Local geoid determination combining gravity disturbances and GPS/levelling: a case study in the Lake Nasser area, Aswan, Egypt

The use of GPS for height control in an area with existing levelling data r equires the determination of a local geoid and the bias between the local l evelling datum and the one implicitly defined when computing the local geoi d. If only scarse gravity data are available, the heights of new data may b e collected rapidly by determining the ellipsoidal height by GPS and not us ing orthometric heights. Hence the geoid determination has to be based on g ravity disturbances contingently combined with gravity anomalies. Furthermo re, existing GPS/levelling data may also be used in the geoid determination if a suitable general gravity field modelling method (such as least-square s collocation, LSC) is applied. A comparison has been made in the Aswan Dam area between geoids determined using fast Fourier transform (FFT) with gra vity disturbances exclusively and LSC using only the gravity disturbances a nd the disturbances combined with GPS/levelling data. The EGM96 spherical h armonic model was in all cases used in a remove-restore mode. A total of 19 8 gravity disturbances spaced approximately 3 km apart were used, as well a s 35 GPS/levelling points in the vicinity and on the Aswan Dam. No data on the Nasser Lake were available. This gave difficulties when using FFT, whic h requires the use of gridded data. When using exclusively the gravity dist urbances, the agreement between the GPS/levelling data were 0.71 +/- 0.17 m for FFT and 0.63 +/- 0.15 for LSC. When combining gravity disturbances and GPS/levelling, the LSC error estimate was +/-0.10 m. In the latter case tw o bias parameters had to be introduced to account for a possible levelling datum difference between the levelling on the dam and that on the adjacent roads.