“What is the difference between Everest Spheroid and WGS 84?”

A suitable geometrical mathematical surface is necessary to be defined for mapping Earth’s surface as its surface is very complex, making it unsuitable for computations and mapping. This geometric surface is called reference surface which is based on the following assumptions:

1. It is an oblate ellipsoid of rotation formed by rotating an ellipse whose major axis and minor axis are nearly equal to that of the equatorial axis and polar axis of the Earth. Rotation of ellipse is about its minor axis.

2. This reference surface is rotating from West to East (anticlockwise) with nearly the same speed as that of the Earth.

3. Mass and Volume of this surface is nearly equal to that of the Earth.

4. Center of the ellipsoid coincides with the center of gravity of the Earth. Minor axis of the ellipsoid coincides with polar axis of the Earth.

India and other countries of the world made measurements in their countries and defined reference surface to serve as Datum for mapping. In India the reference surface was defined by Sir George Everest, who was Surveyor General of India from 1830 to 1843. It has served as reference for all mapping in India. Indian system can be called Indian Geodetic System as all coordinates are referred to it. The reference surface was called Everest Spheroid.

The initial point for mapping on the surface of the Earth was chosen at Kalyanpur in Central India. On realization of the system it was found that our system is in error and the assumptions have not been fully met. It is estimated that center of Everest Spheroid is nearly a kilometer from the center of gravity of the Earth. It is also realized that
minor axis is not parallel to polar axis but inclined to it by a few seconds. The system is therefore a local one and needs to be redefined, as it is not suitable for higher defense and scientific applications. The system will not be suitable for launching Inter Continental Ballistic Missiles.

The United States Of America Defense Department defined World Geodetic System 84 for global navigation and positioning system to cater to their sophisticated defense needs. This global system is named as NAVSTAR GPS. USA has been spending billions of dollars every year
since seventies and will continue to so. The system was extensively during recent Iran – Iraq war successfully. To define WGS 84 similar sets of assumption were made as for Indian system. WGS 84 is an accurate system as its center is estimated to be only + or – 2 meters away from the center of gravity of the Earth. It will be fair to assume that assumptions in this case are almost met. The system is therefore most suitable for higher defense and scientific applications.

To convert the coordinates from Indian to WGS 84 and vice versa 3 translation parameters, 3 rotation parameters and 1 scale parameter are required which can be determined by observations. It is not easy to
determine reasonably accurate transformation parameters.
N. K. Agrawal, nande@rediffmail.com