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From Fellrnr.com, Running tips
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|[[File:910XT Bad.jpg|none|thumb|x500px|Again, this track is recorded on the same run as the image to the left. The {{Garmin 910XT}} gets a little confused at the start, and then again around lap 27.]]
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=Some Devices Are Better Than Others=
Below is a section of two runs showing the same section of the course, both taken at the same time, one from the Garmin 310XT and the other from the Garmin 620. These give a good indication of the accuracy problems I've seen with the Garmin 620.
{| class="wikitable"
|- valign="top"
|[[File:ExampleGarmin310.jpg|none|thumb|x500px| You can see the GPS tracks (thin red line) are close together and the lap markers (yellow diamonds) are clustered nicely. The blue dots on the GPS tracks are the actual GPS recordings.]]
|[[File:ExampleGarmin620.jpg|none|thumb|x500px|By contrast, the 620 has much wider GPS tracks and dispersed lap markers. ]]
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=GPS Short and long measurements=
As you can see from the images below, the GPS track tends to take shortcuts around bends, reducing the length of the measured track.
[[File:GPS Shortcuts.jpg|none|thumb|500px|The GPS tracks in red with blue dots representing each GPS point recorded. The arrow points to the actual path taken.]]
Often GPS measurements of races, especially marathons record a longer distance than the race. This is partly because the USATF technique for measuring the distance takes a path that is no more than 12 inches away from the tangent (corner), and few runners are able to run that close. In a large marathon you can be forced to take a line that is a long way from the tangent. The other factor is that on a straight line, the GPS error tends to give a slightly longer measurement.
[[File:GPS Marathon.jpg|none|thumb|500px|Here you can see the GPS line is not following the straight road, giving a longer reading on the Thunder Road Marathon. Notice that the GPS is also cutting the corner at the top (we didn't run through the building).]]
=Density Plots=
Below are the density plots of the 'normal' condition for each device.
[[File:GPS Normal.jpeg|none|thumb|1024px|Density plots of the GPS accuracy.]]
=Next Steps=
This is an initial analysis of the data I have, and there are a number of further evaluations to do.
* Gather data from some other [[Best Running Watch| GPS Running Watches]].
* 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.
* Look at how accurate the GPS watches are for measuring elevation, and compare with barometric data.
