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) ----------------------------------------------------------------- Unsubscribe from SeeSat-L by sending a message with 'unsubscribe' in the SUBJECT to SeeSat-L-request@lists.satellite.eu.org http://www2.satellite.eu.org/sat/seesat/seesatindex.html
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