Precise geoid determination for the Entire UAEusing three different computation techniques Dr. Adel Khaleefh Alshamsi Military Survey Department (UAE), UAE Email: alshamsi@uaesurvey.ae Prof. Dr. Shahrum Bin Ses University Teknologi Malaysia The main objective for this study was to determine the first precise gravimetric geoid for the United Arab Emirates (UAE). The geoid has been computed using FFT, LSC and Stokes’ Integration techniques. In carrying out the task, substantial gravity data for the entire country has been gathered through land gravity measurement and compilation of the existing marine and land gravity anomalies. A total of nine (9) new absolute gravity stations were established throughout the UAE forming gravity base network to provide control for the detail gravity survey. The land gravity surveys are conducted on a nominal grid of about 5 x 5 km grid. The free-air gravity anomalies have been deduced with a accuracy of ±0.2 mGal. Marine gravity data used in this study is the combination of GECO and Mobil ship-borne gravity data. Comparison with altimetry derived gravity anomalies shows RMS differences of ±5mGal. Residual terrain correction model (RTM) has been used in the geoid computation. Geopotential geoid derived from EGM96, PGM2000A and GGM01C-80 models agree with geometric geoid heights at ±0.38m level of accuracy. The GRAVSOFT package is used in computing the 1-D and 2-D FFT gravimetric geoids for the UAE (21º N ≤ φ ≤27ºN and 50ºE ≤ λ ≤58ºE). Three LSC geoid solutions have been obtained. In the first solution a total of 6,086 gravity anomalies and 48 GPS/Levelling geoid heights have been utilised. The estimated collocation errors remained below ±5cm throughout the country. Stokes Integration geoid solutions have been computed using outer zone cap size of 4°. Generally the geoid is having minimum and maximum values of –34m and –24m, respectively. The higher geoid is observed in the north-eastern part with the steep slope of about 10m over the distance of less than 50km over the mountainous areas. Elsewhere the geoid is very gentle over the fairly flat region of the UAE. GPS-fitted geoid heights that have been computed at selected benchmarks showing agreement with the corresponding geometric geoid heights at ±5cm level of accuracy. The discrepancy between GPS and levelling height difference for 45 GPS/Levelling lines have also been compared to the computed allowable precise levelling miss-closure of . The discrepancies for 50% of the lines are within the specified tolerance of precise leveling practice in the UAE, while discrepancies for the rest of the lines remain below 10cm.