Due to the extensive subsidence in the greater Houston area, stable benchmarks were needed to provide a reference frame. Therefore, stable borehole extensometers were equipped with GPS antennas mounted to the inner pipe to measure subsidence and act as a 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.
