Updates on NGS’s published heights in southeast Texas and NGS GPS on Benchmarks program

January 4, 2023  - By

First, happy New Year to everyone. As a follow up to my November 2022 column on the geodesy crisis, I’d like to highlight that the National Geospatial Advisory Committee (NGAC) of the Federal Geographic Data Committee (FGDC) just adopted a resolution on the need for the federal government to understand and aggressively address the US geodesy crisis. See below.  This is great news and, hopefully, the FGDC and others will follow up with discussions with other organizations such as the Office of Science and Technology Policy (OSTP) in the White House.


Image: FGDC 

Now for this month’s column. Last year the National Geodetic Survey (NGS) started suppressing height information in Southeast Texas (see my April 2021 and June 2021 columns).  See below for more information. Last year’s columns highlighted the potential effects of subsidence on published heights in the Houston, Texas, region which implied that most of the published heights, which are based on older surveys in the region, are not current or accurate.  At the time of NGS’s announcement, only 28 marks with orthometric heights were published on NGS datasheets in southeast Texas. Click here for more information and see below.

This column will provide an update on the following: the current set of published orthometric heights in the southeast Texas region based on recent GNSS surveys performed during 2021 and 2022, NGS’s rules for estimating and publishing GNSS-derived orthometric heights using OPUS Projects, and the status of NGS’s GPS on Benchmarks program.

This provides the benchmarks that are available to users (see also below).


Image: NGS

I always retrieve the latest published coordinates using NGS’s datasheet website routine. See the graphic below of the published NAVD 88 orthometric heights as of Nov. 20, 2022 (I used NGS’s monthly archive by State retrieval option).  There are currently 147 marks with published orthometric heights within NGS’s definition of the southeast Texas zone of subsidence. From mid-October to early December of 2022, another GNSS project sponsored by the Harris-Galveston Subsidence District (HGSD) was performed in the region.  In this project, 154 marks in the southeast Texas region were observed. The results of this project should be published and disseminated by NGS in the spring of this year.


Latest Published Heights in Southeast Texas. Image: Dave Zilkoski

The current version of OPUS projects allows the user to estimate NAVD 88 orthometric heights, providing they adhere to NGS’s recommendations and procedures. A presentation titled “Heights Suppression in Southeast Texas” by Boris Kanazir, NGS, provides guidance on estimating NAVD 88 orthometric heights using OPUS projects.

See below for the requirements for number of occupations, duration of each session, and the spacing of marks with valid NAVD 88 published orthometric heights.

The requirements include:

  • a minimum of two NAVD 88 control marks per new mark observed
  • a mark must be observed twice on different days and at different times of the day
  • the maximum distance between new marks and NAVD 88 control is between 30-50 km, based on session duration
    • 30 km for occupation sessions at least 2 hours
    • 40 km for occupation sessions at least 4 hours
    • 50 km for occupation sessions at least 6 hours

The diagram below depicts how many marks with published NAVD 88 heights are required using a 30 km radius spacing.

I like to think of this concept as drawing Venn diagrams around marks. See below for an example of the concept.


Venn Diagram. Image: Dave Zilkoski

So, what does this mean in the real world? The map below demonstrates the concept in the Houston-Galveston, Texas, region.  As shown, many of the 30 km circles overlap, indicating that in these overlapping areas there are two CORS with published NAVD 88 orthometric height.  This means that a user can occupy a mark for two hours and use the data from two CORS as NAVD 88 control.  Of course, the mark must be occupied twice for redundancy.


30 km Radius Circles around SE TX CORS with NAVD 88 Heights. Image: Dave Zilkoski

Increasing the radius to 40 km includes more overlapping areas.  This means that the user would have more overlapping areas with two CORS that have published NAVD 88 orthometric heights, but the marks would have to be occupied twice for at least four hours each time.


Image: Dave Zilkoski

Now, when you apply a 30 km radius around the current 147 marks that have published NAVD 88 heights, most of the region has overlapping areas (see below).  This means that the user could occupy two of the NAVD 88 marks along with any new marks for at least two hours.


30 km Radius Circle Around all 147 NAVD 88 Marks in SE TX. Image: Dave Zilkoski

The previous figure may seem confusing because of all the circles.  In the example below, based on only two marks, 11 marks fall inside the overlapping sections of the two circles. They could be established using the two NAVD 88 control marks that were used to make the 30 km circles.


Example of Two 30 km Radius Circles. Image: Dave Zilkoski

As depicted in my June 2021 column, the Houston-Galveston, Texas, region is subsiding.  The map below provides the latest estimates of subsidence in Houston-Galveston, Texas, region based on a Harris-Galveston Subsidence District (HGSD) report “Determination of Groundwater Withdrawal and Subsidence in Harris and Galveston Counties – 2021“ published in 2022.  Most of the rates are small, less than 0.5 cm/year, but some are greater than 1 cm/year.  This means that some marks may have subsided around 5 cm in five years.


