[Taxacom] Inappropriate accuracy of locality data

Patrick Alexander paalexan at nmsu.edu
Sat Dec 4 17:56:45 CST 2010

Bob Mesibov wrote:
> Apologies for the long post.
> Dusty wrote:
> "Just to make sure I understand this.."
> Well, you don't, so I'll spell it out again. A position declared by a GPS unit is a measurement. Like any measurement, it's subject to error. For a typical handheld GPS unit that error is minimally what the manufacturer says it is, which in the case of the Garmin eTrex is 15 m RMS. That means (approximately) that 2/3rds of the time the declared position will be within 15 m of the true position, and 1/3rd of the time the declared position will be more than 15 m from the true position. Under non-ideal conditions, the error will be larger.

This is not correct.  I happen to have a Garmin eTrex H handy, and the 
manual says:  "GPS Accuracy:  <10 meters (33 ft) RMS".  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.

Second, as Dusty pointed out, the eTrex (and all other modern GPS units, 
as far as I am aware) will estimate accuracy when in use; it seems 
obvious to me that an estimate of accuracy that includes information 
about current signal strength &c. is to be preferred over one that does 
not.  If the GPS unit in hand says that its present accuracy is 13 ft, 
74 ft, &c., I see no reason to replace that estimate with the user 
manual's reported typical accuracy.  If you're feeling paranoid, then 
test it as Dusty suggested; if in checking the GPS's reported position & 
error you find that the actual position is consistently within the 
reported position & error (which has been my experience with the eTrex 
H), then leave it be and don't worry about second-guessing it.

As another lone datum on that point, my eTrex H placed at the corner of 
my apartment a few minutes ago gave me a position in decimal degrees to 
five decimal points, and estimated error to be 11 ft.  Compared to the 
aerial imagery on Google Maps, the position reported by the GPS was 
about 5 feet off, so the GPS's reported position & error are just fine.  
What would I gain by rounding to four decimal points (with an implied 
error of about 10 m), reporting an error of 15 m, or both?  Why report 
an error that is greater than that reported by the instrument? 

Patrick Alexander

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