Due to the broad extent of subsidence in the Houston-Galveston region, stable benchmarks were needed to provide a reference frame. Therefore, stable borehole extensometers (described subsequently) were equipped with GPS antennas mounted to the inner pipe to measure subsidence and act as stable benchmark. The GPS equipment used at extensometers are permanent stations and are known as continuously operating reference stations (CORS). In the mid-1990s, the HGSD and NGS began developing the use of GPS Port-A-Measure (PAMs), to provide subsidence measurements.
