The strategic importance of the area of Lake Nasser, where the
High Dam exists, utilizes an accurate heights control for many aspects such as
the determination of the lake level and the sedimentation rate at the bottom of
the lake. GPS can be dedicating as a leveling control within this region only if
the geoid undulations are accurately determined.
Computation of geoid heights involves processing of variations in
gravity data, which reflect variations in the geoid surface. The computation
relies on Stokes’ integral. Geoid height accuracy is strongly influenced by how
well the gravity data used in the computation represents the actual gravity field.
Thus, 635 gravity points, on both sides of the lake, were used to describe the
local gravity disturbance in the studied region.
As it is impractical to perform the integration over the entire surface of the Earth, a ”remove restore” approach is often used with the long and short wavelength components of the geoid computed separately. The”removal” is done by computing the long wavelength component as gravity anomalies from a global spherical harmonic model. The short wavelength geoid height is then computed from local gravity disturbance. Finally the short and long wavelength components are added to give the required total geoid height values.