Strategy to establish a Global Vertical Reference System
The present paper proposes to define a global vertical reference system based in two components: a geometricalone, given by a level ellipsoid as a reference surface and ellipsoidal heights as coordinates, and a physical one, defined by a fixed W 0 value as zero height level and geopotential numbers as coordinates. The global W 0 value is estimated by solving the fixed gravimetric geodetic boundary value problem (GBVP) on marine areas, whereas the local reference levels W 0 j existing on continents are determined by solving the free gravimetric GBVP. The relationship between W 0 and the different W 0 j is determined in three approaches: at the reference points (mainly tide gauges) of the classical height datums, on the marine areas closed to the tide gauges, and at fiducial stations of the terrestrial reference frame ITRF. The realization of the new vertical reference system is appointed as a global set of very good (reproducible) benchmarks referred to the ITRS/ITRF and with known geopotential numbers and anomalous potential values. The transformation into physical heights (orthometric or normal heights) and the introduction of the corresponding reference surface (geoid or quasigeoid) are then carried out as the last step in the vertical data analysis.