So what does HGSD gain from using GPS, and how does it differ from previous measurement methods?
One of the most important advantages to GPS is the ability to have constant data. Using dual-frequency, full-wavelength GPS instruments (with geodetic antennas), data is collected at 30-second intervals and averaged over 24 hours. That means that specific stations being monitored can be assessed on a daily basis. And just as important, the measurements are more reliable and handled at a fraction of the cost. Improved GPS techniques and processing have reduced the cost of releveling from millions of dollars to less than $100,000, and the data provided is accurate to + or – one centimeter.
Where are GPS measurements taken ?
GPS measurements are taken using a system of CORS and PAM’s. Sounds complicated, but it’s really quite simple. Because of the broad extent of subsidence in the Houston-Galveston area, there were no stable benchmarks. Therefore, stable borehole extensometers were equipped with GPS antennas to provide a reference frame to measure subsidence at other stations throughout the area. These permanent stations are known as local GPS Continuously Operating Reference Stations, or CORS.
In the mid 1990s, HGSD and NGS began developing the use of GPS Port-A-Measure, or PAM’s., to provide subsidence measurements.