Coastal geoid improvement using airborne gravimetric data in the United Arab Emirates

Abstract

Military survey department (MSD) of The United Arab Emirates (UAE) has undertaken the Airborne Gravity Survey Project for the marine area of the country. The main purpose for the implementation of the airborne gravity survey for the marine area is to improve the existing gravimetric geoid of the UAE. The airborne gravity data was estimated to have accuracy better than 2.0 mGal. The first gravimetric geoid for the country has been established after the completion of nation-wide gravity survey of 5 km grid spacing by the MSD in 2003. The 2003 geoid was hampered by the lack of gravity data along the near coast area (including marine) as well as the eastern part of the country. The geoid has subsequently been computed from all available data: airborne gravity, surface gravity (including older data), satellite altimetry gravity and EGM08 reference field, as well as detailed height data from the shuttle radar topography mission (SRTM). The geoid computations were all done with the GRAVSOFT suite of programs from DTU-Space and University of Copenhagen. The computations involve a rigorous downward continuation from airborne to surface level by least-squares collocation. The result is a gravimetric geoid model “uae2009_gravgeoid2”, that is, a geoid model which refers to a global vertical datum. The change of geoid models computed with or without the new airborne gravity data showed differences of 30 cm or more, largest along the coast of the Northern Emirates and Fujairah. Using available global positioning system (GPS) leveling data from the 2003 report, a new geoid fitted to the UAE height system “uae2009_geoid” has been constructed and made available. The fitted geoid is dependent on the quality of the available GPS-leveling data, which have many apparent errors. Therefore the gravimetric geoid has been transformed by a single constant as well, to yield a shifted gravimetric geoid, “uae2009_gravgeoid”, roughly consistent with the UAE vertical datum and more suitable for any future adaptions of the geoid to local GPS-levelling data. The accuracy of the gravimetric geoid is estimated to be 3 to 5 cm across most of the UAE. Lower geoid accuracy of about 5 cm is expected over mountainous region in the north-eastern part of the country. Key words: Geoid, gravity, airborne gravimetry, global positioning system (GPS), satellite altimetry.