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From Fellrnr.com, Running tips
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We can look at trueness by measuring the average lap length and precision by measuring the standard deviation. (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.)
=Accuracy=
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, 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.
{{:GPS Accuracy-statistics}}
==Progress of newer watches==
|[[File:Garmin620 Offset2.jpg|none|thumb|x500px|This is a simple out and back run of ~3 miles/5 Km, but you can see after the turn around the Garmin 620 records a gradually widening gap, even though it follows the right overall shape. (The outbound track is fairly accurate, the return is messed up.)]]
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=GPS Accuracy and Pace=
[[File:AccuracyAndPace.jpg|none|thumb|500px| A plot of GPS precision against pace. The red line is the correlation.]]
There have been reports of GPS accuracy changing with pace, but as you can see from the graph above, my testing does not show this.
=Device Specific Notes=
For those interested in some of the details of how devices are configured for testing, here are some additional notes.
* Check how GPS accuracy changes over the course of a run, as I've seen a distinct tendency for the watches to say they are good to go when they don't really have an optimal lock on the satellites. I wait for 5+ minutes between the watches saying they have sufficient satellites locked in, so this should not be a problem with the data shown here, but I could do some tests where I turn on the watch from a cold state, then start running as soon as they claim they have a lock.
* Look at how accurate the GPS watches are for measuring elevation, and compare with barometric data.
* 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.
* List the sources of potential error.
* Evaluate the accuracy of the [[Footpod]] alone, both trueness and precision.
