Refinements of regional gravimetric and hybrid geoid models in support of the GeoNetGNSS CORS network in Northern Greece

In the frame of the GeoNetGNSS project, funded by the European Union and National Funds through the Region of Central Macedonia (RCM) in Northern Greece, regional gravimetric and hybrid geoid models have been determined with the main goal being to support a newly established network of Continuously Operating Reference Stations (CORS). The main aim was to assist everyday surveying purposes by delivering accurate orthometric heights based on GNSS/Levelling, i.e., determining orthometric heights without the need to carry out levelling. With that in mind, a regional gravimetric geoid was determined based on historical and newly acquired high-accuracy and density gravity data, employing the Remove-Compute-Restore (RCR) technique and both stochastic and spectral evaluations of Stokes’ integral. Consequently, a hybrid deterministic and stochastic approach was used to model the residuals of the gravimetric geoid solution relative to available GNSS/Levelling geoid heights. The latter refer to 533 geodetic benchmarks in the entire study area, where accurate static GNSS observations and orthometric heights from the Hellenic Military Geographic Service (HMGS) were available. Various parametric models ranging from simple north-south bias and tilt to 2^nd and 3^rd order degree polynomial models were evaluated in terms of the fit residual absolute and relative differences. After the deterministic fit, a collocation approach employing exponential and 2^nd order Gauss-Markov covariance functions was used to model the stochastic residuals. Finally, the hybrid deterministic and stochastic corrector surface, provided as grid corrections for the entire area under study, has been determined to accommodate user needs for orthometric height determination. From the results acquired, absolute differences of the order of 1-2 cm and relative ones at the 3-4 ppm have been achieved after validation against independent GNSS/Levelling observations.