LEE- Geomatik Mühendisliği-Doktora
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ÖgeAssessment of global gravity models in coastal zones: A case study using astrogeodetic vertical deflections in İstanbul(Fen Bilimleri Enstitüsü, 2020) Albayrak, Müge ; Özlüdemir, Mustafa Tevfik ; 619803 ; Geomatik Mühendisliği Ana Bilim DalıAstrogeodetic vertical deflections (VDs) provide valuable information about the structure of Earth's gravity field. For this reason, astrogeodetic VD observations are essential gravity field observables. Several types of astrogeodetic observational instruments have been used to obtain astrogeodetic VD components. Currently, modern imaging instruments such as the Digital Zenith Camera System (DZCS) or the total station (TS)-based QDaedalus system, which are operated at field stations at night, are used to observe astronomical coordinates (astronomical latitude [] and longitude []). Global Navigation Satellite System (GNSS) receivers located at the same benchmark (BM) provide geocentric geodetic coordinates (geodetic latitude [] and longitude []). From these, the North-South (= - ) and East-West (= (-) cos ) components of VDs can be calculated. This thesis aims to introduce a new astrogeodetic VDs data set, which was collected using the QDaedalus system in Istanbul, Turkey to investigate the quality of the Global Gravity Model plus (GGMplus) gravity functional maps and Earth Gravitational Model 2008 (EGM2008). To establish the Istanbul Astrogeodetic Network (IAN), 30 BMs were selected out of 1183 BMs that are part of both the Istanbul GPS Triangulation Network (IGTN) and the Istanbul Levelling Network (ILN). While IGTN provides geodetic coordinates and ellipsoidal heights, ILN provides orthometric heights. Before establishing the IAN, the ellipsoidal heights from the IGTN, the orthometric heights from ILN, and the newly-collected valley cross levelling (VCL) data were used to calculate a new geoid model in Istanbul using soft computing techniques, including the adaptive-network-based fuzzy inference system (ANFIS) and the artificial neural networks (ANNs). The aim of this calculation is to show the current status of the Istanbul geodetic geoid. After the calculation of the Istanbul geoid, which is very weak in coastal and mountainous areas, the IAN was established. The first astrogeodetic VD observations campaign taken in Istanbul were made using the Leica TCRM1101 TS integrated QDaedalus system. The measured VDs are unique in that, not only were they measured for the first time in Istanbul, but they also form Turkey's first dense astrogeodetic network. A total of 21 out of 30 BMs are located in the coastal zone allowing us to investigate the quality of global gravity models along the coast of Istanbul. Preliminary steps are required before the QDaedalus system can be used in the IAN to investigate the precision and accuracy of the system. One such activity is to test the QDaedalus system at the same BM several nights in a row to determine the precision of the system. For this thesis, these test observations were carried out at a control site at Technical University of Munich (TUM) test station, the Istanbul Technical University (ITU) test station, and at six densely-spaced pillars of the geodetic control network at the Geodetic Observatory Wettzell (GOW). The accuracy should also be established, and in this thesis was determined by comparing the Hannover DZCS TZK2-D VDs results at 10 field stations located in the Munich region to independently observed VDs data from the QDaedalus. The ITU test station was also used by the Turkish DZCS's—Astrogeodetic Camera System 2 (ACSYS2)—test station to determine the precision of this DZCS by repeated observations. The VDs results comparison of the QDaedalus and ACSYS2 at the ITU test station provide us to determine the accuracy of the ACSYS2. The initial test observations with the Leica TCRM1101 TS integrated QDaedalus system showed that it is capable of producing highly accurate VDs data (~0.20"). After establishing these satisfactory results, the astrogeodetic VDs in the IAN were measured for follow-on campaigns. The standard deviations (SDs) for the IAN are approximately 0.20" for both the North-South () and East-West () components. This new VD data set was compared with modelled VDs from the GGMplus gravity functional maps and EGM2008. The differences between the VDs from QDaedalus and those from GGMplus and EGM2008 tend to increase towards the coastlines, where discrepancies of several arcseconds amplitude between the observed and modelled values are encountered. We interpret these spurious differences as weaknesses in the modelled VDs along the Istanbul coastlines, most likely reflecting increased errors in the altimetry-derived marine gravity field the GGMplus model depends on (via EGM2008 and Danish National Space Center [DNSC2007]). The central finding of this thesis is that astrogeodetic VDs are valuable tools for independently investigating the quality of coastal-zone gravity data sets and gravity field products. The new VD data set is useful for the quality assessment of future EGMs, such as the EGM2020. The results and findings presented in this thesis were supported by grants and scholarships from several fundings and research support sources. The Turkish DZCS modernization process, determination of the precision and accuracy of the system, and the IAN's fieldwork by the QDaedalus system were supported by the Scientific and Technological Research Council of Turkey (TUBITAK) Project (grant number 115Y237). The Leica TDA5005 TS integrated QDaedalus system test observations at TUM were supported by the German Academic Research Council Scholarship (DAAD) short-term grant. The Leica TCRM1101 TS integrated QDaedalus system's test observations at TUM and the Munich region observations were supported by a TUBITAK BIDEB 2214-A scholarship. The GOW observations were supported by the Institute of Astronomical and Physical Geodesy (IAPG) at TUM and GOW. Finally, the data analysis of the IAN was supported by the Fulbright Foundation.