Harris-Galveston Subsidence District’s (HGSD) initial form of measurement consisted of the establishment of permanent benchmarks. Included in these benchmarks were precise elevations, latitudes and longitudes for each point.
As the land surface began to subside due to groundwater withdrawal, the need to relevel benchmarks became necessary. Over the years, new benchmarks were added (for a total of more than 2,500) and “relevelings” were conducted in 1978, and again in 1987. Although this measurement method provided excellent spatial subsidence data, the cost of the releveling procedure for a single epoch prohibited HGSD from accessing up-to-date data at a rate necessary to sufficiently monitor the sometimes monthly, weekly, or even daily effects of subsidence.
In 1987, in conjunction with the conventional releveling, an experimental GPS releveling was initiated.