Ideally, coastal tide gauges measure sea level with respect to a benchmark on land. Knowledge of vertical land movement is required in order to determine absolute sea level changes. For some applications of tide gauges such as the monitoring of long-term sea level changes, the secular vertical motion of the land needs to be known with an accuracy of better than 1 mm/yr. The tool most readily available to measure the secular vertical land motion with that accuracy is a co-location of the tide gauge with a continuously operated GPS (CGPS). In many cases, the co-location of a tide gauge with CGPS can also ease the task of monitoring potential motion of the tide gauge with respect to the benchmark.

We will review key details of the methodology of co-locating tide gauges with GPS including the requirements for site selection, length of record, GPS data analysis and analysis of the time series that have to be met in order achieve the 1 mm/yr accuracy with respect to a global, geocentric reference frame.