Validation of the 2008 Earth Gravity Model Using Gps/Leveling Data – A Case Study Around Debre Birhan

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Addis Ababa University


The accuracy of the 2008 Earth Gravity Model (EGM08) is evaluated using GPS/ Leveling data around Debre Birhan town. Now, the advents of satellites gravimetery and Global Positioning System have revolutionized the way Geopotential number is computed on the earth’s surface. The Space Domain Spherical Harmonic Analysis of the EGM08 Stokes’ dimensionless coefficients Cn,m and Sn,m provides Geopotential height at GPS observation points. This study compared the EGM08 derived orthometric height, normal height, geoid and height anomaly with GPS/Leveling data. Three first order Blue Nile leveling benchmarks available around Deber Birhan town with a tie to the port of Alexandria mean sea level tide gauge datum were used for evaluating the quality of EGM08. A total length of 1200 m first order geodetic leveling work was done to shift all the three old Blue Nile leveling benchmarks to new locations that have good GPS satellite visibility. The spatial GPS coordinates of these benchmarks through which the Geopotential heights are computed were determined within few millimeter accuracy using GAMIT/GLOBK software. The accuracy of the EGM08 derived Geopotential heights are in general good to few centimeters when compared to GPS/Leveling data. The agreement between the orthometric height derived from EGM08 and level heights has a standard deviation of 2.41cm with a mean of -3.14cm. This error is reduced to 1.09cm when the prey assumption for mean gravity is constrained by surface gravity observation. Besides, the comparisons of the gravimetric geoid and gravimetric height anomaly (zeta) with the geoid undulation derived from GPS/Leveling, have a standard deviation of 2.24cm & 2.26cm, respectively. In general, the comparison of orthometric height with level height gave the best result with datum offset of -3.73 cm between the gravimetric geoid defined by Wo=Uo and leveling datum. The datum offset may be attributed to the choice of the Wo value and real displacement between the mean sea level and the geoid. Key words: Geopotential height, leveling height, EGM08, GPS coordinates.



Geopotential height, leveling height, EGM08, GPS coordinates