The Global Positioning System (GPS) provides the surveyor with three-dimensional coordinates with respect to the World Geodetic System 1984 (WGS84). The heights obtained from GPS are above an ellipsoidal model of the earth, WGS84. However, it is unlikely that ellipsoidal heights will ever be used for practical surveying, engineering or geophysical applications, as they have no physical meaning. Therefore, it will always necessary to transform GPS-derived ellipsoidal heights to orthometric heights. Conversion of GPS heights requires a high-resolution geoid height model. In this context, orthometric heights can be obtained without leveling by combining the geoidal undulations with the ellipsoidal heights derived from GPS. The goal of this research is to compare orthometric heights derived from GPS observations with orthometric heights obtained from spirit leveling observations over small areas in Egypt. For this purpose, five test areas were chosen, the first one is in Helwan city [12 stations], the second one is in Al-Obour city [7 stations], the third one is in Al-Dabaa city [16 stations], the fourth one is in Domiate port [8 stations] and the fifth one is in Toshkii [10 stations]. GPS measurements, spirit-leveling measurements, were performed at all stations in each test area. The geoid undulations were computed using three techniques. One of them GPS/Leveling consists of computing the differences between the ellipsoidal and orthometric heights, the second technique was by calculating the undulations from the gravimetric Geoid-95 and the third technique was by deriving the geoidal undulations from the global geopotential earth model, EGM96. The analysis of the obtained results shows that with available accurate geoid, the determination of geoidal heights is currently at least as accurate as the determination of ellipsoidal heights by GPS-phase measurements. This opens interesting possibilities for orthometric height determination in the mountains and in remote areas without traditional vertical control procedures. |