Estimate of subsidence in SE TX. Image: Dave Zilkoski

The surveying and mapping community has done a tremendous job of increasing the number of published heights in the Houston-Galveston, Texas, region (from 28 to 147).  That said, the amount of movement in the Katy region is more than -2 cm/year (see box titled “Estimate of Subsidence in the Katy Area”).  That means, the marks in this area may subside 10 cm in five years.


Estimate of subsidence in the Katy Area. Image: Dave Zilkoski

Heights that change 10 cm cannot be considered NAVD 88 control marks. NGS’s OPUS Projects User Guide states the following about superseding a mark’s coordinates:

“Users should review the newly adjusted coordinates on user marks to decide whether they recommend that the user mark be re-determined (re-published). Typically, this would happen if the coordinates have shifted by more than 2 centimeters horizontally or 4 cm vertically from the published coordinates marks.”

Therefore, these marks in the Katy region may not be valid NAVD 88 control marks in about two years.  Even marks that are subsiding at 1 cm/year may not be valid NAVD 88 control marks in about four years.

The community needs to maintain these marks to account for movement in the region. As previously stated, the Harris-Galveston Subsidence District (HGSD) has marks, denoted as PAMS, that are occupied continuously for a week several times throughout the year.  These PAMS and the CORS in the area could be used to estimate crustal movement rates and maintain a set of valid, published heights in the region.  See the boxes titled “ArcGIS Online HGSD Subsidence Rates” and “PAM 98 Subsidence Rate.” Additionally, the Texas Spatial Reference Center (TSRC) could provide the appropriate services to help maintain the published coordinates. The TSRC website states, “The technical mission of TSRC is to conduct basic and applied research contributing to NGS’s national Height Modernization program. TSRC is a repository for information used by researchers to develop improved understanding of elevation, geodetic and vertical datums in the state of Texas. The TSRC goal is to re-establish accurate evaluations throughout Texas in cooperation with qualified  geospatial scientists, professional engineers, and professional land surveyors.”


ArcGIS Online HGSD Subsidence Rates. Image: Harris-Galveston Subsidence District


PAM 98 Subsidence Rate. Image: Harris-Galveston Subsidence District

In 2025, NGS will replace all three North American Datum of 1983 (NAD 83) frames and all vertical datums, including the North American Vertical Datum of 1988 (NAVD 88), with four new terrestrial reference frames and a geo-potential datum. As stated in my previous columns – April 2022, April 2021, June 2020 – the new reference frames will rely primarily on Global Navigation Satellite Systems (GNSS) as well as on a gravimetric geoid model.  These new reference frames will be easier to access and to maintain than the current NSRS. NGS will provide tools similar to the OPUS suite of routines that will facilitate users’ ability to submit data to NGS to maintain and publish survey marks. See the graphic below.


Processing Data in the New NSRS. Image: NGS

I would like to highlight that NGS has extended the cut-off date for submitting data for use in the 2022 Transformation Tool. The new cut-off date is Sept. 30 (see below).

In support of the GPS on Benchmarks program, on Jan. 12, NGS is hosting a webinar on using RTN data in OPUS Projects 5 for submitting GPS on Benchmarks data (see the box titled “NGS Webinar on OPUS Projects 5”)


NGS Webinar on OPUS Projects 5. Image: NGS

This column provided an update on the current set of published orthometric heights in the southeast Texas region based on recent GNSS surveys performed during 2021 and 2022, NGS’s rules for estimating and publishing GNSS-derived orthometric heights using OPUS Projects, and the status of NGS’s GPS on Benchmarks program. Additionally, it highlighted that the NGAC of the FGDC adopted a resolution on the need for the federal government to understand and aggressively address the United States geodesy crisis. This is a good step forward, and I hope that others will follow up with discussions with other organizations such as the OSTP in the White House.  Finally, “The Geodesy Crisis” white paper can be downloaded from the American Association for Geodetic Surveying (AAGS) website.

I hope everyone has a happy new year filled with optimism, happiness and a generous amount of enthusiasm and fun.

This article is tagged with , , and posted in Latest News, Opinions, Survey

About the Author: David B. Zilkoski

David B. Zilkoski has worked in the fields of geodesy and surveying for more than 40 years. He was employed by National Geodetic Survey (NGS) from 1974 to 2009. He served as NGS director from October 2005 to January 2009. During his career with NGS, he conducted applied GPS research to evaluate and develop guidelines for using new technology to generate geospatial products. Based on instrument testing, he developed and verified new specifications and procedures to estimate classically derived, as well as GPS-derived, orthometric heights. Now retired from government service, as a consultant he provides technical guidance on GNSS surveys; computes crustal movement rates using GPS and leveling data; and leads training sessions on guidelines for estimating GPS-derived heights, procedures for performing leveling network adjustments, the use of ArcGIS for analyses of adjustment data and results, and the proper procedures to follow when estimating crustal movement rates using geodetic leveling data. Contact him at dzilkoski@gpsworld.com.