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From Fellrnr.com, Running tips
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* It takes time for the GPS watches to acquire the satellites. The watches tended to say they are ready to go before they have an optimal lock. Therefore, to improve accuracy try to give them a little more time. Note that some newer GPS watches such as the [http://www.amazon.com/Garmin-010-01128-30-Forerunner-620-Bundle/dp/B00FBYYC90 Garmin 620] have the ability to be preloaded with the satellite positions, reducing this startup time and start up in accuracy dramatically.
* The GPS watches are not accurate enough to perform any type of [Practical Interval Training| interval training]. Instead, use a track or measure out the distance using some other mechanism.
=Even GPS Watches have Bad Days=
While it's tempting to take the various GPS watches on a single run and simply compare the totals, this is a flawed approach. Evaluating the devices GPS accuracy on the basis of a single sample does not tell you much. It's a bit like evaluating an athlete's ability on the basis of one event; everyone has good days and bad days, and that applies to GPS watches as well. To illustrate this, the images below are from two runs, recorded on 9/20 and 9/22. In each run I recorded data on both the 310 and 910 watches, hitting the lap button on both at as close to the same time as is humanly possible. On 9/20 the 910XT was far more accurate than the 310XT, but on 9/22 the situation is reversed. If you were to have evaluated the two watches on the basis of a single run, you would conclude that one is much better than the other. But which device would win would depend on the particular day. This is why I've accumulated a lot of data to do a statistical analysis to work out which is really better.
{| class="wikitable"
|- valign="top"
|[[File:310XT Bad.jpg|none|thumb|x500px| The {{Garmin 310XT}} having a bad day. You can see on the upper half of the course where it got a little confused and off track. ]]
|[[File:910XT Good.jpg|none|thumb|x500px|The {{Garmin 910XT}} on the same run having no problems, and only the standard, expected level of inaccuracy.]]
|- valign="top"
|[[File:310XT Good.jpg|none|thumb|x500px|Two days later and it's the turn of the {{Garmin 310XT}} to have a good day.]]
|[[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.]]
|}
=Next Steps=
* Evaluate how a [[Footpod]] influences accuracy. Most of my runs are with a Footpod, and this may improve the accuracy if GPS is lost.
* Write up general GPS accuracy and talk about WAAS.
* Check if the power lines have any impact of accuracy.
* List the sources of potential error.
* Evaluate the accuracy of the [[Footpod]] alone, both trueness and precision.
