[Taxacom] Inappropriate accuracy of locality data

Bob Mesibov mesibov at southcom.com.au
Sat Dec 4 22:10:14 CST 2010

Hi, Patrick.

"A quick check reveals that among current eTrex models, Garmin is in all cases giving accuracy as either "GPS Accuracy: <10 meters (33 ft) RMS" or "GPS: <10 meters (33 feet) 95% typical", or, in more straightforward language, "usually less than 10 m"."

Please don't quote selectively. <10 m, 95% typical applies 'with DGPS corrections' (eTrex H manual), which very few fieldworkers can apply. The manufacturer is claiming 10 m RMS under favourable conditions, which means that weather, terrain and satellite access, satellite geometry problems can push this 68%-believable threshold out past 10 m. Your  word 'usually' conflates two separate issues: (1) 'usually' meaning 68% of readings under favourable conditions, and (2) 'usually' under most field conditions. Experienced users will make a judgement on both.

"In the meantime, if the eTrex H I have in hand tells me that the its present measurement is accurate within 11 feet I see no reason for me to dispute that, regardless of the various rigors of field work in Tasmania."

Then you don't understand what your GPS is telling you. The accuracy reading on your Garmin is an EPE, or Estimated Position Error. For most Garmin GPS units, the EPE is a 50% threshold: there is an equal chance that the error is greater or less than the declared accuracy. This is even worse than 68/32 RMS. Again: the 'accuracy' declared by your GPS after locking onto satellites is not a circle within which the true position lies. It is a likelihood cut-off. If you want 95% certainty, double the RMS.
Dr Robert Mesibov
Honorary Research Associate
Queen Victoria Museum and Art Gallery, and
School of Zoology, University of Tasmania
Home contact: PO Box 101, Penguin, Tasmania, Australia 7316
Ph: (03) 64371195; 61 3 64371195
Webpage: http://www.qvmag.tas.gov.au/?articleID=570

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