The alignment of PPP-derived coordinates to a local geodetic reference system through a reliable velocity field: case study in Greece

The main scope of this work is to present a reliable and rigorous method for efficiently converting the coordinates estimated by precise point positioning (PPP) to Greece's official geodetic reference system, i.e. the Hellenic Geodetic Reference System of 1987 (HGRS1987). The HGRS1987 is a hybrid geodetic reference system, assimilating classical (spatial distances, angles, directions, and azimuths) and space observations (global positioning system and Doppler measurements). For this purpose, it is necessary to know the velocities at each given arbitrary point in the Hellenic region (continent and islands). Therefore, we built a velocity model derived from a recent study for Greece. Appropriate coordinate conversions between various reference frames (global, regional, and local) are crucial in addition to a reliable velocity field. The method for transforming PPP-derived coordinates to the Greek geodetic system was assessed by comparing the coordinates (official versus computed) at 130 benchmarks located throughout the country. The computed and official coordinates coincided by 7.4 cm, which can be considered acceptable given the following factors: a) the official acceptance of the 8.3 cm root mean square; b) the use of a designated velocity model; and c) the application of several coordinate transformation algorithms. This kind of methodology could also be applied in any old geodetic reference system, worldwide.