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

Bob Mesibov mesibov at southcom.com.au
Fri Mar 20 16:38:28 CDT 2009


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.
-- 
Dr Robert Mesibov
Honorary Research Associate
Queen Victoria Museum and Art Gallery
and School of Zoology, University of Tasmania
Home contact: PO Box 101, Penguin, Tasmania, Australia 7316
Ph (03) 64371195; 61 3 64371195
Webpage: http://www.qvmag.tas.gov.au/mesibov.html





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