Fitting Gravimetric Local and Global Quasi-Geoids to GPS/Levelling Data: The Role of Geoid/Quasi-Geoid Variations

Abstract. The Global Positioning System (GPS) has been extensivelyutilized in surveying and mapping applications worldwide and inEgypt, which results in a crucial need for precise geoid models.Traditionally, global and local gravimetric quasi-geoid models aredetermined and, then, fitted to GPS/leveling data. In the context ofprecise geoid determination, a quasi-geoid determination has to beconverted to a geoid model prior to fitting process. Neglecting such atransforming process leads to the absorption of these effects in thefitting residuals that, hence, may produce misleading results andinterpretations. The geoid/quasi-geoid separations in Egypt have beencomputed and found to range from –0.074 m to 0.367 m, with anaverage of 0.060 m. Additionally, when the quasi-geoid to geoidconversion is performed, it has been found that the undulation of theEIGEN-CG01C global quasi-geoid model when compared to trueundulations of HARN stations, have been decreased, in the meansense, from 0.52 m to 0.46 m. That means an average improvement ofaccuracy of about 11%. Based on the obtained results, it can beconcluded that the conversion of the quasi-geoid to the geoid is quitesignificant and it should be a regular step in precise geoiddetermination in Egypt.