Lt. Col. (Dr.) M. N. Kulkarni
Geodetic & Research Branch, Survey of India, Dehradun
As the uneven and undulating physical surface of the Earth is not suitable for the mathematical computations of geodetic data reduction and also as a map datum, a hypothetical geometrical reference surface, called ‘Geodetic Datum’ must be defined. The accuracy of such computations and mapping is directly affected by the suitability of the datum used. Several hundred local geodetic datums are in use in different parts of the world, and many of these have been / are being redefined to meet the ever-increasing accuracy requirements.
The Global Positioning System (GPS) yields positions of survey points on a global reference surface called World Geodetic Sysetm 84 (WGS 84). This geodetic datum is geocentric, and defined to high accuracy by the Defense Mapping Agency, USA (DMA). In order to correlate the coordinates obtained by using GPS, on this datum, to the coordinates of the point on s on Survey of India topographical maps, the relationship between the WGS 84 and Indian map datum must be defined.
The Indian Geodetic Datum adopted in 1880 is the Everest ellipsoid, which is based upon the adjustment of geodetic data available at that time, computed using the models, techniques and methods in practice then. Due to the numerous limitations inherent to this Datum, including the lack of modern computers, least squares technique, sufficient data etc., this datum is not suitable for the geodetic & other scientific applications requiring high accuracy. Hence, a modern techniques methods has been taken up in the Geodetic Research Branch of Survey of India, under a DST Project titled “Redefinition of the Indian Geodetic Datum-cum-Integrated adjustment of Geodetic Network of India.”
The salient features of this adjustment and proposed redefinition of the Datum, including the need for redefinition, overview of few such projects undertaken in other countries, the mathematical models, methodology and software / hardware being used, and the progress made so far, will be presented in this paper. Some sample results obtained will be discussed, and future plan will be outlined. The complex issue of establishing the relationship between the Indian Datum and WGS 84 will also be discussed in detail.
