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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 36,500 000 miles/59,600Km and recording over 1425,000 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. However, '''none of the watches have GPS accuracy that is good enough to be used for displaying your current pace'''. For current pace, the only viable option is to use a [[Footpod]], and my [[Best Running Watch| review of running watches]] lists those that can display current pace from a Footpod while still using GPS for your course.
[[File:Accuracy.jpg|none|thumb|600px|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 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}}
The values used are simply 10 minus the value for trueness and standard deviation. The overall is 10 minus the standard deviation from true values.
=Methodology=
''Main article: [[GPS Testing Methodology]]''
* Under normal conditions the GPS accuracy is quite good for most devices.
* The accuracy of a calibrated [[Footpod]] is far better than any GPS device. Without calibration the Footpod is more accurate than any watch currently on the market with the exception of the 310XT/910XT with a Footpod backing up the GPS.
* The [[Polar M400]], [[Garmin Fenix 2]], and [[Garmin 10 ]] are noticeably poorer than the other devices. I found the accuracy of the M400/Fenix2/10 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 Fenix2/10 would have poor accuracy compared with the 610 or 310XT on the same run. * The Fenix2 would repeated loose satellite reception, something I've not seen with any other watch(the M400 has done this once). The statistics do not reflect just how bad the Fenix2 is, as some of the data is too bad to analyze.
* The results of the Garmin 610 & 620 indicate the problems with the 10 are not inherent in a smaller device.
* The improvement in GPS accuracy of the 620 with updated firmware shows just how important the software can be. With the earlier firmware the 620 lost over a mile over a 20 mile run!
* '''The accuracy of all devices is better in a straight line than on curves or bendy twisty routes'''. My course is a tough test for GPS devices with many curves and only a few relatively straight sections. * Not surprisingly, for many devices accuracy drops going under the bridge. However, some devices do great in this section, probably because it's fairly straight.
* More interestingly the trueness just after the bridge is even lower, suggesting that the GPS watches are struggling to reacquire the satellites.
* The turnarounds are even less accurate than going under a bridge, but Power Lines do not seem to impact accuracy noticeably.
* 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 older Garmin 205 does remarkably well.
=Recommendations=
Here are some recommendations for GPS watches.
* Most GPS watches are accurate enough for casual running. However, the M400, Fenix2, and 10 have such serious problems that I would not recommend them even for casual usage.
* The better devices are accurate enough for most runners if their limitations are understood.
* None of the devices were accurate enough for a runner to trust the display of current pace for training or race pacing.
* Garmin devices are set to 'smart recording'. I did try an informal test with the 620 using 1-second recording, but it appeared to make no difference.
* For details of the calibration of the [[Footpod]] see [[GPS Testing Methodology]].
* The Fenix 2 had was tested with and without WAAS support activated, which should provide better accuracy; WAAS helped slightly.
=Next Steps=
This is an initial analysis of the data I have, and there are a number of further evaluations to do.
