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From Fellrnr.com, Running tips
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{{DISPLAYTITLE:GPS Accuracy of Garmin, Polar, and other Running Watches}}<div style="float:right;">__TOC__</div>I evaluated the real world accuracy of GPS watches while running over 23,500 miles/45,000Km 600Km and recording over 1014,000 data points as part of my evaluation of the [[Best Running Watch]]es. Under good conditions most of the watches are remarkably good, but when things get a little tough the differences become more apparent. The table below is a vastly simplified summary of the results, rating the best device as a 10 then comparing the others. (Note that '10' does not indicate perfect, just the best I've measured so far.)
{| class="wikitable"
! Device
! Overall
|-
| {{Garmin 205910XT}}with [[Footpod]]| 7.8
| 10.0
| 10.0
|-
| {{Garmin 910XT205}} with [[Footpod]]| 710.40| 8.87| 9.04
|-
| {{Garmin 310XT}} with [[Footpod]]
| 5.9| 8.3| 8.1|-| iPhone 4s| 4.5
| 8.3
| 7.95
|-
| {{Polar RC3 GPS}}| 5.9| 6.6| 6.8|-| {{Garmin 610}} (preliminary)| 4.01| 108.05
| 7.4
|-
| {{Garmin 310XT}} no [[Footpod]]
| 4.67
| 7.2
| 6.6|-| {{Polar RC3 GPS}}| 5.7| 6.2| 6.59
|-
| {{Garmin 620}}
| 1.9
| 7.63| 45.30
|-
| {{Garmin 10}} (preliminary)| 1.87| 56.17| 34.54
|}
=Methodology=
Simply taking a GPS watch on a single run does not provide sufficient data to reasonably evaluate its accuracy. So to gather the data for this test I marked my usual running route at quarter-mile intervals, using a [http://www.jonescounter.com/ Jones counter] to provide an accurate measure of distance. The Jones counter is the device used to certify courses, and I followed the [http://www.usatf.org/Products-/-Services/Course-Certifications/USATF-Certified-Courses/Certify-Your-Course.aspx USATF course certification process]. The course I run along is a little challenging for GPS, with lots of twists, tree cover, power lines, and one bridge that I go under. The bridge carries a 4 lane road, so it's wide enough to cause the watches to briefly loose GPS signal. However, I believe that it's reasonably representative of real-world conditions, and probably less challenging than running in the city with skyscrapers. At both ends of the course there is a turnaround, and I set the mark an eighth of a mile from the end. That way I can evaluate how well the watches handle an about turn.
What do these statistics mean? This is my interpretation:
* Under normal conditions the GPS accuracy is quite good for most devices.
* The Garmin 620 is and Garmin 10 are noticeably poorer than the other devices and the preliminary results of the Garmin 10 are even worse. I found the accuracy of the 10/620 in general usage to be rather grim, and I did some testing pairing them up with the 610 or the 310XT. In all cases the 10/620 would have poor accuracy compared with the 610 or 310XT on the same run. On one run, the 620 lost over a mile over a 28 mile distance.* The results of the Garmin 610 and Garmin 10 results are preliminary, but I've included them to see if indicate the problems with the Garmin 620 & 10 are not inherent in a smaller device. The reasonable accuracy I've seen with the Garmin 610 indicates that a small device can have good accuracy.
* The accuracy of all devices is better in a straight line than on curves or bendy routes. My course is a tough test for GPS devices with many curves and only a few relatively straight sections.
* Not surprisingly, accuracy drops going under the bridge.
