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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 6,000 miles/9,600Km and recording over 25,000 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|600px800px|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. '''Important''': Manufacturers do not typically release the type of GPS chipset used, so the information in this table is based the best available data, but it should be treated with caution.
=Methodology=
''Main article: [[GPS Testing Methodology]]''
=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. For details of how the Footpod calibration was done, see [[GPS Testing Methodology]].
=Which Chipset? =
While the specific chipset used in a GPS watch will impact its accuracy, there are many other factors that come into play. The physical packaging of the chipset, the antenna used, the particular features that are implemented, and the software that interprets the raw data will influence the overall accuracy. It's important to note that the SiRF chipsets such as "SIRFstarIV" are not a single chipset, but rather an overall architecture with several specific chipsets bearing the same name.
=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.
