Investigating the Effect of the Iranian Ground Gravity Data on Local Geoid Determination

By increasing the accuracy and resolution of gravity data derived from terrestrial, airborne and satellite methods, the accuracy and resolution of global geopotential models have significantly been improved. For example, EGM2008 and EIGEN-6C4 are among the most accurate global geopotential models which have been expanded up to degree 2190. Nevertheless, global geopotential models do not have an adequate accuracy everywhere. Therefore, the local gravity field modeling based on the geodetic boundary value problem approach and the local gravity data has always been an interesting subject. In Iran, first-, second- and third-order gravity networks with the spatial resolutions 30ʹ, 15ʹ, and 5ʹ have been designed for geodetic applications. Now, these important questions arise: (1) How important is the spatial resolution of the ground gravity data in the local modeling? (2) Can local gravity data with any resolution improve the global models? To answer these questions, the effect of the spatial resolution of the Iranian ground gravity data to determine the local geoid based on the geodetic boundary value problem solution by the remove-compute-restore technique is studied. In this line, four regions over Iran with different spatial resolutions are selected as test regions. Region 1 consists of 1738 gravity data with spatial resolution 5.7ʹ, region 2 consists of 165 gravity data with spatial resolution 21.6ʹ, region 3 consists of 234 gravity data with spatial resolution 18ʹ and region 4 consists of 1728 gravity data with spatial resolution 5.7ʹ. Then, the geodetic boundary value problem is separately solved for each region, where the EGM2008 global model up to degrees 360, 720, 1080 and 2160 is used as reference model. Finally, the computed local geoids and the global geoids are compared with the GPS/Leveling geoid. From results we found that the local geoid in the regions 1 and 4 has an accuracy of about 23 cm in terms of the root mean square error (RMSE), while the local geoid in the regions 2 and 3 has an accuracy of about 32 cm. This means that the local geoid in the regions 1 and 4, where the spatial resolution of gravity data is higher, is more accurate than the local geoid in the regions 2 and 3. Moreover, we found that the local geoid of region 1 is more accurate than the global geoids up to degrees 360, 720 and 1080, while the accuracy of the local geoid is consistent with the global geoid up to degree 2160. Such a result is obtained for the region 4. For the regions 2 and 3, the local geoid is more accurate than the global geoid only up to degree 360, while the accuracy of the local geoid is consistent with the global geoids up to degrees 720, 1080 and 2160. This is due to the fact that the spatial resolution of gravity data in the regions 1 and 4 is 5.7ʹ which is equivalent to degree about 2160, while the spatial resolution of gravity data in the regions 2 and 3 are 21.6ʹ and 18ʹ, respectively, which are equivalent to degrees about 500 and 600. Therefore, it is concluded that when the spatial resolution of ground gravity data is lower than the corresponding degree of the reference model, the local geoid does not outperform the corresponding global geoid.