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

Karl Magnacca kmagnacca at wesleyan.edu
Mon Mar 23 05:03:54 CDT 2009

On Sat, March 21, 2009 5:30 pm, Richard Arnold wrote:
> For some reason I was unable to respond via taxacom to your post, but
> I second Dave Watts comments that datums definitely do matter.

Sigh...did no one read beyond the first half-sentence of my post?  I'm
well aware that datums matter, my point was that the datum set on the
unit when it's *recording* is of (relatively) little importance.

> Even going from NAD 1927, which is printed on most USGS 7.5' topo maps
> to NAD 1983 (which is printed on newer USGS topo maps along with NAD
> 1927) can cause a shift of several to a few hundred meters depending
> upon where you are in the US.  Pull out a USGS maps and plot the same
> pair of coordinates using these different datums to see the positional
> shift for your area.

But the shift between datums isn't the same as the error introduced in
converting between them.  Last night, I took the same point twice, once
natively in WGS84, and the second time in NAD27-Caribbean (since this is
in Ireland, presumably there is a considerable shift).  Once back in
WGS84, the first showed up as 53.31820N, 6.26460W and the second as
53.31820N, 6.26463W, which is a difference of <2m at a time when the GPS
showed an error of 5.8m.  After resetting to WGS84 the display showed
53.31822N, 6.26462W, so the difference could easily have been entirely
due to GPS error.

Karl Magnacca
Postdoctoral Researcher
Department of Zoology
Trinity College, Dublin 2

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