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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. (Note that accuracy is the ability to measure distance, not the moment in time position, though the two are obviously related.)
[[File:Accuracy.jpg|none|thumb|800px|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 (average) and precision (standard deviationfrom true). The overall is 10 minus the standard deviation from true valuescombination of trueness and precision. Repeatability is how consistent a watch is in providing the same value for the same course segment. '''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]]''
|[[File:High accuracy Low precision.svg|none|thumb|x200px|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.]]
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We can look at trueness by measuring the average lap length and precision by measuring the standard deviation. I use the traditional approach to standard deviation (variation from mean) as well as a modified approach that uses variation from the true value. (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. If a company sold 'accurate' 12 inch pipes and shipped half of them as 6 inches and half as 18 inches, they would meet the traditional definition of accuracy, but few people would be happy with the product. ) In addition, I calculate a value for "repeatability", which is a measure of how likely a watch is to give the same distance measurement for a specific course. I calculate the standard deviation for each segment of the course, and then take the average. A high repeatability score can mask poor accuracy and can convince users they have a good device.
=Accuracy=
The table below shows summary data for each device. The count field is how many measurements I have for that combination of condition and device, with each measurement being a quarter mile distance. I generally aim for over 1,000 data points to even out the effects of weather, satellite position and other factors. The Trueness is the absolute of the mean, though nearly all watches tend to read short. The standard deviation is provided based on the variance from the mean and the variance from the known true value. The average pace error is shown to give a sense of how much error you're likely to see in the display of current pace. This is an average error not a worst case. The data shown below is a summary the accuracy based on all the sections. If you'd like more detailed information, I've split off the [[Detailed Statistics for GPS Running Watches]] for the results under different conditions.
