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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 below for details).
{{:GPS Accuracy-g620}}
I have come to suspect that the 620 has two issues. * Firstly, Garmin has compromised GPS accuracy for size and battery life. This is then compounded by high levels of smoothing in an attempt to compensate. * I'm told believe that the newer Garmin watches have changed from a SiRFstar chipset to a MediaTek chipset. This is backed by the way the newer Garmin watches support the pre-caching of satellite position data using an "EPO.BIN" file that is a MediaTek trademark. I wondered if problems in the EPO ([http://www.gtop-tech.com/LNG/product/AGPS-Assisted-GPS/Software_Services_07.html Extended Prediction Orbit]) data could be causing some of the problems, so I ran a test without any EPO data. To do this, though I did a factory reset and did not connect the watch to the internet while testing. The obvious impact was that the 620 now takes an age to acquire a satellite signal. As you can't confirm thissee from the accuracy above, the 620 appears to be slightly more accurate without EPO data. However, one run I did without EPO data recorded only 19.5 miles on a 21 mile run, and analyzing the file shows a general poor quality rather than a specific section of the run being bad. (If that run is excluded, the 620 without EPO showed Trueness of 2.68% and SD of 5.62%). I thought that the 620 had a problem with its [http://en.wikipedia.org/wiki/Wide_Area_Augmentation_System WAAS] processing, but I have it on good authority that no Garmin Forerunner has WAAS support. Below are a couple of examples where you can see that it appears that the track has been offset, rather than the 620 simply becoming 'lost', which I'd attributed to WAAS error.
{| class="wikitable"
|- valign="top"
|[[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.)]]
|}
=Garmin Fenix 2 Issues=
I've had similar GPS accuracy issues with the Fenix 2. In fact, the Fenix 2 is the only device I've ever had that has given the "lost satellite reception" message on my usual running route. Because of these issues Garmin replaced my Fenix 2 under warranty, and below are the results for the original and new watches.
{{:GPS Accuracy-Fenix2}}
=GPS Accuracy and Pace=
[[File:AccuracyAndPace.jpg|none|thumb|500px| A plot of GPS precision against pace. The red line is the correlation.]]
* Garmin devices are set to 'smart recording'. I did try an informal test with the 620 using 1-second recording, but it appeared to make no difference.
* For details of the calibration of the [[Footpod]] see [[GPS Testing Methodology]].
* The Fenix 2 had WAAS support activated, which should provide better 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]].
* Test Fenix 2 without WAAS.
* Test Fenix 2 and 620 with footpod to see if that helps.
* Test Fenix 2 and 620 to see if there is any variation in accuracy with the satellite pre-cache data.
* Add in more graphs of the distribution of accuracy, and possibly a [http://en.wikipedia.org/wiki/Q%E2%80%93Q_plot Q-Q plot] (which shows reasonably [http://en.wikipedia.org/wiki/Normal_distribution normal distribution]).
* 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.