The 2021 gravimetric and hybrid geoid models for Taiwan and offshore islands: Application to a seamless vertical datum

We construct the 2021 gravimetric and hybrid geoid models that cover the island of Taiwan and the offshore islands of Kinmen, Matzu, Penghu, Liuqiu, Lyudao, and Lanyu, where new gravity data and observed geoidal heights are available. The mean sea levels around Matzu, Penghu, and Lanyu are re-defined by better tidal records, resulting in new gravity anomalies. The gravity measurements from airborne, shipborne, and terrestrial gravimeters and from altimeters are combined to form a 30" × 30" grid of free-air gravity anomalies. The terrain corrections are computed using updated land DEMs and ocean depths and then used to generate Faye gravity anomalies. A gravimetric geoid is created by the remove-compute-restore technique using the 1-D FFT implementation of Stokes' integration with a modified kernel. A hybrid geoid is then created using the gravimetric geoid and the observed geoidal heights. The means and the standard deviations of the differences between the model geoidal heights from the gravimetric geoid and from the observations are up to few dm and to less than 10 cm, and are reduced to few cm and 0 - 6 cm respectively when the model values are from the hybrid geoid. The hybrid geoid in the Taiwan Strait is assessed against the sea surface heights from the DTU18 mean sea surface model and the TOPEX-Jason series of altimeters. The hybrid geoid is one essential surface in a seamless vertical datum around Taiwan that define land elevations and ocean depths in a unified way. We discuss the potential application of the hybrid geoid to orthometric heighting in the TWVD2001 system in offshore wind farms in the Taiwan Strait.