks that have a NAD 83 (2011) ellipsoid height in their state by using an ESRI query builder statement; for example, “STATE” = ‘NC’ AND “DATUM_TAG” = ‘(2011)’ AND “POS_DATUM” = ‘NAD 83’. Now the user can compute the GPS on BMs residual using the following formula: GPS on BMs Residual = Geoid12B value minus [NAD 83 (2011) Ellipsoid Height – NAVD 88 Orthometric Height)]. The user can perform this operation in the ESRI ArcGIS program or download the ArcGIS “NGS_Bench_Marks.dbf” file into Excel (or another spreadsheet program) and compute the computation in that spreadsheet program. The user can then import the file back into ArcGIS or their own GIS software. Once you have the GPS on BMs Residuals you can plot them and look for outliers. This is what I denote as “Strategically Occupying Stations to Support the GPS on Bench Mark Program.” I performed the above operation for the entire “NGS_Bench_Marks” file.

The file can be downloaded as an Excel document here and as a text document here.

So, what do I really mean by strategically occupying station to support the GPS on Bench Mark Program. Once you plot the GPS on Bench Marks residuals, the user should use the plots to identify stations that should be re-occupied because of large residuals or new stations that should be occupied in areas void of control. Figure 1 is an example of the GPS on BMs residuals for the State of North Carolina.

Looking at figure 1, the reader should notice some large red circles (negative GPS on BMs residuals) are located near some large blue circles (positive GPS on BMs residuals). In my opinion, these regions should be analyzed to determine if stations should be re-observed during a GPS on Bench Mark campaign. This doesn’t mean that if other stations are occupied that they will not help improve the hybrid geoid model and the NAVD 88 transformation model to the new 2022 Vertical Reference Datum, it just means that these previously occupied stations are questionable and re-observing these stations may help to explain why the residuals are so large. I’ve provided a couple of examples in North Carolina to explain what I mean.

Figure 2 depicts a station with a large negative residual (-7.9 cm) surrounded by stations with smaller residuals (mostly positive residuals). This station’s published NAVD 88 height may be an invalid height; that is, the station may have moved after the leveling-derived orthometric height was determined. In my opinion, this station should not be used in the development of a hybrid geoid model or any transformation model from NAVD 88 to another vertical reference datum. It would be useful information to know if the NAVD 88 orthometric height is invalid. In this example, the user could re-observe station Z 183 (PID = FA0997) with a long GNSS session, or simultaneously observe station FA0997 and another nearby station (such as AH5641) during the same long session. The second option allows the user to estimate a new ellipsoid height difference between the two stations that can be compared with the published ellipsoid height difference.

The ArcGIS NGS_Bench_Marks layer includes when the station was first recovered (e.g.,1967) and last recovered (e.g., 2009), and the condition of the station (e.g., good condition). The NGS dataset provides the network and local accuracies for published NAD 83 (2011) stations. (See box titled “Excerpt from NGS’ Datasheets for Stations FA0997 and AH5641.”) We discussed NGS’ datasheet and published local and network accuracy values in the August 2015 column.

The stations’ local and network accuracy values are highlighted in the box titled “Excerpt from NGS’ Datasheets for Stations FA0997 and AH5641.” Station AH5641 local ellipsoid standard error value (0.51 cm) is much better than station’s FA0997 value (2.47 cm). Next, we should look at the local network accuracies to determine which stations were simultaneously observed during a GNSS survey. Once again, these options on the NGS’ datasheets were discussed in the August 2015 column.

The box titled “Excerpt from NGS’ The Local and Network Accuracy Data Sheet for Stations FA0997 and AH5641” provides the local and network accuracy data sheet for stations FA0997 and AH56412. The readers should notice that Station FA0997 only has one local accuracy to another station and that station is not AH5641. This implies that these two stations were not observed during the same session. The large relative difference in residual could be due to an invalid NAVD 88 orthometric height but it could also be due to an undetected error in the ellipsoid height due to a weak GNSS survey design. Let’s look at another example where there’s more than one outlier in a small group.

Figure 3 depicts two stations (AI7070 and AI7073) that appear to be inconsistent with their neighboring stations (FB3216 and FB3222). If we look at the datasheets for these stations, it can be determined that stations AI7070 and AI7073 were observed in the same session but neither station was occupied in a session with FB3216 or FB3222. The datasheets do indicate that FB3216 and FB3222 were observed during the same session. In this case, I would recommend simultaneously observing stations FB3222 and AI7073 to determine an accurate ellipsoid height difference to determine if the relative ellipsoid height difference computed from the published ellipsoid heights are really as accurate as their published network and local accuracy values. If these stations do not get re-observed, I would not recommend using stations AI7070 and AI7073 in the hybrid geoid model.

I have focused on North Carolina but this analysis can be performed on any state or region. Figure 4 is a plot of GPS on BMs residuals using Geoid 12B for the State of Florida. Looking at figure 4, there appears to be a lot of stations with large GPS on Bench Mark residuals.

