Mapping the geoid for Iberia and the Macaronesian Islands using multi-sensor gravity data and the GRACE geopotential model

A new gravimetric geoid model (ICAGM07) has been determined for the North-East Atlantic Ocean, Iberia, and the Macaronesian Islands using multi-sensor gravity data and a GRACE derived Earth geopotential model. A high resolution gravity model, determined using least squares optimal interpolation of marine, land, and satellite derived gravity anomalies, was used to resolve the medium and short wavelengths of the geoid. Long wavelengths of the geoid were provided by the GRACE derived Earth geopotential model. The topographic effects were computed in the spectral domain using a high resolution (100 m) digital terrain model derived from SRTM mission data and cartographic charts. The remove-restore technique was used to compute the geoid model on a 1.5′ grid, and the residual geoid height was computed using spherical FFT and a modified Stokes' kernel. The effects of different Earth tide models on the geoid were computed and analysed. Comparison over the sea with an oceanographic geoid determined from CLS01 MSSH and Rio05 MDT yield a relative accuracy of 8 cm, with larger differences close to the shoreline. Over land, comparisons with 1646 GPS/levelling marks indicate an overall precision of 8-10 cm and relative vertical datum offsets of up to 2 m. This new geoid model represents a significant improvement over existing geoid models, with a homogeneous relative accuracy of 8-10 cm over both marine and land areas, and exposes the inaccuracies of local vertical datums as references for studies of vertical deformation.