Seamless Geoids across Coastal Zones: Comparison of Satellite and Airborne Gravity across the Seven Continents – and an Azores Heritage Case

An accurate coastal geoid model is important for determination of near-shore ocean dynamic topography and currents, as well as for land GPS surveys and global geopotential models. Since many coastal regions across the globe are regions of intense development and coastal protection projects, precise geoid models at cmlevel accuracy are essential. The only way to secure cmgeoid accuracies across coastal regions is to acquire more marine gravity data; here airborne gravity is the obvious method of choice due to the uniform accuracy, and the ability to provide a seamless geoid accuracy across the coastline. Current practice for gravity and geoid models, such as EGM2008 and many national projects, is to complement land gravity data with satellite radar altimetry at sea, a procedure which can give large errors in regions close to the coast. To quantify the coastal errors in satellite gravity, we compare results of a large set of recent airborne gravity surveys, acquired across a range of coastal zones globally from polar to equatorial regions, and quantify the errors as a function of distance from the coast line for a number of different global altimetry gravity solutions. We find that accuracy in satellite altimetry solutions depend very much on the availability of gravity data along the coast-near land regions in the underlying reference fields (e.g., EGM2008), with satellite gravity accuracy in the near-shore zone ranging from anywhere between 5 to 20 mGal r.m.s., with occasional large outliers; such errors may typically propagate into coastal geoid errors of 5-10 cm r.m.s. or more, and highlight the needs for airborne gravity surveys in the coastal zones. One of the first examples of such a dedicated effort is the Azores airborne gravity campaign, carried out more than 20 years ago as part of the AGMASCO (Airborne Geoid Mapping System for Coastal Oceanography) EU project 1996-98.