การศึกษาเปรียบเทียบความเหมาะสมของพื้นหลักฐานประเทศไทย : WGS84 กับอินเดียน 1975 / ศรายุทธ อยู่สำราญ = A comparative study on suitability of datums for Thailand : WGS84 versus Indian 1975 / Sarayut Yousamran
Currently, the geodetic network for surveying and mapping of Thailand is based on Indian 1975 datum. The horizontal control network is computed from triangulation network. The reference coordinate system is computed on Everest spheroid and used as a local geodetic datum. Now the technology of surveying and mapping is developed very quickly. The coordinate system is determined by satellites, especially GPS satellites which give more accuracy and convenience in surveying than the triangulation. A coordinate system obtained from satellites is World Geodetic System 1984 or WGS84 datum. If a location of the center, orientation of axes and a shape of the ellipsoid are changed, the coordinate system will be changed too. From the investigation on suitability to the geoid in Thailand, between WGS84 datum and Indian 1975 datum, it was found that WGS84 datum fitted to the geoid better than the Indian 1975 datum. The change of the geodetic datum changed the coordinate system. It was therefore necessary to determine parameters for datum transformation between two coordinate systems. The transformation between different terrestrial reference system was performed by three dimensional (3D) similarity transformation. The investigation in this thesis shows that only 3 translations were needed in the datum transformation for Thailand from Indian 1975 to WGS84 datum, they are deltaX = 204.4 m. : deltaY = 837.7 m. : deltaZ = 294.7 m. One important effect for the changed datum was that coordinates on the map would change too. In case of Thailand, the change on UTM grid was much more than could be accepted. The Royal Thai Survey Department considered the mater and then decided to make the new UTM grid for the new map series called L7018. The new series was expected to complete in 2003. The change of the datum for Thailand was important because it would bring the surveying and mapping of Thailand to international standards.