Figure 5 is a plot of the GPS on Bench Mark residuals using GEOID12B in the Lynn Haven, Florida, area. Looking at figure 5, the reader can see that station BE1497 has a large relative difference between its neighbors (BE0604 and AA9918). This station and one of its neighboring station should be re-observed in a GNSS survey. In my opinion, if this station is not re-observed then it should be rejected and not included in the development of the hybrid geoid model.

Some States have enough bench marks that have been occupied by GPS that re-observing a station may not improve the hybrid geoid model. It may be sufficient to reject the station so it doesn’t distort the hybrid geoid model. Figure 6 is a plot of the GPS on BMs for the State of Missouri. If you compare figure 1 (plot of GPS on BMs in North Carolina) with figure 6 (plot of GPS on BMS in Missouri), it’s obvious that the State of North Carolina has more bench marks occupied by GPS than Missouri. Most of the residuals in figure 6 seem reasonable but the user should investigate those stations that are greater than +/- 5 cm. An example of a station that should be re-observed is station C 10 (KD0210). Figure 7 is a plot of the GPS on BMs surrounding station C 10 (KD0210). The NGS data sheet for station C 10 states that the station was incorporated into NAD 83 (2011) in May 2015; therefore, it wasn’t used in the creation of GEOID 12B. The data sheet also provides the Network and Local Accuracy values for the station. [See the box titled “Excerpt from NGS’ Datasheets for Station KD0210.”] The network and local ellipsoid height accuracy values (6.49 cm) are larger than most published NAD 83 (2011) stations.

This is an area that is void of GPS on bench mark control so this station is extremely important. However, this station has a large GPS on BM residual and a large local accuracy value which makes the station’s published orthometric height and ellipsoid height questionable. I would recommend that this bench mark and several nearby bench marks be observed in a GNSS survey to provide additional estimates of the relationship between the NAVD 88 orthometric heights and NAD 83 (2011) ellipsoid heights in this area. Saying that, it is very important that users perform procedures that result in an accurate GNSS-derived ellipsoid height. This means that users may have to observe stations for several hours and repeat observations on different days and at different times of the day. Of course, I realize that this may be too expensive for most surveyors but the end result may not be sufficient to determine why the station has a large GPS on BM residual.

I stated in my October 2016 column that step 2 was to use the latest experimental geoid model in the analysis. (See box titled “Excerpt from October 2016 column – Analyzing Stations in the GPSBM Table.”) I have focused this column on using data that can easily be obtained from the NGS’ website. Saying that, in my next example I have computed the GPS on Bench Marks residuals using a detrended xGeoid16b that is consistent with NAD 83 (2011) [i.e., a bias and trend has been removed from the differences]. This information is not currently available from NGS’ website but I want to show the differences between the hybrid model residuals and the experimental geoid model, xGeoid16b.

It’s very difficult, if not impossible, to identify how much the hybrid geoid model has been distorted to fit a GPS/Leveling station by looking at published data from NGS data sheets. Figures 8 and 9 demonstrate how some large GPS on Bench Marks residuals using GEOID12B may be distorting the hybrid geoid model. Figure 8 is a plot of the GPS on BM residuals using GEOID 12B in an area in Rockbridge County, Virginia, and Figure 9 is a plot of the same stations using a detrended scientific geoid model xGeoid16b that is consistent with NAD 83 (2011). Looking at figure 8, stations GW2113 and GW0934 appear to be large outliers, -8.8 cm and 11.8 cm, respectively. Station GW0934 was rejected by the geoid team. However, looking at figure 9, using a detrended xGeoid 16b model, the GPS on BM residual of station GW2113 is -19.3 cm and the residual of station GW0934 is only 3.4 cm. What is very important to notice on figure 8 is that nearby stations GW1042 and GW0822 residuals are only -3.3 cm and -2.0 cm, respectively; but, on figure 9, using the detrended xGeoid16b model, the residuals of stations GW1042 and GW0822 are -12.2 cm and -11.5 cm, respectively. Some of these stations need to be re-observed to determine if the NAVD 88 orthometric heights are no longer valid or if there are undetected errors in the published ellipsoid heights. This is why the experimental geoid model should also be used when analyzing GPS on Bench Mark residuals; and why some GPS on BM stations that are inconsistent with their neighboring stations should not be included in the development of a hybrid geoid model. This means that analyzing GPS on Bench Marks residuals using just the hybrid geoid model will only identify outliers that are significantly different from their neighbors. Some outliers will be missed but the procedure does help to prioritize those stations that should be re-observed to help support NGS’ GPS on Bench Mark Program.

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 the current NAVD 88. 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. I want to emphasize that I’m not criticizing NGS process for creating their hybrid geoid model. NGS’ goal is to create a hybrid geoid model that is consistent with published NAVD 88 values. I believe NGS is using all the data and information available to them. I am trying to emphasize to users the importance to strategically occupy stations to help support the GPS on Bench Marks Program and create a hybrid geoid model that accurately represents the current NAVD 88.

This 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. This information will be useful to NGS when developing hybrid geoid models and the 2022 Vertical Transformation model.