In a simplified sense, borehole extensometers are used to measure vertical displacements in the ground. More specifically, they are deeply anchored wells that are used to measure compaction of the interval between the bottom of the extensometer and land-surface.
. Using dual-frequency, full-wavelength GPS instruments (with geodetic antennas), data is collected at 30-second intervals and averaged over 24 hours.
Stable borehole extensometers (described subsequently) were equipped with GPS antennas mounted to the inner pipe to measure subsidence and act as stable benchmark.
The traditional method used to measure changes in land surface elevation consisted of the establishment of permanent benchmarks that include precise elevations and coordinates for each point.
