A regional gravimetric geoid model in support of the GeoNetGNSS CORS network

Within the GeoNetGNSS project, funded by the European Union and National Funds through the Region of Central Macedonia (RCM), the main goal is to establish a dense network of Continuously Operating Reference Stations (CORS) in Northern Greece. Accuracy, reliability, wide coverage, ease of use and cost effectiveness are among the main advantages of differential positioning supporting mapping, surveying, geodetic, and large infrastructure projects. However, as ellipsoidal heights from GNSS measurements have no physical meaning, they should be transformed to orthometric heights. This transformation requires an accurate gravimetric geoid model to be readily available, in order to carry-out the so-called GNSS/Leveling, i.e., the determination of 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 that have been collected through dedicated gravity campaigns. These were focused not only around the CORS stations but also targeted the entire area of RCM. The gravity observations have been carried out with the CG5 gravity meter relative to absolute gravity stations established using the A10 (#027) absolute gravity meter. The development of the geoid was based in the classical Remove-Compute-Restore (RCR) technique and an FFT-evaluation of Stokes’ integral. The long and short wavelengths of the gravity field spectrum were removed from the available input gravity data, then prediction of the residual geoid was carried out and finally the effects removed have been restored to derive the final model. To model the long-wavelength part of the spectrum, XGM2019e has been used as reference while the topographic effects were evaluated based on a spherical harmonics expansion of the Earth’s potential and ultra-high resolution residual terrain correction (RTC) effects from a global model. The prediction of the geoid model was carried out using the classical 1d-FFT spherical Stokes convolution with the Wong-Gore modification for the Stokes kernel and 100% zero-padding in all directions. Various tests against available collocated GNSS/Leveling observations have been performed to find the optimal cut-off degree while the evaluation of the model was carried out over 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.