Wasn't it Peter J Seymour who wrote:
>What I find a bit disconcerting with GPS measurements is the question
>of datums. Different measurements may be relative to different dataums.
>For instance on Google Maps, the Greenwich meridian and 0 degrees
>longitude do not coincide. Has any one found cases of confusion and
>inaccuaracy in practice, such as not correctly locating a grave?
You're very unlikely to find anything other than the WGS 84 datum being
used in any recent software. The GPS satellites themselves use WGS 84,
and so do things like Google Earth and Google Maps. (Although Google did
use the ancient Tokyo Datum for locations in Japan when they first
started those services).
[There have been slight improvements in things like the WGS 84 ellipsoid
and its Earth Gravity Model since WGS 84 came out, but they only make a
difference of a few millimetres to locations on the Earth's surface.
They refinements are mainly useful for extremely accurate positioning of
satellites.]
Before 1960, it wasn't particularly easy to obtain lat/lng coordinates
accurate to within 100 feet, so moving the 0 degree line away from the
historical Prime Meridian with WGS 60 didn't cause many problems.
The differences between WGS 60, WGS 66, WGS 72 and WGS 84 changed
locations by only a few feet in the worst case, so finding a grave that
was marked using one of those datums would be affected more by the GPS
error than by the difference in datum.
Before 1 May 2000, the non-military GPS signals contained a random error
of up to 10 metres anyway, so that would swamp any error due to a datum
difference from records made before May 2000.
--
Mike Williams
Gentleman of Leisure
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