The Digital Elevation Models (DEMs) are used to represent the topographic surface of the earth as it is used in many applications. Nowadays, many free DEMs are available online. This study aims to assess the accuracy of six free DEMs. This research uses the GPS observations to determine the elevation stations in the study area by using the Post-Processing Kinematic (PPK) technique and compares these elevations against the elevations computed at the same stations by using six free online Digital Elevation Models (DEMs): SRTM 1 Arc-Second, ASTER 1 Arc-Second, NASADEM 1 Arc-Second, SRTM 3 Arc-Second, GTOPO30, and ALOS PALSAR 12.5 m to find the best results from the free DEMs. The elevations of all stations of different data sources were calculated using the geoid model EGM96. The results indicate that the DEMs that give the best results in this study are obtained from NASADEM 1 Arc-Second, where (RMSE is 2.99 m, and the SD is 2.97 m), and SRTM 3 Arc-Second, where the (RMSE is 3.70 m, and the SD is 3.53 m).

The orbital error is one of the errors in GPS which affect the accuracy of GPS positioning. In this research GPS broadcast, ultra-rapid, rapid and precise satellite ephemerides are used for processing different baseline lengths among some CORS stations by using the Trimble Business Center software (TBC) and different satellite ephemerides (NRCan ultra-rapid, NRCan rapid and IGS final) are tested in CSRS-PPP online application at the same CORS stations. In this research, when using TBC software for processing the different baseline lengths by using the different satellite ephemerides and compared the coordinates of CORS stations which obtained from the different satellite ephemerides with each other. The results showed that the best satellite ephemerides closest to rapid and final satellite ephemerides are the ultra-rapid (00 UTC) and ultra-rapid (06 UTC). When processing the same CORS stations which used at TBC on CSRSPPP online application by using the different satellite ephemerides it is found also that the NRCan ultra-rapid closest to final satellite ephemerides.

The heights determined by  Global Positioning System (GPS) refer to the ellipsoid called the World Geodetic System 1984 (WGS84).  Global Geopotential Models (GGMs) that are available on GNSS commercial software are generally used to transform  ellipsoidal heights to orthometric heights. In this study, the geoid heights of GPS/Levelling were computed to evaluate the accuracy of the geoid heights obtained from two GGMs, namely, the Earth Gravitational Model 96 (EGM96) and the Earth Gravitational Model 08 (EGM08). Seventeen (17) GPS/Levelling stations of the High Accuracy Reference Network (HARN) over Egypt were used for this purpose. The standard deviations for the differences between the geoid heights obtained through GPS/Levelling and those obtained from EGM96 and EGM08 were determined as  ± 1.212 m and ± 0.543 m, respectively. This research confirms that the geoid heights obtained from EGM08 are closer to the geoid heights determined using GPS/Levelling over Egypt.