[Taxacom] Inappropriate accuracy of locality data

Patrick Alexander paalexan at nmsu.edu
Mon Nov 29 14:49:59 CST 2010

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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