[Taxacom] GPS units that record the time a waypoint was taken, accuracy, datum
taxacom3 at achapman.org
Mon Mar 23 18:19:14 CDT 2009
Whatever is done, it is important to record the accuracy/uncertainty
(not just the precision of the instrument which is often wrongly called
accuracy) and the Geodetic Datum.
I refer you again to the document edited by John Wieczorek and myself on
georeferencing which makes a number of recommendations on what and how
to record this information. Also there is some discussion on Geodetic
datums - it would appear that the maximum variation between datums is
around 3.5 km, but that is the exception - more regularly is ranges from
zero (or nearly so - centimeters at least) to 200-300 meters. The
document is available from the GBIF web site.
Chapman, A.D. and Wieczorek, J. (eds). (2006). /Guide to Best Practices
for Georeferencing/. BioGeomancer Consortium. Copenhagen: Global
Biodiversity Information Facility. 90pp. ISBN: 87-92020-00-3.
Karl Magnacca wrote:
> On Mon, March 23, 2009 2:35 pm, Curtis Clark wrote:
>> If your unit records in datum A, but you assume in all further work
>> with the coordinates that they were recorded instead in datum B,
>> wouldn't your results always be off?
> Only if you're hand-writing the coordinates; otherwise, the unit
> displays and downloads all points in whatever datum it's currently set
> for (at least mine does, I assume that they all do). Obviously there
> are still ways to screw it up (e.g. by not paying attention to what
> datum is set when you're downloading different batches of points, or by
> thinking you're using a different datum from what you are), but the
> point was that the datum that's set when you actually push the button to
> mark a waypoint isn't important.
> Karl Magnacca
> Postdoctoral Researcher
> Department of Zoology
> Trinity College, Dublin 2
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