[Taxacom] Data quality of aggregated datasets

Doug Yanega dyanega at ucr.edu
Mon May 6 12:28:58 CDT 2013


On 5/6/13 12:10 AM, Quentin Groom wrote:
> Unfortunately, a common problem is that taxonomists fail to record the
> Geodetic System or Datum they are using, which leaves the aggregators to
> guess, sometime wrongly.
> Worst still there have been many occasions where GPS are not set up
> correctly and give the wrong output.
> Before the recent advent of the the GPS I would not guarantee the
> accuracy of any grid reference made in open country.
> This also underlines the importance of collectors/recorders creating an
> accurate site name with which the grid reference can be validated.
> Quentin
>
It's funny how often I hear people make this claim, yet the reality of 
it is absurd in its precision. I once asked a friend of mine - an actual 
world authority on georeferencing - what sort of offset would result for 
a position in the US that was recorded in one map datum and displayed in 
a different one. He said, with ominous tones, "Why, that could easily be 
100 meters error!". I laughed in his face, and offended him terribly. As 
someone who manages a museum full of specimens we are presently 
georeferencing, and as someone who studies mobile organisms, I find 
several things about this absurd: (1) with very, very, very few 
exceptions, one will typically not find a specimen label that can be 
said to be accurate to within 100 meters. That actually includes most 
labels that have GPS readings, since most specimen labels in existence 
are attached to *insect* specimens, and since most entomologists who 
carry GPS units take a single reading for a collecting site but wander 
beyond that exact point, sometimes with a wandering radius approaching a 
kilometer. Accordingly, the intrinsic uncertainty on a legacy 
georeference (generally from 1-5 km) is typically MUCH greater than 
whatever error could possibly result from using the wrong datum. (2) 
More to the point, perhaps, is that prior to the invention of GPS units, 
virtually no specimen labels actually GAVE a latitude/longitude, so the 
odds of a label containing coordinates from any datum other than WGS 84 
are very, very slim. That means that legacy material being georeferenced 
NOW is almost all being given coordinates based on a resource like 
Google Earth, /which uses WGS 84/. So, it doesn't make any difference at 
all whether a data provider says anything about it; an aggregator that 
*assumes* WGS 84 as default is almost never going to be wrong (and even 
if so, the error radius is almost always large enough to encompass 
this). (3) For a mobile organism, if one is using a georeference either 
as an indicator of habitat (i.e., for GIS analyses) or for trying to go 
back to re-collect additional specimens, a 100 meter error is, with 
very, very, very few exceptions, utterly trivial. I have this vision of 
a biologist with a GPS unit, staring intensely at the screen, walking to 
the exact point the database gave them, looking up, and noticing that 
they've just walked off the edge of a cliff, and then gravity takes 
hold, /a la/ Wile E. Coyote. If you're within 100 meters of the spot a 
specimen was collected, any failure to re-collect it is NOT going to be 
because you were looking in the wrong place.

In a practical sense, since the error radius is an essential parameter 
of any legitimate georeferencing effort, and since that radius always 
takes the *maximum* possible value, potential datum error almost never 
would exceed that radius, and can therefore be safely ignored. Yes, I 
know there will be cries of "Heresy!", but of all the myriad things that 
are LEGITIMATE causes for concern when one is producing or using 
georeferenced data, the map datum is the *least* of our worries. Sure, 
it would be NICE if we didn't introduce that sort of error, but it isn't 
worth WORRYING about it - if I'm paying a data entry technician by the 
hour, I'm not going to have them waste a single minute of their time 
dealing with map datum issues as opposed to, say, making sure the label 
doesn't give the wrong county, or have spelling errors. I can't imagine 
ever being persuaded otherwise.

Sincerely,

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




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