![]() ![]() So, clearly, to go from a toposheet to physical data requires knowledge of both the datum parameters or the ground control data and the projection parameters. The datum used currently is called the Revised Everest Spheroid.Ĭartography or plane projection of this data as maps involves appropriate choice of projection parameters. The Indian geodetic datum has been constantly improved upon by determining the "deflection of the vertical" more precisely. That is to say, mathematical computations are carried out on the Everest Spheroid while physical measurements are made on a geoid defined with respect to that ellipsoidal surface. Geodetic datum series#The coordination of geographical data is done by a series of triangulations using the GCPs. The geodetic datum for the Indian region is defined (through actual physical measurements) with respect to the mathematically defined ellipsoidal surface (by specifying the semi-major axis and flatness) called the Everest Spheroid (first defined in 1830). Besides more accurate determination of the GCPs themselves, this "deflection of the vertical" is a key gravimetric measurement - which is constantly improved upon and updated - to define the geodetic datum. ![]() The normal to the spheroid would, therefore, be differently oriented to the plumbline vertical. However, actual geodetic measurements are made with respect to the datum defined by the geoid - the (irregular) surface of equal gravitational potential - because the plumbline is perpendicular to the tangents to this surface. This is a mathematically smooth surface with respect to which all computations of coordinates are carried out. For geodetic surveys, an appropriate spheroid for the area of survey (which approximates the curvature and flattening of the earth locally) is chosen. These GCPs define the "geo-referencing" for the country. ANY mapping is geo-referenced with respect to certain ground control data, which comprise a network of Ground Control Points (GCPs) across the country, whose co-ordinates are determined accurately through geodetic surveys. ![]()
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