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From Fellrnr.com, Running tips
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I evaluated the real world accuracy of GPS watches while running over 3,500 miles/5,600Km and recording over 14,000 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.
[[File:Accuracy.jpg|none|thumb|600px|A graph of the accuracy of the GPS running watches.]]
The table below is a vastly simplified summary of the results, rating the best device as where a '10 then comparing the others' would be a perfect device. (Note that For an explanation of the ISO 5725 terms '10trueness' does not indicate perfect, just the best I've measured so farprecision' and 'accuracy', see below.)
{{:GPS Accuracy-summary}}
=Methodology=
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 ISO 5725 definition of [http://en.wikipedia.org/wiki/Accuracy_and_precision Accuracy as the combination of trueness and precision]:.
{| class="wikitable"
|- valign="top"
|[[File:High accuracy Low precision.svg|none|thumb|x300px|This shows good trueness, as all the hits are around the center. On average they are on target, but there is poor precision, as the hits are scattered.]]
|}
We can look at trueness by measuring the average lap length and precision by measuring the standard deviation. (It is more common in many fields to use "accuracy" to mean closeness to true value and "validity" to mean the combination of accuracy and precision. However, I feel that the meanings used by ISO 5725 are closer to the common usage.)
=Accuracy=
I've divided the laps into those that go under the bridge, those that are immediately after a lap that goes under the bridge, those that go next to or under the power lines, those that turn around and then all the rest (normal). I've also included a 'mixed' category that reflects all the sections and a category for the straightest segment. The count field is how many measurements I have for that combination of condition and device. I generally aim for over 1,000 data points to even out the effects of weather, satellite position and other factors.
{{:GPS Accuracy-statistics}}
==Interpretation and Conclusions==
