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From Fellrnr.com, Running tips
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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).]]
=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 (see NoEPO row below) and with a Footpod (+FP row below for details).
{{:GPS Accuracy-g620}}
I have come to suspect that the 620 has two issues.
=Next Steps=
This is an initial analysis of the data I have, and there are a number of further evaluations to do.
* 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.
* Write up general GPS accuracy.
