The traditional method used to measure changes in land surface elevation (also called “geodetic differential leveling,”) consisted of the establishment of permanent benchmarks that include precise elevations and coordinates (i.e., latitude and longitude) for each point. Benchmarks, according to the NGS, are a specific type of survey mark that has a known elevation above or below an adopted surface or datum.
As the land surface began to subside in the early 1900s due to groundwater and hydrocarbon withdrawals, 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 by the HGSD in cooperation with the NGS in 1978, 1987 and 2000. Although the geodetic differential leveling method provided excellent spatial subsidence data, the cost and time of the releveling procedure for a specific period in time (an epoch) limits the accessibility of current data at a rate necessary to effectively monitor the effects of subsidence.
In the 1980s, new technologies emerged that provided the same accuracy as the geodetic differential leveling and while also allowing for constant monitoring in a cost-effective way. In 1987, in conjunction with the conventional releveling, an experimental Global Positioning Systems (GPS) releveling was initiated throughout the HGSD. The class-A benchmarks established for the GPS releveling have proven to be valuable benchmarks in the Houston area. Since 2000, all land surface elevation data is obtained through GPS measurements throughout the HGSD and surrounding counties.
