European Vertical Height Datums: Unification and Time variation

Estimation of the fundamental geodetic parameter, w0, the potential value of the Gauss-Listing geoid, is required to define the geoid, the best fitting ellipsoid to the geoid and as a means of unification of the vertical datums in use by various mapping agencies throughout the world. Consideration should also be given to the time derivative of w0 given sea-level change etc. Methodologies to quantify w0 include (i) the use of tide gauge time series and collocated GPS and (ii) satellite altimetry. The first methodology enables global isostatic adjustment and other local deformation to be removed from the tide gauge time series. The tide gauge time series, levelling heights and the GPS coordinates provide a measure of the normal height at the tide gauge benchmark. By reference to a global geopotential model or high accuracy and high resolution local geoid model the equipotential value corresponding to mean sea-level can be recovered for each of the tide gauge locations. In this manner the equipotential surface that provides the best fit to mean sea-level is used as a reference for the unification purposes. Through annual investigations the methodology has the potential to estimate the rate of change of w0. In particular this study is used to connect UK and European datums through analysis of seven tide gauges distributed around the UK coastline. GPS positioning is quantified by using the GIPSY OASIS II software. In the second methodology, the geodetic parameter w0 can be recovered from altimetric mean sea-level models from TOPEX/Poseidon and ERS. This facilitates unification of datums by estimating local differences from the global altimetric value. This study will report and inter- compare results from the two methodologies.