The fiducial GPS technique is now well established in geodesy and geophysics, for determining horizontal and vertical land movements. In particular, the technique is used in crustal dynamics and in monitoring changes in mean-sea-level. The main limiting factor to achieving accuracies of a few millimeters is the reference frame (or datum), as defined by the adopted coordinates of the fiducial stations. This paper summarises a series of tests which have been carried out to assess the quality of alternative global reference frameworks, in order to achieve the highest accuracies for tide gauge heights in the UK. ¿ American Geophysical Union 1993