Determination of Geoid Undulation by Astro-Geodetic Method

摘要

Civil engineering projects require the orthometric height of points (i.e., the height of points above geoid). Thus, to make use of the Global Navigation Satellite System (GNSS) in engineering surveying, geoid undulation (i.e., the separation between an ellipsoid and geoid surface) must be known. In this study, an attempt has been made to determine the geoid undulation using the astro-geodetic method. Observations were taken in a network of seven stations spread over 300 km(2) around Dehra Dun, India. Astronomical observations of seven stars belonging to different constellations were taken with a theodolite having a least count of 1.0 s. Global positioning system (GPS) observations were taken using dual-frequency geodetic GPS receivers with a 0.1-mm-accurate digital level. Values of astro-geodetic geoid undulations (of stations) have been found close to that computed from field levelling. Further, the values have been compared with those obtained from global geoid models such as EIGEN6C3stat and EGM2008, and the astro-geodetic method was found to provide better accuracy than the global geoid models. Therefore, it can be concluded that the astro-geodetic method is the better choice for reducing GPS observation to orthometric height. In the future, the astro-geodetic method may be used to develop an accurate geoid model in general and for Indian topography in particular.