Gravity field mapping from satellite altimetry, sea-gravimetry and bathymetry in the Eastern Mediterranean

Abstract

SUMMARY Using data from the Eastern Mediterranean it has been shown how different data types may improve the quality of a gravity field approximation, and a detailed geoid has been computed for the area. The following data types were available: (a) bias-adjusted satellite altimetry, (b) sea-gravimetry, (c) bathymetry and heights and (d) the spherical harmonic coefficient set OSU81, complete to degree 180. A comparison of the satellite altimetry with geoid heights computed from the OSU81 coefficients with and without the additional use of gravity data revealed substantial errors due to unmodelled orbit tilts. A trend analysis of the altimeter data showed a tilt of 0.0026 m km−1 in eastern direction and 0.0067 m km−1 in northern direction. The standard deviation (s.d.) of observed altimeter values minus OSU81 geoid heights decreased from ±1.53 m to ±1.07 m after a correction for the tilt. Least-squares collocation was then used to predict subsets of the data types (a) and/or (b) from subsets of the same data types. The results given here are in terms of the s.d. of observed minus computed values. Using every third value of the corrected altimeter data for gravity prediction a s.d. of ±17 mGal was found. When gravity values spaced 10′ apart were used to predict, (1) gravity values in a grid with the same mesh width shifted 5′ and (2) altimeter observations, a s.d. of ±10 mGal and ±0.42 m, respectively, were found. A combination of gravity and altimeter data made the first s.d. of ±10 mGal decrease to ±7 mGal, showing the impact of combining the two data types. The use of topographic information to smooth the locally strongly-varying gravity field gave substantial improvements in the gravity values predicted from either altimeter observations or gravity data. Error estimates of geoid heights predicted in between the altimeter tracks showed that a 65 per cent improvement had been achieved.