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Summary
A synthetic Earth gravity model (SEGM) is a hypothetical, but realistic, gravity field for which the associated geopotential and its derivatives can be determined with arbitrary precision. The purpose of an SEGM is to verify and compare various techniques and approaches for interpreting gravimetric data. In physical geodesy, an SEGM provides the only independent and unambiguous means to validate geoid solutions obtained from gravity measurements, spirit levelling heights and global geopotential models.

In practice, an SEGM is used to generate self-consistent sets of gravity values, topographic heights and geoid undulations over a test region. Existing Stokesian techniques are applied to the synthetic gravity and height data to produce geoid solution. The techniques are evaluated by comparing this geoid solution with the 'correct' geoid heights. Errors of various magnitudes and characteristics are introduced into the synthetic data to investigate their effects on the final geoid values.

SEGMs are of great interest to the international geodesy community. In April, 1997 the International Association of Geodesy's (IAG) Executive Committee created a special study group (SSG 3.177) to investigate various issues pertaining to the construction of the SEGM. This group is chaired by Will Featherstone. Simon Holmes is an associate member of the group.

Recent research at Curtin University has resulted in the development of algorithms for the rapid and accurate evaluation of very high degree spherical harmonic expansions. These algorithms facilitated the construction of simple SEGMs based on a hybrid empirical/synthetic geopotential models up to degree and order 2700 and 5400. These SEGMs were used to compare the performance of various modified Stokes kernel functions in gravimetric geoid determination (Novak et al. (1999), Holmes and Featherstone (1999, 2000a, 2000b)).

Current efforts at Curtin University are focused upon investigating the convergence and divergence properties of spherical harmonic expansions of terrestrial geopotential. It is predicted that this research will allow the hypothetical gravity fields associated with high resolution synthetic mass distributions to be described, both within and without the synthetic bodies, by high degree spherical harmonic expansions.

Results
Some preliminary results obtained from a spherical harmonic SEGM were presented at the 22nd General Assembly of the International Union of Geodesists and Geophysicists (IUGG), Birmingham, UK, July 1999. A postscript version of this presentation is available below.

References
Holmes, SA and Featherstone, WE (1999) Using a synthetic Earth gravity model to assess the performance of modified kernels in gravimetric geoid computation, poster presented at the 22nd General Assembly of the IUGG, Birmingham, July.
(download postscript version)
Holmes, SA and Featherstone, WE (2000a) Synthetic Earth gravity models from ultra-high degree spherical harmonic expansions, in preparation.
Holmes, SA and Featherstone, WE (2000b) The performance of modified kernels over a synthetic gravity field, in preparation.
Novak P, Vanicek P, Veronneau M, Holmes S, Featherstone WE (1999) On the accuracy of Stokes’s integration in the precise high-frequency geoid determination, poster presented to the AGU spring meeting, Boston, USA, June.

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