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
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.