Validation Of The 2008 Earth Gravity Model Using Gpsleveling Data A Case Study Around Debre Birhan

The accuracy of the 2008 Earth Gravity Model (EGM08) is evaluated using GPS/rnLeveling data around Debre Birhan town. Now, the advents of satellites gravimeteryrnand Global Positioning System have revolutionized the way Geopotential number isrncomputed on the earth’s surface. The Space Domain Spherical Harmonic Analysis ofrnthe EGM08 Stokes’ dimensionless coefficients Cn,m and Sn,m provides Geopotentialrnheight at GPS observation points. This study compared the EGM08 derivedrnorthometric height, normal height, geoid and height anomaly with GPS/Leveling data.rnThree first order Blue Nile leveling benchmarks available around Deber Birhan townrnwith a tie to the port of Alexandria mean sea level tide gauge datum were used forrnevaluating the quality of EGM08. A total length of 1200 m first order geodeticrnleveling work was done to shift all the three old Blue Nile leveling benchmarks tornnew locations that have good GPS satellite visibility. The spatial GPS coordinates ofrnthese benchmarks through which the Geopotential heights are computed wererndetermined within few millimeter accuracy using GAMIT/GLOBK software. Thernaccuracy of the EGM08 derived Geopotential heights are in general good to fewrncentimeters when compared to GPS/Leveling data. The agreement between thernorthometric height derived from EGM08 and level heights has a standard deviation ofrn2.41cm with a mean of -3.14cm. This error is reduced to 1.09cm when the preyrnassumption for mean gravity is constrained by surface gravity observation.rnBesides, the comparisons of the gravimetric geoid and gravimetric height anomalyrn(zeta) with the geoid undulation derived from GPS/Leveling, have a standardrndeviation of 2.24cm & 2.26cm, respectively. In general, the comparison ofrnorthometric height with level height gave the best result with datum offset of -3.73 cmrnbetween the gravimetric geoid defined by Wo=Uo and leveling datum. The datumrnoffset may be attributed to the choice of the Wo value and real displacement betweenrnthe mean sea level and the geoid.rnKey words: Geopotential height, leveling height, EGM08, GPS coordinates.

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