[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


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


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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