Iran is one of the most complicated areas in the world from the view of rough topography, tectonic activity, large lateral density and geoidal height variations. The computation of a regional gravimetric geoid model with high accuracy in mountainous regions, especially with sparse data, is a difficult task that needs a special attention to obtain reliable results which can meet the needs of the today geodetic community. In this research different heterogeneous data has been used, which includes gravity anomalies, the high-resolution SRTM Digital Elevation Model (DEM), recently published GRACE Global Geopotential Models (GGMs), geological maps and GPS/levelling data. The above data has been optimally combined through the least-squares modification of Stokes formula with additive corrections. Regarding the data evaluation and refinement, the cross-validation technique has been used for detection of outliers. Also, several GGMs and DEMs are evaluated with GPS/levelling data. The impact of utilizing a high resolution SRTM DEM to improve the accuracy of the geoid model has been studied. Also, a density variation model has been established, and its effect on the accuracy of the geoid was investigated. Thereafter a new height datum for Iran was established based on the corrective surface idea. Finally, it was found that there is a significant correlation between the lateral geoid slope and the tectonic activities in Iran. We show that our hybrid gravimetric geoid model IRG04 agrees considerably better with GPS/levelling than any of the other recent local geoid models in the area. Its RMS fit with GPS/levelling is 27 cm and 3.8 ppm in the absolute and relative senses, respectively. Moreover, the relative accuracy of the IRG04 geoid model is at least 4 times better than any of the previously published global and regional geoid models in the area. Also, the RMS fit of the combined surface model (IRG04C) versus independent precise GPS/levelling is almost 4 times better compared to the original gravimetric geoid model (IRG04). These achievements clearly show the effect of the new gravity database and the SRTM data for the regional geoid determination in Iran based on the least-squares modification of Stokes’ formula. / <p>QC 20100906</p>

Developed countries are striving to achieve a cm geoid model. Most developing countries/regions think that the situation in their areas does not allow even a few decimetre geoid model. GNSS, which provides us with position, is one of the greatest achievements of the present time. Conversion of ellipsoidal height to orthometric height, which is more useful, requires an accurate geoid model. In spite of the sparse terrestrial gravity data of variable density, distribution and quality (a typical situation in developing countries), this study set out to develop as accurately as possibly achievable, a high quality geoid model of Tanzania. Literature review of three more preferred geoid methods came to a conclusion, that the Royal Institute of Technology of Sweden (KTH) method of least squares modification of Stokes formula (LSMS) with additive corrections (AC) is the most suitable for this research. However, even with a good method, the accuracy and the quality of a geoid model depend much on the quality of the data. In this study, a procedure to create a gravity database (GDB) out of sparse data with varying density, distribution and quality has been developed. This GDB is of high density and full coverage, which ensures presence of high and low gravity frequencies, with medium frequencies ranging between fair and excellent. Also an alternative local/regional Global Gravitational Model (GGM) validation method based on quality terrestrial point surface gravity anomaly has been developed. Validation of a GGM using the new approach of terrestrial point gravity and GPS/Levelling, gave the same results. Once satisfactorily proved, the method has extra advantages. The limits of Tanzania GDB (TGDB) are latitudes 15 ° S to 4 ° N and longitudes 26 ° E to 44 ° E . Cleaning and quality control of the TGDB was based on the cross validation (XV) by the Kriging method and Gaussian distribution of the XV residuals. The data used in the LSMS with AC to develop a new Tanzania gravimetric geoid model 2008, TZG08, are 1′ ×1′ clean and statistically tested surface gravity anomalies. 39,677 point gravity in land and 57,723 in the ocean were utilised. Pure satellite ITGGRACE03S GGM to degree 120 was used to determine modification parameters and long-wavelength component of the geoid model. 3′′ Shuttle Radar Topographic Mission (SRTM) Digital Elevation Model (DEM), ITG-GRACE03S to degree 120 and EIGENCG03C to degree 360 combined GGM qualified to patch the data voids in accordance to the method of this research. TZG08 is referred to Geodetic Reference System 1980 (GRS80), and its extents are latitudes 12 ° S to 1 ° N and longitudes 29 ° E to 41 ° E . 19 GPS/levelling points qualified to assess the overall accuracy of TZG08 as 29.7 cm, and upon approximate removal of GPS and orthometric systematic effects, the accuracy of TZG08 is 27.8 cm. A corrector surface (CS) for conversion of GPS height to orthometric height referred to Tanzania National Height Datum (TNHD) has been created for a part of TZG08. Using the CS and TZG08, orthometric height of Mt. Kilimanjaro is re-established as it was in 1952 to be 5,895 m above the TNHD, which is still the official height of the mountain. / <p>QC 20100813</p>