Comparison of the geoids determined with different methods and data for Turkey

Geoid can be used for different purposes, but in practice it is needed mainly to obtain orthometric heights. Obtaining orthometric heights through traditional measurements is very laborious and time-consuming. Converting ellipsoidal heights obtained with GPS measurements to orthometric heights is easier. However for this conversion, geoid heights must be known with a certain level of accuracy.In this study, different geoid models that have been determined for Turkey by the authorized entities are compared with each other. To this end, ten Turkish local geoid models (1976 Turkish Geoid, Turkish Geoid-1991, Turkish Doppler Geoid, Turkish PseudoWGS84 Doppler Geoid, Turkish Astrogeodetic Geoid-1994, GPS/Leveling Geoid, Updated Turkish Geoid-1999, Turkish Geoid-2003, Turkish Geoid-2009, Turkish Hybrid Geoid Model-2009) and twelve global geoid models (OSU91A, EGM96, GGM01S, GGM01C, EIGEN-GRACE01S, EIGEN-GRACE02S, EIGEN-CHAMP03S, GGM02S, GGM02C, EIGEN-CG01C, EIGEN-CG03C, EGM08) are used.For numerical application, 30 points in the Turkish National Fundamental GPS Network were selected in a way that they are distributed as homogeneously as possible. The geoid heights of the points in different geoid models are calculated. These models are compared in terms of the criteria based on their developments and in terms of different geoid heights calculated using these models at the test points. The geoid height comparisons are made among the local geoid models themselves as well as between global geoid models and local GPS/Levelling geoid. The differences of geoid heights calculated from various models at the points are evaluated and some findings are obtained. Taking these findings and the above-mentioned criteria into account, the causes of discrepancies between various geoid models are examined. It is seen that converting orthometric heights through geoids determined after 1999 years can be made the order of decimeter.