Five terrestrial techniques, i.e., sea level variations, repeated gravity surveys, tilt variations, repeated airborne terrain profiling, and repeated levelings appear to have sufficient accuracy to give useful information for detection of crustal movements of the order of a few centimeters. They are either being used already or have a clear potential of becoming useful, which cannot now be said of other available techniques, such as trigonometric heighting, barometric heighting, or photogrammetry.
The techniques generally adopted for analyzing the complex ocean signal are numerical filtering and regression analysis. The aim of such an analysis is to eliminate those components of the signal which can be interpreted as caused by tides, barometric pressure changes, wind stress, steric topography, and other phenomena associated with marine dynamics, thus leaving the remaining variations, both secular and episodic, available for study. In spite of the limited understanding of ocean processes a certain measure of success has been achieved in this field, so that given good quality data, at least the secular trend can be identified. The work of the Permanent Service for Mean Sea Level in its current task of publishing mean sea level data in a new format should provide assistance in evaluating the eustatic contribution in the secular trend.