[Taxacom] Inappropriate accuracy of locality data

Bob Mesibov mesibov at southcom.com.au
Sat Dec 4 19:02:53 CST 2010

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.

'it seems obvious to me...Why report an error that is greater than that reported by the instrument?'. Please learn more about GPS errors. They arise from atmospheric conditions (signal slowdown), multipath tracking (bounces), ephemeris (timing) errors and receiver noise. The accuracy declaration by a GPS unit in your hand cannot account for all these sources of error, and does *not* represent the diameter of a circle within which the true location lies. What the GPS accuracy declaration can do is indicate that one reading ('3 m') is better than another ('10 m').

"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."

I'm glad you're Google Maps images are so nicely georegistered. By GPS'ing landmarks, I find that Google satellite images in my field area in NW Tasmania are as much as 20 m out of register with the ground. I'm also glad that both you and Dusty do your fieldwork around mailboxes and apartments in town. Mine's down in more challenging conditions of topography, access to satellites. I sometimes have to wait 5 minutes to get a GPS reading under forest cover, and with a large accuracy declaration. Sometimes the unit says 'Weak signal. Are you indoors?'
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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