Re: flare/mapblast/gps

From: Chris Olsson (olsson@globalnet.co.uk)
Date: Sun Sep 02 2001 - 09:39:41 EDT


There is an excellent web-page which describes and quantifies the effect of
averaging GPS data:
http://users.erols.com/dlwilson/gpsavg.htm

Whatever precision is obtained, I would urge anyone who lists their terrestrial
co-ordinates to also mention the geodetic basis of those co-ords.

For celestial co-ordinates, there is only one reference point.  It is
sufficient to list an RA/Dec without declaring any reference.

Not so for terrestrial co-ordinates. A Latitude and Longitude only has meaning
if it is referred to some particular spheroid and geodetic datum.  There are
dozens of spheroids in use around the world and well over a hundred different
datums.  Apparently precise co-ordinates are quite meaningless if the geodetic
basis is missing because different spheroids/datums can produce markedly
different co-ordinates for the same point in space.  The difference between
various geodetic datums is frequently several hundreds of metres and should not
be ignored when listing ground positions to the IAU/Cospar conventional
precisions of 0.0001 or 0.1" of Latitude and Longitude..

If starting from scratch, WGS84 is as good a spheroid and datum as any, so long
as that fact is declared.  If using "geodetic maps" then a careful note should
be made of the spheroid and datum to which that map refers.  Not all countries
have a single national mapping datum.  Some countries, especially those which
have been occupied by invading armies during the past couple of centuries, have
two or more datums, usually quite incompatible with eachother.  For such
co-ordinates to make any sense, it is necessary to mention which spheroid/datum
is used in their production.


Cheers,       Chris Olsson
57 02' 30.9"N   3 10' 25.9"W  314m (WGS84)

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