hBG2018 is the hybrid quasi-geoid model for Belgium, including both the Belgian mainland and the Exclusive Economic Zone. The hBG18 quasi-geoid heights are referred to the ETRS89/GRS80 ellipsoid. It is computed using radar altimetry data, terrestrial gravity anomalies, airborne gravity disturbances, and shipboard gravity anomalies. The quasi-geoid computation is based on the remove-compute-restore procedure. The long-wavelength signal content in the data is reduced by removing the contribution of the GOCO05S global gravity field model complete to degree 280. At the very short wavelengths, residual terrain modelling (RTM) is applied to the shipboard, terrestrial and airborne gravity datasets using EuroDEM as input data. The residual disturbing potential is parameterized over the target area using Spherical Radial Basis Functions (SRBF). The SRBF coefficients and bias parameters for the sets of gravity anomalies and disturbances are estimated using weighted least-squares with regularization, assuming white noise.

To support the exploitation of the hBG2018 gravimetric quasi-geoid for the conversion of GNSS derived heights to the TAW/DNG height system, several post-processing steps were applied. First, as the TAW/DNG height system is a mean-tide height system (i.e., mean-tide crust = zero crust over mean-tide geoid) the quasi-geoid was transformed from the zero-tide to the mean tide system. Thereafter, a corrector surface (also called 'innovation function') has been estimated from the differences between the geometric quasi-geoid at 3707 GNSS/leveling points and the gravimetric quasi-geoid. This surface also accounts for the difference between the fictitious datum point of the gravimetric hBG18 and the datum point of the TAW/DNG. Finally, the transformation from the tide-free permanent tide system adopted in the GNSS community and the mean-tide system adopted in TAW/DNG, has been applied. hBG18 replaces hBG03 as the official Belgian model since August 1, 2018.

The geoid model is provided in ISG format 2.0 (ISG Format Specifications), while the file in its original data format is available at the model ISG webpage.

The International Service for the Geoid (ISG) was founded in 1992 (as International Geoid Service - IGeS) and it is now an official service of the International Association of Geodesy (IAG), under the umbrella of the International Gravity Field Service (IGFS). The main activities of ISG consist in collecting, analysing and redistributing local and regional geoid models, as well as organizing international schools on the geoid determination (Reguzzoni et al., 2021).