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From Fellrnr.com, Running tips
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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.
[[File:Course Overview.jpg|none|thumb|500px|This is the course I use to evaluate the accuracy of [[Best Running Watch| GPS Running Watches]].]]
To prevent startup problems each device was turned on, satellites acquired, and then left for around 5 minutes before the run starts. This is to counter the problem of a device claiming to have acquired the satellites when it only has a minimal lock. The watches are worn on the left wrist or held in the left hand in roughly the same orientation as it would be on the left wrist. (The iPhone was also on the left wrist.)
=Accuracy, Trueness and Precision=
For this evaluation I'll use the definition of [http://en.wikipedia.org/wiki/Accuracy_and_precision Accuracy as the combination of trueness and precision]:
* Gather data from some other [[Best Running Watch| GPS Running Watches]].
* Add in more graphs of the distribution of accuracy, and possibly a [http://en.wikipedia.org/wiki/Q%E2%80%93Q_plot Q-Q plot] (which shows reasonably [http://en.wikipedia.org/wiki/Normal_distribution normal distribution]).
* Check how GPS accuracy changes over the course of a run, as I've seen a distinct tendency for the watches to say they are good to go when they don't really have an optimal lock on the satellites. I wait for 5+ minutes between the watches saying they have sufficient satellites locked in, so this should not be a problem with the data shown here, but I could do some tests where I turn on the watch from a cold state, then start running as soon as they claim they have a lock.
* Look at how accurate the GPS watches are for measuring elevation, and compare with barometric data.
* Check for any correlation between accuracy and running pace.
