[Taxacom] Inappropriate accuracy of locality data

Arthur Chapman taxacom3 at achapman.org
Wed Dec 1 16:40:52 CST 2010

Someone has already mentioned that the"accuracy" as reported by a GPS 
being not the reportable accuracy, but the precision of the instrument.

This is indeed correct, and is one thing that creates a lot of confusion 
when I train people in GPS use.

What is reported as 'accuracy' by the instrument is mislabelled and 
should be called 'precision' - or more correctly 'instrument 
precision'.  The instrument may conduct 100 repetitions  and report an 
error of 2m - that is the repeatability, or the error between those 100 
readings by the instrument. That 'accuracy' reported by the GPS of 2m, 
however, may bear no resemblance whatsoever to the accuracy of the 
reading to the actual position on the earth, which may be up to 30m or 
more away. To get a better fix on the real accuracy one could calibrate 
the GPS against a known fixed TRIG point; or use a differential GPS, or 
even use several GPSs and average the results.  For most of our 
biological work, an accuracy of 10-30m or so is suitable.  If you 
regularly need greater accuracy, then you should be using differential 
GPSs (which links to a GPS Base station, and provides a form of 
calibration) or link to a Wide Area Augmentation System (WAAS) as used 
by aircraft landing systems - or  Local Area Augmentation Systems (LAAS).

Somebody mentioned the difference between different DATUMs, and 
especially NAD83 and WGS84.  Indeed - for North America, there is little 
difference between these, however, the further you move away from 
continental North America, the greater the difference.  As far away as 
Hawaii - there may not be a great difference - I've never checked it, 
but NAD was never intended for use so far out into the Pacific. Many 
older maps, and older lats and longs would have been produced prior to 
1983, and in North America probably used NAD27 (North American Datum 
1927), and this can be up to hundreds of meters out (around 480 around 
the Aleutian Islands).  Indeed - on a recent trip to Alaska, I regularly 
found the map used in my GPS showed the road I was on to be consistently 
about 80 meters from my position.  As one reached a town, the maps had 
obviously been updated and I was back on the road.  This is a DATUM 
problem in the maps being used by the GPS company. More and more 
institutions are locking their GPSs to WGS84 or the local equivalent 
(NAD83 in North America; AGD84 in Australia), but I often see people 
using GPSs and have no idea what Datum they are using.  In the Guide to 
Best Practices for Georeferencing we provide a table showing some the 
variations between a number of DATUMs.  We also refer to the MANIS 
Georeferencing Calculator which will calculate the error for any 
position using a large range of different DATUMs.

I hope this helps

Arthur D. Chapman
Toowoomba, Australia.

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