The old Indian Datum
Under the new Map Policy, it seems that India has decided to retain the old Indian Datum, which has been identified by “Everest”, for DSMs. The following geodetic definition issues and “specifi cations” are worth commenting:
Vintage – 1880s.
Name – On a recent enquiry, four SOI experts provided four different names. However, it cannot be “Everest”.
Spheroid – Inherited from British times, this nomenclature should have been corrected decades back to “ellipsoid”.
Everest Ellipsoid – The semi-major axis “a” was defined originally by the Indian Yard. After the last calibration with the International Meter, the 1956 conversion factor has significantly changed the “scale” in meters.
Longitude Definition – The original zero definition was changed by applying a “correction” in 1905. Even with this correction, it is obvious that the Indian Datum has a significant defi ning bias with respect to the latest IERS realized zero longitude.
Multiple Datums – With the changing semi-major axis in meter, the Everest Ellipsoid has changed within India and also in other countries, which still use it to defi ne their datums.
It is pertinent to note that there are many versions of the old Indian Datum, e.g., Indian 1916, 1954, 1960, and 1975. And, a few more versions exist due to arbitrary or unintentional modifications of the semi-major axis in many other countries, hidden within the classified records. In one case, nobody has any information who defi ned it, but one country is using it as its national datum; in another, the starting scale defi nition is unknown.
Accuracy – The accuracy is no match to the currently achievable level(s). A new adjust or transformation will not make any improvement.