Current Geoid Studies in Turkey and the need for Local High-Precision Astrogeodetic Geoid Determination Using CCD/Zenith Cameras

During the last few years, the development of CCD image sensors at a reasonable price made the instruments of astrogeodetic observation possible to use for local high-precision astrogeodetic geoid and gravity field determination. Generally, the geoids of most European countries are in centimeter level accuracy except in mountainous regions. Turkish geoid also has accuracy problems in mountainous regions especially in the eastern parts of Anatolia and around boundaries of Marmara Sea. Studies performed in Europe in last decade indicate that, to reach the centimeter level accuracy in mountainous areas, astrogeodetic vertical deflections are more effective than gravimetric and other geoid determination methods. Turkey had started the geoid determination studies in 1976 with 13 absolute gravity points. Turkish National Fundamental Gravity Network (TNFGRN), densificated with 1st and 2nd order 66245 gravity points in Potsdam Gravity datum. TG03 has a final internal precision of 1 cm at the observation points and the external accuracy is within decimeter level. High precision in astrogeodetic geoid determination techniques are scarcely published by some universities around Europe using CCD/Zenith cameras. There are various zenith camera systems developed as state-of- art instrumentations using both CCD sensors for imaging stellar objects and GPS receivers for ellipsoidal coordinates, in order to determine the direction of the plumb line. These systems are designed and tested where conventional techniques are not sufficient. In this study, increasing accuracy of Turkish geoid is subjected to using CCD/Zenith cameras in the province of Istanbul. The planning test area is going to use the data available on the GPS/Leveling geoid of Istanbul and produce astrogeodetic data on a profile starting from the north shore of Marmara region, passing through the Marmara Sea to the south. The astrogeodetic instruments will be designed for engineering studies that are needed to determine vertical deflections to use ellipsoidal heights derived from GPS.