[Taxacom] GPS units that record the time a waypoint was taken, accuracy, datum

Arthur Chapman taxacom3 at achapman.org
Mon Mar 23 21:51:29 CDT 2009

Here are some references worth looking at for Position shifts for 
different Geodetic Datums


Also see the table on page 24 of the Biogeomancer Guide



Bob Mesibov wrote:
> Hi, Karl.
> Yes, the datum can matter. Here in Australia the shift from AGD66 to
> GDA94 a few years ago was ca. 200m, which is more than 10x the accuracy
> now expected of hand-held GPS units.
> When I was getting initial feedback on my website on spatial data basics
> http://www.utas.edu.au/spatial/locations/index.html
> in 2005 I found to my amazement that some recorders thought the datum
> shift only applied to UTM coordinates, and that lat/long was unaffected.
> Another problem has been that many recorders use the printed,
> UTM-gridded 1:25000 map series in Tasmania. This older series is based
> on AGD66, hence is ca. 200m off the UTM reading for the same spot on a
> GPS unit. I'm sometimes asked, 'Which is right?'
> I agree with Finn that databases and publications should include datums
> with records. See also the GBIF publication on best practice in
> georeferencing:
> http://www.gbif.org/prog/digit/Georeferencing
> It's also interesting to see published location data given to so many
> decimal places that the bird/tree/nest has apparently been located to
> the nearest 1 mm, which is what happens when you mindlessly transcribe
> what appears on the GPS screen.

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