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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. 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]]''
What do these statistics mean? This is my interpretation:
* 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 Garmin 620 and Garmin 10 are noticeably poorer than the other devices. I found the accuracy of the 10/620 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 10/620 would have poor accuracy compared with the 610 or 310XT on the same run. On one run, the 620 lost over a mile over a 28 mile distance. When Garmin replaced my 620, the new watch would lose over a mile on a 16 mile run!
* The results of the Garmin 610 indicate the problems with the 620 & 10 are not inherent in a smaller device.
* For the {{Garmin 610}} there was no difference with and without the Footpod. (Trueness was 3.33%/3.32%, Precision was 3.54%/3.68%, with/without).
* It takes time for the GPS watches to acquire the satellites. Some watches tended to say they are ready to go before they have an optimal lock. Therefore, to improve accuracy try to give them a little more time. Note that some newer GPS watches such as the {{Garmin 620}} have the ability to be preloaded with the satellite positions, reducing this startup time and start up in accuracy dramatically.
=Footpod Accuracy=
The accuracy of a Footpod is far higher than GPS, as well as more consistent and quicker to react to changes in pace. For any given run, the average pace error from the Footpod is only 7 seconds/mile (at a 9:00 min/mile pace) or 5 seconds/Km (at a 5:30 min/Km pace). In practical terms, I've found that I always have to use a Footpod to pace a marathon or for critical speedwork.
=Even GPS Watches have Bad Days=
While it's tempting to take the various GPS watches on a single run and simply compare the totals, this is a flawed approach. Evaluating the devices GPS accuracy on the basis of a single sample does not tell you much. It's a bit like evaluating an athlete's ability on the basis of one event; everyone has good days and bad days, and that applies to GPS watches as well. To illustrate this, the images below are from two runs, recorded on 9/20 and 9/22. In each run I recorded data on both the 310 and 910 watches, hitting the lap button on both at as close to the same time as is humanly possible. On 9/20 the 910XT was far more accurate than the 310XT, but on 9/22 the situation is reversed. If you were to have evaluated the two watches on the basis of a single run, you would conclude that one is much better than the other. But which device would win would depend on the particular day. This is why I've accumulated a lot of data to do a statistical analysis to work out which is really better.
For those interested in some of the details of how devices are configured for testing, here are some additional notes.
* 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]].
=Next Steps=
This is an initial analysis of the data I have, and there are a number of further evaluations to do.
* Look for any correlation between accuracy and the use of [[Heart Rate Monitor]]. The radio signal from the heart rate monitor could interfere with accuracy.
* Write up general GPS accuracy.
