ASSESSING THE ACCURACY OF THE GEOID HEIGHTS CALCULATED BY INTERPOLATION AND EXTRAPOLATION FROM GLOBAL GEOID MODELS IN NORTHERN EGYPT

Global Geoid Models (GGMs) have an important role in height transformation to convert the ellipsoidal height into orthometric height in several engineering applications. Based on recent research findings, and by pre-assessment of five GGMs; namely EGM2008, EIGEN-6C4, GECO, SGG-UGM-1, and XGM2019e_2159, it was found that the global models XGM2019e_2159 and GECO showed promising results by comparing against 165 GNSS/Level control points in the north of Egypt. In light of the above, the geoid heights calculated from the XGM2019e_2159 and GECO using interpolation and extrapolation methods have been assessed in this region . For each GGM, Geoid heights have been computed and, then, compared with the known control points. In the interpolation part, statistical analysis has been performed with accomplished results indicating that, the errors were about 0.04 m and 0.05 m in the Mediterranean coast and for the Nile River in the delta area were about 0.06 m and 0.05 m, respectively. In the region of Northern Egypt, the leveling of the 3rd class could be replaced by the XGM2019e_2159 or GECO model after aligning it with the control points every 50 km by interpolation approach. On the other approach, the geoid height for a distance of 25 km can be obtained from the global models XGM2019e_2159 and GECO using the extrapolation method with an error of 0.08 m. In order to improve the results obtained by the extrapolation method, Artificial Neural Network (ANN) has been used to create a local geoid in the area of the Mediterranean coast and evaluated over the Delta region. The results showed small error values of about 0.04 m and 0.05 m in the first 50 km using XGM2019e_2159 and GECO.