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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 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 An infographic of the accuracy of the GPS running watches.The top right corner represents the most accurate watches. (This graphic uses ISO 5725 terminology.)]]The table below is a vastly simplified summary of the results, where a '10' would be a perfect device. (For an explanation of the ISO 5725 terms 'trueness', 'precision' and 'accuracy', see below.)
{{:GPS Accuracy-summary}}
=Methodology=
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}}
==Progress of newer watches==
I expected GPS watches to improve with time, but the opposite appears to be happening. With the Garmin devices especially, you can see that the older watches generally do far better than the newer ones. I suspect this is due to compromises to get better battery life and smaller packaging and the cost of GPS accuracy.
==Interpretation and Conclusions==
What do these statistics mean? This is my interpretation:
* The [[Footpod]] improves the accuracy of the 310XT.
** Note that I'm intentionally using an uncalibrated Footpod (factor = 1.000) to gather data for a comparison of Foodpod and GPS.
* The Garmin 205 does remarkably well, especially for an older device.
=Recommendations=
Here are some recommendations for GPS watches.
