AN OPTIMUM LOCAL GEOID MODEL FOR IRAN BASED ON THE LEAST-SQUARES MODIFICATION OF STOKES’S FORMULA

The modification of Stokes’s formula allows the user to compensate the lack of a global coverage of gravity data by a combination of terrestrial gravity and a global geopotential model. The minimization of the errors of truncation gravity data and potential coefficients could be treated in a least-squares sense as is the basic ingredient in the Royal Institute of Technology (KTH) approach as proposed by Sjöberg in 1984. This article presents the results from a recent research, whose main purpose is to evaluate the KTH approach numerically and to compute a gravimetric geoid model for Iran. The new geoid model (IRG04) was computed based on the least-squares modification of Stokes’s formula, the GRACE global geopotential model, a high-resolution digital terrain model and the NKG gravity anomaly database. The IRG04 was fitted to 247 GPS/levelling points by a 7-parameter transformation, yielding an all-over fit of 43 cm and 3.5 ppm. Also, we found a significant expected difference between the IRG04 and TUG06 models in rough topographic areas (up to ± 4 m). As the major ground data and global geopotential model were almost same in the two models, we believe that there are different reasons that come into play for interpreting the discrepancies between them, as the method for eliminating outliers from the gravity database, the interpolated denser gravity observations using the high-resolution digital elevation model before Stokes's integration, the potential of the LSM kernel, which matches the errors of the terrestrial gravity data, GGM and the truncation error in an optimum way, and the effect of applying more precise correction terms in the KTH approach compared to the remove-computerestore method. It is concluded that the least-squares modification method with additive corrections is a very promising alternative for geoid computation.

Keywords:

Gravity database , least-squares modification of Stokes’s function , regional geoid determination , SRTM , GRACE , GPS/levelling , Iran