The need for data and the distribution of that data is crucial to understand regional and localized subsidence. As early as 1906, surveys were conducted throughout the Houston area to establish permanent benchmarks, which are survey marks with a known surface elevation. Since the creation of the HGSD in 1975, land surface measurements were developed with guidance from the National Geodetic Survey (NGS), which is part of the National Oceanic and Atmospheric Administration (NOAA) for the greater Houston-Galveston region. Geodetic differential leveling surveys were conducted in 1978, 1982, 1987, 2000, and 2007 in a collaborative effort between the HGSD and the NGS to measure specific benchmarks throughout the region.
Over the years, subsidence measurement methods have evolved from manual site measurement of benchmarks (e.g., geodetic differential leveling) to satellite-based technology (e.g., Global Positioning System for the United States). Since the mid-1990s, the HGSD has utilized GPS to obtain land-surface elevation measurements throughout Harris and Galveston counties.
Since 2012, the HGSD has been a contributing member of the Houston GPS Network (HoustonNet). The HoustonNet project was established by University of Houston (UH) to create a dense GPS network in the greater Houston-Galveston region to study natural hazards and partnered with the HGSD.
The HoustonNet comprises over 220 permanent GPS stations that include:
- CORS and periodic monitoring GPS stations operated by the HGSD
- CORS and periodic monitoring GPS stations operated by the FBSD
- CORS operated by the UH
- CORS operated by Texas Department of Transportation
- CORS operated by the City of Houston
- CORS in Angleton operated by the United States Coast Guard
- WAAS (Wide Area Augmentation System) CORS in Houston operated by the Federal Aviation Association
- Cooperative CORS throughout the area
The HGSD also co-funds the UH Geodetic Laboratory that processes the GPS data from HoustonNet to monitor subsidence and faulting in the greater Houston-Galveston region.
Considering the surface area of the HGSD, remote sensing of subsidence throughout the region will provide a more resolute picture of current land-surface deformation. Remote sensing can be accomplished through the use of Light Detection and Ranging (LiDAR) or Interferometric Synthetic Aperture Radar (InSAR) methods. When combined with the data collected from the HoustonNet, remote sensing can provide coverage between the GPS stations.
Since 2019, the HGSD has sponsored research conducted by Southern Methodist University to utilize a novel InSAR methodology to evaluate land-surface deformation from 2008 to 2020. This cutting-edge research will provide land-surface deformation at the meter scale over Harris, Galveston, Fort Bend, and surrounding counties. These data, when combined with the HoustonNet, create the densest network in North America.
As of 2020, the Harris-Galveston Subsidence District (HGSD) maintains subsidence data from a global positioning system (GPS) network of over 220 permanent GPS stations throughout Harris, Galveston, and surrounding counties. This network is constantly evolving and requires consistent investment to ensure it adequately captures the greater Houston-Galveston region. Current information on the location and data from the GPS stations is provided in the Annual Groundwater Report.