[Taxacom] Inappropriate accuracy of locality data

fautin at ku.edu fautin at ku.edu
Tue Nov 30 11:11:09 CST 2010

Dear Colleagues,

Two threads on Taxacom that may seem unrelated concern, to my mind, pretty 
much the same things -- accuracy and precision.  We have had several posts 
about accuracy and precision in locality data (being a marine scientist, I 
am aware that most places are defined not just by lat/long but also by 
depth -- and then, of course, in 4D there is time).  The same applies to 
identifications -- taxon names may differ in precision and accuracy.

Not all data are equally reliable.  Fitness for use is one of the 
judgments we experts must apply in determining which data to use.  (In my 
own database, an example I cite is that an occurrence recorded only as 
"Australia" -- which is presumably accurate but is very imprecise -- is 
usable if one is assembling a biota of Australia, but is not if one is 
assembling a biota of New South Wales: same datum, fit to use or not 
depends on circumstance.)

It seems to me that concerns about fitness for use did not provoke the 
sorts of reactions we are seeing about electronic data when data were 
available only in hard copy.  Electronic data are much more readily 
accessible than the same data in print; because those assembled by 
aggregators may be several steps removed from their source, their fitness 
may be difficult to judge.  One of our jobs in this brave new world is to 
apply our knowledge to the mass of data out there.  In the days of few 
alternatives to print outlets, editors and peer reviewers helped winnow 
the mass of data potentially available, at the cost of some data not 
seeing the light of day, and many publications showing only derived 
products, not the underlying data (so we could not really judge the 
validity of an author's conclusions).  In the spirit of free access, now 
we are overwhelmed by data, good and bad.  We must understand which among 
them have the accuracy and precision appropriate to the purposes to which 
we want to put them.

Daphne G. Fautin
Professor, Ecology and Evolutionary Biology
Curator, Natural History Museum and Biodiversity Research Center
Haworth Hall
University of Kansas
1200 Sunnyside Avenue
Lawrence, Kansas 66045-7534  USA

telephone 1-785-864-3062
fax 1-785-864-5321
evo user name fautin
website www.nhm.ku.edu/~inverts

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On Mon, 29 Nov 2010, Patrick Alexander wrote:

> I agree in general principle, but there are some minor errors here.
> Tom Schweich wrote:
>> I use a Garmin 76csx for location data collection.  It's native
>> geographic system is WGS 1984 and it collects in decimal degrees.   I
>> suspect this is true of nearly all common GPS receivers we use in the
>> sciences.   Yes, the Garmin 76 can report location in
>> degrees-minutes-seconds, or in meters using a UTM system.  However, all
>> of those other numbers are calculated from the GPS' native coordinates
>> via algorithms that may be simplified and give less accurate results
>> than the native system.  You're adding error to your data.    Secondly,
>> if some wants to use your data in a different system it will now have
>> been transformed twice: once by your GPS and once by your user.  Each
>> transformation has the potential of adding error.
> Translating from degrees-minutes-seconds to decimal degrees isn't a
> lossy approximation, it's just simple arithmetic.  For instance, the
> formula to get from DMS to DD is just D + M/60 + S/3600.  If you don't
> trust your GPS with such simple math, you shouldn't be trusting it to
> give you a location in the first place!
> Translating to and from UTM, on the other hand, is a more complicated
> procedure.  I don't know if it is reasonable to expect error to be added
> by the process, but it seems plausible.
>> Always keep the datum with the original collection data.  In my case,
>> it's WGS 1984.   In North America, some like to use NAD 1983.  They're
>> both good systems.  At Mono Lake, though, the same coordinates in WGS
>> 1984 and NAD 1983 are 86 meters apart.  If the location is simply "Mono
>> Lake," then it's no big deal.  However, if the location is the highly
>> faulted terrain south of the lake, the 86 meter difference could easily
>> put you in the wrong drainage.
> Specifying the datum is certainly an important point that is often
> missed; most herbarium specimens I have seen that include UTM or
> lat/long coordinates do not specify the datum and have relatively high
> uncertainty as a result.  However, NAD83 & WGS84 are for most purposes
> interchangeable.  The difference between the two is on the order of a
> meter at most, and most of us aren't using GPS receivers that are
> accurate enough to ever tell the difference.  The problem in the US is
> typically whether NAD 27 or NAD83/WGS84 is being used.  The difference
> here is often on the order of 86 meters.
> Patrick Alexander
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