Downward continuation and geoid determination in Mongolia from airborne and surface gravimetry and SRTM topography

In this paper we outline the principles of downward continuation and geoid determination in Mongolia, based on the 2004-5 airborne gravity survey, surface gravimetry from various sources, and detailed digital terrain models from satellite radar interferometry (SRTM). A remove-restore method is used for terrain and global spherical harmonic reference models, with the residual gravity field signal downward continued by least-squares collocation, and the geoid and quasi-geoid computed by spherical Fourier methods. For reference field a GRACE and EGM96 combination field is used, and the geoid assessed using a subset of available GPS-leveling data, indicating that recent GRACE fields still have detectable problems in the harmonic wavelength band 50-100. The gravimetric geoid model computed refers to a global vertical datum; the Kronstadt Baltic Sea level data used in Mongolia shows a difference of more than one meter to the global datum, and therefore it is essential to fit the gravimetric geoid to local height data to obtain an operational geoid model for GPS use.