Astrogeodetic geoid determination in İstanbul

Astrogeodetic deflections of the vertical (DoV) provide significant information about the Earth's gravity field, as they define the direction of the gravity vector, and have traditionally been determined by observing stars using astrogeodetic instruments. Today, DoV data are used for various purposes, such as geoid modeling, showing possible deficiencies of official geoid models or height systems and validating Global Geopotential Models (GGMs). In this thesis, DoV data obtained in 2018 using the robotic total station-based QDaedalus system, developed by ETH Zurich, were utilized for geoid determination in Istanbul. The Istanbul Astrogeodetic Network, consisting of 30 points, includes these DoVs, as well as geodetic coordinates, ellipsoidal heights, and orthometric heights information at each point. Astronomical leveling (computed using DoV data) was compared with GNSS/leveling (calculated using ellipsoidal and orthometric height data), and the results of these two methods were analyzed for geoid height differences (ΔN) and geoid heights (N). To determine geoid height differences, 134 baselines were created by triangulating the 30 points. The distances between the baselines ranged from 2.44 to 37.96 km. Geoid heights were calculated using DoV data, with 435 possible baselines formed among the 30 points in the network, via Least Squares Adjustment; and for GNSS/leveling, using the ellipsoidal and orthometric height differences (N = h – H). The results revealed that astronomical leveling yielded significantly better results than GNSS/leveling for both geoid height differences (ΔN) and geoid height (N) calculations. The primary reason for this is that the network, with baseline distances on the order of kilometers, is unsuitable for GNSS/leveling. However, in an adjustment involving 435 baselines in a network of 30 points, astronomical leveling proved to be a highly advantageous method, providing an accuracy of 1.6 cm in the Bosphorus and 3 cm in peripheral regions of the network, compared to GNSS/leveling. In the scope of the study, four different traverses within the network consisting of 30 points were analyzed. Comparisons between the astrogeodetic method and GNSS/leveling revealed that the astrogeodetic method provided higher accuracy. This study demonstrates the critical importance of astrogeodetic observations for geoid determinations in regions with low point density. In this context, it is anticipated that astrogeodetic DoV data could be utilized to verify or validate the Turkish Geoid Model-2020 (TG-20) in areas where it is needed.