Satellite mapping enabled obtaining Global Digital Elevation Models (GDEMs) to wide areas of the earth surface. Those DEMs have low accuracy, so they need to be evaluated and improved. In this paper, ground orthometric and GPS heights have been used to evaluate two GDEMs: Shuttle Radar Topography Mission (SRTM) with 30 m resolution; and ALOS Phased Array type L-band Synthetic Aperture Radar (ALOS/PALSAR) with 12.5 m resolution. Toshka in the south of Egypt and another site in the southwest of Egypt were both available for the study. The evaluation technique involved comparing the ellipsoidal and orthometric heights of the two GDEMs with the corresponding GPS and orthometric terrestrial data. The GDEMs have been through three steps of improvement. The first step includes converting the ellipsoidal heights of the used GDEMs to orthometric ones by using the High-Resolution Earth's Gravity Field Model (SGG-UGM-2) instead of the Earth Gravitational Model 1996 (EGM96). The second step has been performed by shifting the model heights using one point in the middle of the study area. The third step compromises shifting the model heights using the average value of well distributed five ground control points (GCPs). The results proved that the proposed simple shifting process is effective in improving the performance of the used GDEM. On the other hand, using GDEMs ellipsoidal heights is much better than using their orthometric heights. As well, using an accurate geoid model, instead of EGM96, in converting the ellipsoidal heights into orthometric heights can improve the GDEM performance.