[Taxacom] Accuracy of GPS Receivers, was...

Robert Mesibov mesibov at southcom.com.au
Thu May 9 21:28:21 CDT 2013


GPS manufacturers aren't often clear on this point.

>From that dataset of 50 readings I linked to, you can calculate a mean location - a 'best estimate' by the GPS unit. You can then summarise the distances from this mean location as a measure of the *precision* of the locating process. On average, the readings were 3 m from the mean; the standard deviation was 2 m, the maximum 11 m.

How close was the mean location to the true location? That's the *accuracy* of the averaged GPS readings, and Garmin claimed for that eTrex that 95% of readings would be within 15 m of the true location under best field conditions.

This is a good thing. It means that if I'm getting quick and stable satellite fixes and low 'declared accuracy', I can be reasonably confident that a single GPS reading - taken after patiently waiting for the reading to settle down - is within the '95% circle'.

If I'm deep in a canyon under a tall, dense tree canopy, and even after patiently waiting for 10 minutes my GPS position still isn't stable and the declared accuracy is up around 80 m and I suspect satellite signals are reaching the GPS after bouncing off the canyon walls.... Well, maybe a paper map, or some field notes followed by a look at Google Earth would be a better option than a GPS.

Last year (http://www.pensoft.net/journals/zookeys/article/4195/) I mentioned in a paper the results of checking the location of a permanent survey mark in my neighbourhood. Google Earth put it 2-3 m from the surveyed location, which is pretty impressive georegistration of a satellite image.

I've been using Google Earth more and more in recent years, first to check that a GPS reading is for the site I thought it was, second to get an approximate elevation above sea level for the site, and third to 'displace' the GPS reading when required - like when I couldn't get a reading deep in a forested gully, and had to move 50 m to a nearby clear spot to get a reading. In the latter cases, again, I'm finding the Google Earth position is usually well within that 95%/15m circle advised by the GPS manufacturer.

The biggest variable in my sampling is me. A single collecting site is almost always a patch of bush, in which I take specimens from several scattered spots found while wandering around the patch. The point+radius circle defining that site normally has a radius of 25 m, and often 50 m. My GPS and Google Earth 'errors' are well within that.
-- 
Dr Robert Mesibov
Honorary Research Associate
Queen Victoria Museum and Art Gallery, and
School of Agricultural Science, University of Tasmania
Home contact: PO Box 101, Penguin, Tasmania, Australia 7316
Ph: (03) 64371195; 61 3 64371195




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