Changes

To 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, 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. So far I have recorded 4,658 GPS points, 3,881 using a {{Garmin 310XT}} and 777 using a {{Garmin 910XT}}. This represents 1,164 miles/1,874 Km of running.

What do these statistics mean? Is This is my interpretation:

* Not surprisingly, the trueness drops going under the bridge and is 1.27% (201"/500cm). More interestingly, the accuracy trueness just after the breach is even lower at 2.24% (355"/885cm). This suggests that the GPS watches are struggling to reacquire the satellites.* The turnarounds are even less accurate true than going under a bridge, resulting in an average distance that is out by 4.01% (635"/1,582cm).* The precision of the GPS watches is 3-4%, which means that 95% of samples will fall within 12-15% of the correct distance. For normal laps, that is 95% of laps are within 159'/4.8m per quarter mile.