A global gravity model (GGM) is a mathematical function describing the gravity field of the Earth. The assessment of GGMs involves identifying the best-fitting model to local gravity field for geodetic and geophysical applications. Thus, highly accurate independent datasets are required to obtain the appropriate model. In general, GPS/levelling data have been used for this purpose. If these measurements are not performed simultaneously, they may not be reliable due to vertical deformations especially in seismically active countries. Therefore, we used highly precise absolute and vertical gravity gradient measurements obtained by the Scientific and Technological Research Council of Turkey’s (TUBITAK) National Metrology Institute and General Directorate of Mapping within the frame of Turkish Height System Modernization and Gravity Recovery Project over the period of 2016–2018 from Turkey to choose the best GGM for the whole of Turkey among the 19 latest tested satellite-only and combined GGMs (2004–2018). Our results showed that vertical gravity gradient measurements could also be used for the regional validation of GGMs as an independent in situ dataset. The GOCE based XGM2016 combined model and GO_CONS_GCF_2_TIM_R5 satellite-based model were found to be the best-fitting models. The results also showed improvements over the widely used EGM2008 up to the spherical degree 270 for Turkey. The improvements of the GOCE models over the EGM2008 model are mostly seen in mountainous areas such as the Black sea, Aegean Sea, some parts of the Mediterranean, and South-eastern Anatolia regions with maximum improvements in the coastal areas of the Eastern Black Sea. The best-fitting GGMs to local gravity field identified with these measurements could be used for further geodetic and geophysical purposes in Turkey.
<p>For the quasi-geoid determination by 3-D Least Squares Collocation (LSC) in the context of Molodensky&#8217;s approach, there is no need to measured or modelled vertical gravity gradient (VGG) as the 3-D LSC takes the varying heights of the gravity observation points into account. However, the use of measured or modelled VGG instead of the thereotical value is expected to improve the quasigeoid-geoid separation term particularly in mountainous areas. The VGG measurements are found to be different from the theoretical value in the range of - % 25 and + % 39 in western Turkey. Previously there has been no study using modelled VGGs for gravimetric geoid modelling in Turkey. VGGs are modelled by 3-D Least Squares Collocation (LSC) in remove-restore approach and validated by terrestrial VGG measurements in western Turkey. The effect of using modelled VGG instead of the theoretical one in quasigeoid-to-geoid separation term is found to be significant. The quasi-geoid computed by 3-D LSC in western Turkey is converted to geoids using theoretical or modelled VGG values and compared with GPS/levelling geoid-undulations.</p><p>&#160;</p>
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