As described in the previous web page, vertical land motion can be caused by a number of processes. As the tide gauges are attached to the land, this motion will be reflected in the tide gauge records. For some studies, for example determining rates in the context of inundation at a given location, the tide gauge record is the ideal measurement. However, other studies want to remove the impact of crustal motion to obtain a measurement of sea surface height. This can be achieved by removing estimates vertical land motion derived from global positioning system (GPS) records. This can also reduce (though not remove) the impact of local, non-oceanographic processes in a global analysis of tide gauge records.
Considerable progress has been made in obtaining a global set of GPS estimates at tide gauge locations. The International GNSS Service (IGS) has a GPS Tide Gauge Benchmark Monitoring pilot project page, while Continuous GPS at Tide Gauges (CGPS@TG) is the classic website for this effort. A set of CGPS-derived rates, along with a survey on the CGPS sites near tide gauges, can be found at the SONEL website. Read further for a history of GPS-at-tide-gauges efforts.