The Indian system is a topocentric system which is realized by
Choosing an initial point (origin)
Specifying, the latitude and longitude of the initial point
Azimuth of a line from this point
Two parameters of a reference surface (ellipsoid):
Components of deflection of vertical
Geoidal undulation at the initial point
Indian Geodetic datum, using Everest spheroid is a local geodetic datum, which best fits to certain extent the Indian subcontinent. It is non-geocentric ellipsoid, and its origin is far away from the geocentre (C. G. of the Earth).
The geodetic coordinates based on Everest spheroid differ considerably (in many cases even hundreds of meters) as compared to WGS 84 and other International ellipsoids.
Details of the Indian topocentric system (Agrawal, 2004)
