[Taxacom] Inappropriate accuracy of locality data

Patrick Alexander paalexan at nmsu.edu
Sat Dec 4 20:26:09 CST 2010

Bob Mesibov wrote:
> Patrick Alexander wrote:
> "The claim that a Garmin eTrex has a minimum error of 15 m is in direct contradiction to Garmin's claims. Ignoring that 15 and 10 are different numbers, it is not clear to me why exactly we should interpret a number reported as a maximum to instead be a minimum."
> From my Garmin eTrex manual: 'Position Accuracy: 15 meters (49 ft) RMS.' RMS is *not* a maximum. RMS (root mean squared) means that positions indicated by my GPS receiver will be within 15 m of the actual position about 68% of the time. The other 32% of the time, positions may not be within 15 m of the actual value.

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".  Your suggestion that a Garmin eTrex 
has a minimum error of 15 m remains in contradiction to Garmin's claims, 
at least for current models. 

It seems appropriate to repeat, with slight modification, an earlier 

"If you want to argue that an ordinary handheld GPS unit is less 
accurate and has more uncertainty than the manufacturer and all the GPS 
user guides are claiming, then I suggest you test that idea in a range 
of situations and publish your results, so that the GPS manuals and GPS 
user guides can be revised."

If the accuracy reported by Garmin's GPS units is incorrect, you may 
want to take that up with Garmin.  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.

Patrick Alexander

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