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Turkish National Vertical Control Network (TUDKA)
The Turkish Vertical Reference System has been realized by the adjustment of geopotential numbers at ~26000 benchmarks. The current realization is called the Turkish National Vertical Control Network-1999 (TUDKA-99). Almost 30000 km spirit-leveling measurements between 1935-1999 periods supplemented by surface gravity observations along the traverses were used to determine the gravity potential differences, and hence the geopotential numbers at benchmarks. Great majority of the traverses were resurveyed between 1972 and 1992 as the second period. To express geopotential numbers in terms of orthometric heights, they were divided by the mean value of gravity taken along the plumbline. Since a pure orthometric height cannot be practically realized, the approximation given by the Helmert orthometric heights has been used. The mean value of gravity was computed using the simplified Poincare-Prey reduction, that crudely approximates the vertical gravity gradient by the linear free-air gradient and models the topography by a spherical shell with a constant mass density of 2670 kg/m3. The local Mean Sea Level (MSL) of Antalya tide gauge, located at the Eastern Mediterranean coast of Turkey (Latitude: 36.8844N, Longitude: 30.7016E), has been selected as a reference (zero) surface. Local MSL has been estimated from annual means of sea level between 1936 and 1971. Due to the sea surface topography that was not taken into account in the datum definition, the point heights are supposed to be systematically biased.
Turkish National Vertical Control Network-1999 (TUDKA-99)