NGS releases beta tool for obtaining geodetic information

NGS has developed a new beta tool for obtaining geodetic information about a passive mark in their database. This column will highlight some features (available as of Oct. 5, 2020) that may be of interest to GNSS users. It provides all of the information about a station in a more user-friendly format. The box titled “Passive Mark Lookup Tool” is an example of the webtool. The tool provides a lot of information so I have separated the output of the tool into several boxes titled “Passive Mark Lookup Tool — A through D.”

I will highlight several attributes that I believe will be very useful to users, especially users of leveling-derived and GNSS-derived orthometric heights. I’ve highlighted several attributes in the box titled “Passive Mark Lookup Tool — A” that are important to users such as published coordinates, their datum and source, Geoid18 value, GNSS Useable, and the date of last recovery. All of these values are available on a NGS datasheet but, in my opinion, this provides the information in a more user-friendly format.

Passive Mark Lookup Tool — A (https://beta.ngs.noaa.gov/datasheets/passive-marks/index.html) Image: National Geodetic Survey

One calculation that the user can easily compute for marks that have been leveled to and occupied by GNSS equipment, is the difference between the published leveling-derived orthometric height and the computed GNSS-derived orthometric height. This may indicate that the mark has moved since the last time it was leveled to or that its height coordinate has been readjusted since the creation of the published geoid model.

The table below provides the calculation using the data from the box titled “Passive Mark Lookup Tool — A.” The calculation [HGNSS = hGNSS — NGeoid18; Difference = HGNSS — HNAVD 88] has been described in several of my previous columns (this one, for example).

Data: National Geodetic Survey

In this example, the difference between the GNSS-derived orthometric height and the Published NAVD 88 height is 6.1 cm. NGS is looking for comments on this beta webtool so if users would like this computation added to the tool, they should send a comment to NGS using the link provided on the site (This is a beta product. NGS is interested in your feedback concerning its function and usability as well as how users would like to interact with NGS datasheet information in the future. Email us at ngs.feedback@noaa.gov.) So, the user should ask the question, did the station move since the last time it was leveled?

Another attribute that would be nice to be part of this tool is which station was used to create the hybrid geoid model. As of Oct. 5, 2020, users have to go to the Geoid18 webpage to get the information. The Excel file and shapefiles provide whether the station was used to create the Geoid18 model. In the case of this example, KK1531, CHAMBERS, the mark was not used in the creation of Geoid18 so NGS felt that the station may have moved and/or the GPS on Bench Mark residual was large relative to its neighbors. See NGS’s technical report on Geoid18 for more information on the creation of Geoid18. The GPS on Bench Mark residual analysis was described in several of my previous columns (see “The differences between Geoid18 values and NAD 83, NAVD 88 values” and “NGS 2018 GPS on BMs program in support of NAPGD2022 — Part 6” for examples).

The webtool provides a map depicting the location of the station, photos (if available), and previously published, superceded values of the mark. See the box titled “Passive Mark Lookup Tool — B.”

Passive Mark Lookup Tool — B Image: National Geodetic Survey

In the example of Chambers, KK1531, no photos were available. It would be helpful if a user would provide photos to NGS when visiting this station. (Note: NGS has a webtool for users to submit recovery information about a mark as well as to provide current photos of the station.) The new Passive Mark webtool also provides information about the survey projects that the mark has been involved in such as leveling and GNSS projects.

In this example, mark CHAMBERS was leveled to in a 1984 first-order, class 2 leveling project (Leveling Line number L24838/6) and, in 1995, the mark was part of a GNSS project (GNSS Project GPS1010). It also provides all the descriptive text and recovery information (See boxes titled “Passive Mark Lookup Tool – C” and “Passive Mark Lookup Tool – D”).

Passive Mark Lookup Tool — C Data: National Geodetic Survey Passive Mark Lookup Tool — D Image: National Geodetic Survey

I want to highlight a few other attributes of this webtool. The station, PID AA3862, has an interesting attribute that users should take note of; that is, the NAD 83 (2011) position source is NO CHECK. See box titled “Passive Mark Page for PID AA3862.”

This means that the mark’s NAD 83 (2011) coordinates were determined without redundant observations. This is not a good survey practice but there are times that a project may contain check observations for some purpose or, more likely, the mark did contain other GNSS vector but they were rejected in the final adjustment. Either way, a good survey practice would be for users to verify the coordinates of these marks before using them.

Passive Mark Page for PID AA3862 Data: National Geodetic Survey

As previously mentioned, the tool provides the location of the station on a map and photos if they are available. This is a really nice feature for anyone searching for the mark. The map can be enlarged as well reduced by clicking on the box. See boxes titled “Passive Mark Page for PID AA3862” and “Photos of Mark PID AA3862.” The box titled “Photos of Mark PID AA3862” provides all three photos of mark PID AA3862.

Photos of Mark PID AA3862 Photo: National Geodetic Survey Photo: National Geodetic Survey Photo: National Geodetic Survey

It should be noted, according to the Geoid18 GPS on BMs dataset that users can download, this station, AA3862, was not used in the creation of Geoid18. The table below provides the difference between the GNSS-derived orthometric height and the published NAVD 88 height.

In this example, the difference between the GNSS-derived orthometric height and the published NAVD 88 height is 9.9 cm. Also, the webtool provides the network accuracy values for the station. In this example, the horizontal network accuracy is 20.65 cm and the vertical network accuracy value is 14.50 cm (see highlighted values in box titled “Passive Mark Page for PID AA3862”). These are very large network accuracy values. This should be a flag to anyone that is using this station as control.

Data: National Geodetic Survey

As I previously mentioned, as a beta site, users should verify all information from the site. NGS is requesting feedback on this tool so they can improve it and make it an operational webtool. I encourage everyone to access the tool and check out a few of their favorite marks, and then send an email to NGS informing them of what you like, what you would like to change, and what you would like to see added to the tool.

NGS is releasing this tool as a beta product to get feedback from users. As NGS states in the heading of the tool, they are interested in your feedback concerning its function and usability as well as how users would like to interact with NGS datasheet information in the future. Email NGS at ngs.feedback@noaa.gov.

One last item that may be of interest to GNSS users is that NGS, working with the University Corporation for Atmospheric Research (UCAR), developed another online GNSS lesson (see box titled “New GNSS Lesson by NGS and UCAR”). These lessons are free but users must sign up to access the website and lesson.

New GNSS Lesson by NGS and UCAR Image: National Geodetic Survey