The aim of this research is to develop a new precise and high-resolution geoid model for Egypt by refining the Global Geopotential Models (GGMs) through a process named tailoring, where the existing spherical harmonic coefficients of geopotential model are fitted to the Egyptian gravity field by integral formulas using an iterative algorithm to improve the accuracy of the obtained harmonic coefficients.
The satellite-only model GOCO05s (complete to degree and order 280) and ultra-high degree geopotential model EGM2008 (complete to degree and order 2190) have been tailored to the Egyptian 5′×5′ mean gravity anomalies in order to select the optimal model that can be used for the reference gravity field for the new geoid model. The results show that both EG1GOC5s and EGTM0818 tailored geopotential models give less and better residual gravity anomalies, where the EGTM0818 tailored model has been improved significantly by about 27% than the EGM2008 model similarly, the EG1GOC5s tailored model better than the GOCO05s model by about 17%.
Gravimetric and combined (gravity and astrogeodetic data) geoids solutions for Egypt have been computed using both tailored geopotential models in the remove-restore technique through Least-squares Collocation (LSC). The computed geoids are fitted to the GPS/levelling stations. The results show that no significant variance between the gravimetric and combined geoids solutions exists. In addition, both combined geoids solutions are given the same accuracy, where RMS ≈ ± 15 cm.
Finally, a comparison between GPS/levelling stations and both NGRAV-A and NGRAV-B gravimetric geoids computed by using both EG1GOC5s and EGTM0818 models, respectively, give nearly the same external accuracy, where the RMS of the differences ± 13 cm for NGRAV-A and ± 15 cm for NGRAV-B. Therefore, we recommend that both tailored geopotential models as reliable models for geoid heights over Egypt.