The marine geoid (actually geoid undulation, but called simply geoid) is a distance above the reference ellipsoid. It is an equipotential surface of the Earth’s gravitational field that is closely associated with the location of the mean sea surface.
|fig 1.Geoid height (more properly geoid undulation) from Envisat GDR, in a 3D and in a plate carrée projection. These maps are drawn using the BRAT.|
The reference ellipsoid is a bi-axial ellipsoid of revolution. The centre of the ellipsoid is ideally at the centre of mass of the Earth although the centre is usually placed at the origin of the reference frame in which a satellite orbit is calculated and tracking station positions are given. The separation between the geoid and the reference ellipsoid is the geoid undulation.
The geoid undulation, over the entire Earth, has a root mean square value of 30.6 m with extreme values of approximately 83 m and -106 m. Although the geoid undulations are primarily long wavelength phenomena, short wavelength changes in the geoid undulation are seen over seamounts, trenches, ridges, etc., in the oceans. The calculation of a high resolution geoid requires high resolution surface gravity data in the region of interest as well as a potential coefficient model that can be used to define the long and medium wavelengths of the Earth’s gravitational field. For ocean circulation studies, it is important that the long wavelength part of the geoid be accurately determined. Thanks in particular to gravity missions, new geopotential models (EGM96 and EGM2008) have become available as an improvement of the initial JGM3 and OSU91A models (JGM3 is described in [Tapley et al., 1994]; OSU91A is described in [Rapp et al., 1991.])
- Rapp, R. H. et al., 1991, Consideration of Permanent Tidal Deformation in the Orbit Determination and Data Analysis for the TOPEX/POSEIDON Mission, NASA Tech. Memorandum 100775, Goddard Space Flight Center, Greenbelt, MD.
- Rapp, R. H., Y. M. Wang, and N. K. Pavlis, 1991, The Ohio State 1991 geopotential and Sea Surface Topography Harmonic Coefficient Models, Rpt. 410, Dept. of Geodetic Science and Surveying, The Ohio State University, Columbus, OH.
- Tapley, B. D. et al., 1994, Accuracy Assessment of the Large Scale Dynamic Ocean Topography from TOPEX/POSEIDON Altimetry, J. Geophys. Res., 99 (C12), 24, 605-24, 618.