As of 2015 the Harris-Galveston Subsidence District (HGSD) maintains an 80 node network of GPS land deformation monitoring sites (PAM) throughout the HGSD and surrounding counties. This network is constantly evolving and requires consistent investment to insure it adequately captures the intended dataset.
Recently, HGSD observed compaction in portions of regulatory area 1. Compaction in this area, where water use has been converted to 90% alternative water and 10% groundwater is anomalous. Further investigation into this area and the extensometers is needed.
Considering the surface area of the HGSD, remote sensing of subsidence throughout the region will provide a more resolute picture of current land surface change. Remote sensing can be accomplished through the use of LIDAR or INSAR methods. When combined with the periodically and continuous stations, this type of information would periodically provide coverage between the GPS network nodes.
The need for data and the distribution of that data is key. As early as 1906, surveys were conducted throughout the Houston area to establish permanent benchmarks (some of which are still used today). Over the years, subsidence measurement methods have evolved from manual site measurement of benchmarks to satellite-based technology. For the HGSD, the goal has always been the same: to monitor the effects of groundwater withdrawal within our regulatory areas, and to take appropriate actions based on those measurements.
All land measurement systems have been developed and controlled by the National Geodetic Survey (part of the National Oceanic and Atmospheric Administration – NOAA). From the creation of the HGSD to present-day, the NGS has been an integral partner to the HGSD; serving as counselor, setting standards, studying and housing data, and much more.