[Taxacom] Geodetic datums do matter

Doug Yanega dyanega at ucr.edu
Mon Mar 23 17:47:00 CDT 2009

Fred Schueler wrote:

>* I don't know: certainly there are many situations where 200m doesn't
>matter, but there have been many times when a GPS has led me to the spot
>where a single specimen of a nearly-sessile snail was taken by a
>deceased colleague, or from which declining frogs had been heard
>calling, or where a particular rare plant had been discovered, or a
>colony of an invasive plant had been found. In these cases a 200m error
>or uncertainty would have ruined the replication I was trying to
>achieve. [snip]
>For herpetology, malacology, and invasive plants, I'd say that 10m of
>error is acceptable, but not much more.

Karl Magnacca wrote:

>I agree for many cases in entomology as well; 20m accuracy isn't great,
>but the difference between 20m and 200m error is between being able to
>eyeball the location of a particular patch of plants from the point, and
>blundering around in dense forest for hours trying to find it.

and Erast Parmasto wrote:

>In Estonia,
>we have on collecting labels the data of some very rare fungal species
>with coordinates determined using older types of GPS up to ten years
>ago already. Now these species are protected by law in this country,
>and it is really important where a locality is actually situated. In a
>small forest of one owner of the land plot, or in a neighbouring
forest owned by another man.

I would have hoped it was clear that I was not implying that you or I 
couldn't come up with dozens of scenarios where precision is 
important - but rare plants, rare fungi, rare sessile snails, and 
invasive plants that only occur in a SINGLE PATCH, are all *extremely 
exceptional* cases. There are not going to be very many species on 
this planet that will be found ONLY in an area that is less than 50m 
in diameter, never straying beyond that boundary. Just because there 
are exceptional cases doesn't invalidate the existence of a general 
pattern, and the general pattern is that if you give a biologist a 
GPS reading and let them go in search of an organism, even a 200m 
error is almost never - *in and of itself* - going to make it 
impossible for them to find what they're after, because most 
organisms don't occur as isolated individuals or "particular patches" 
that could never be found unless you go back to the exact square 
meter they were first seen (and most biologists wouldn't give up 
looking after searching only a 20m radius). Certainly, if you happen 
to work on organisms that *are* in that exceptional category, then by 
all means you should take all necessary steps to record your 
geolocations as precisely as possible, but for the vast majority of 
cases, that level of precision is meaningless.


Doug Yanega        Dept. of Entomology         Entomology Research Museum
Univ. of California, Riverside, CA 92521-0314        skype: dyanega
phone: (951) 827-4315 (standard disclaimer: opinions are mine, not UCR's)
   "There are some enterprises in which a careful disorderliness
         is the true method" - Herman Melville, Moby Dick, Chap. 82

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