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

Frederick W Schueler bckcdb at istar.ca
Mon Mar 23 09:52:16 CDT 2009

Curtis Clark wrote:
> On 2009-03-23 03:03, Karl Magnacca wrote:
>> 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.
> I guess I'm having a bit of trouble understanding that. To me, an 
> extreme analogy is not knowing whether your measuring stick is graduated 
> in inches or centimeters: the conversion is indeed precise, but not 
> knowing which you started with is problematic. 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?

* no this is more like a measuring stick that's calibrated in both 
inches and centimetres, and the waypoint is like a mark made on the 
stick to record a measurement. You can read inches or cm off one side of 
the stick or the other -- the GPS software does the conversion between 
datums, and Karl's question was how much of an error was introduced by 
switching from co-ordinates recorded in one datum to another. This is 
like rounding error in a calculation, and depends on the software.

The big problem is authors (and editors!) who don't know about datums, 
and submit mss with co-ordinates to ever-so-many decimal places, without 
mentioning which datum they're based on.

             Bishops Mills Natural History Centre
           Frederick W. Schueler & Aleta Karstad
        RR#2 Bishops Mills, Ontario, Canada K0G 1T0
     on the Smiths Falls Limestone Plain 44* 52'N 75* 42'W
       (613)258-3107 <bckcdb at istar.ca> http://pinicola.ca

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