The geodetic datum

Print edition : August 26, 2005

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. These GCPs define the "geo-referencing" for the country. For geodetic surveys, an appropriate spheroid for the area of survey (which approximates the curvature and flattening of the earth locally) is chosen. This is a mathematically smooth surface with respect to which all computations of coordinates are carried out. 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. The normal to the spheroid would, therefore, be differently oriented to the plumbline vertical. 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 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). The coordination of geographical data is done by a series of triangulations using the GCPs. 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. The Indian geodetic datum has been constantly improved upon by determining the "deflection of the vertical" more precisely. The datum used currently is called the Revised Everest Spheroid.

Cartography or plane projection of this data as maps involves appropriate choice of projection parameters. 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 Survey of India holds these confidential.

Like the data based on the Everest Spheroid, different geodetic data are in use all over the world and each country, for security reasons, makes an appropriate choice of the datum and projection parameters. Therefore, GCPs based on one set of parameters would be shifted with respect to the Everest Spheroid parameters and the horizontal shift can be several tens of metres to even hundreds of metres. Correspondingly, the positional accuracy would be shifted.

Similarly, GCPs based on Global Positioning System-based coordination, which may have high relative accuracy, will be shifted with respect to the GCPs of the Everest Spheroid because they are based on a different geodetic datum called the World Geodetic System (WGS), which is under constant refinement. The present datum is called WGS-84.

With the removal of selective availability (S/A) scrambling codes - hitherto available only to the United States military - a Precise Positional Survey (PPS) with an accuracy of about 1-10 m on a single receiver GPS and up to 1 mm-1 cm accuracy in `differential' GPS is now possible. But it must be emphasised that all these are on WGS-84 with its overall shift with respect to other geodetic data. There are methods to go from a coordinate set on one datum to a set on another. Therefore, if GCPs on the Everest Spheroid are known to great accuracy across the country, the same on another datum can be mathematically worked out. It is for this reason that GCP data on WGS-84 and the corresponding transformation parameters will continue to be held confidential by the Survey of India. From a national security point of view, therefore, there is no threat if topomaps and digital data of any scale, geo-referenced on WGS-84, are freely available.