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[[File:GPS Shortcuts.jpg|none|thumb|500px|The GPS tracks in red showing the tendency to cut the corners on the curves.]]
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.
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[[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).]]
|[[File:GPS MarketSt.jpg|none|thumb|x300px|Here's another example of running down Market Street in San Francisco, where you can see the errors that would add to the distance. ]]
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=Garmin 620 Issues=
The Garmin 620 has become rather notorious for its poor GPS quality. I raised the issue with Garmin support and they kindly sent me a replacement device, but as you can see below, the replacement is actually worse than my original unit. I've also broken down the readings by firmware version, and you can see some slight improvement going from V2.90 to V3.00, but it's only slight. I also tested the 620 without EPO data (NoEPO row below) and with a Footpod (+FP row below).