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.
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