Evaluation of the EGM2008 and GOCE global geoid models versus the Laplace points in Iran

The deflection of vertical components, are the second order spatial derivatives ofthe gravity potential, efficiently counteract signal attenuation at the low andmedium frequencies. Regional gravimetric geoid and quasi-geoid models are now commonly fitted to GPS-leveling data, which simultaneously absorb GPS/leveling and quasi/geoid errors due to their inseparability. We propose that independent vertical deflections are used instead as they are not affected by this inseparability problem. The formulation is set out for geoid slopes and changes in slopes. In this research, 10 Laplace points form the Iranian astro-geodetic networks were utilized for calibration and combination of the EGM2008 and GOCE global geoid models. Several two-, three- and four-parameter- models were used as a correction surface for the combination and evaluation of the geoid models. The standard deviation of the deflection of vertical components before and after fitting in geoid models evaluated with independent data. The results showed a significant improvement in the N-S direction of the GOCE model from 0.095 to 0.003 and in the E-W direction of the EGM2008 model from 0.246 to 0.008. To sum up, the two-parameter models worked best among the other corrective surface models. Also, the EGM2008 model gave slightly better results versus the GOCE model. For any future researches, use of homogenous and high quality zenith-nadir digital camera data is strongly recommended.