Changes

=Methodology=

Simply taking a GPS watch on a single run does not provide sufficient data to reasonably evaluate its accuracy. So to gather the data for this test I marked my usual running route at quarter-mile intervals, using a [http://www.jonescounter.com/ Jones counter] to provide an accurate measure of distance. The Jones counter is the device used to certify courses, and I followed the [http://www.usatf.org/Products-/-Services/Course-Certifications/USATF-Certified-Courses/Certify-Your-Course.aspx USATF course certification process]. The course I run along is a little challenging for GPS, with lots of twists, tree cover, power lines, and one bridge that I go under. The bridge carries a 4 lane road, so it's wide enough to cause the watches to briefly loose GPS signal. However, I believe that it's reasonably representative of real-world conditions, and probably less challenging than running in the city with skyscrapers. At both ends of the course there is a turnaround, and I set the mark an eighth of a mile from the end. That way I can evaluate how well the watches handle an about turn.

[[File:Course Overview.jpg|none|thumb|500px|This is the course I use to evaluate the accuracy of [[Best Running Watch| GPS Running Watches]].]]

=Accuracy, Trueness and Precision=

For this evaluation I'll use the definition of [http://en.wikipedia.org/wiki/Accuracy_and_precision Accuracy as the combination of trueness and precision]:

|}

We can look at trueness by measuring the average lap length and precision by measuring the standard deviation.

=Accuracy=

{| class="wikitable"

! Device

! 95% within

|-

| {{Polar RC3 GPS}}

|-

| {{Garmin 310XT}} with [[Footpod]]

|-

| {{Garmin 310XT}} no [[Footpod]]

|-

| {{Garmin 910XT}} with [[Footpod]]

|-

|-

| {{Polar RC3 GPS}}

|-

| {{Garmin 310XT}} with [[Footpod]]

|-

| {{Garmin 310XT}} no [[Footpod]]

|-

| {{Garmin 910XT}} with [[Footpod]]

|-

|-

| {{Polar RC3 GPS}}

|-

| {{Garmin 310XT}} with [[Footpod]]

|-

| {{Garmin 310XT}} no [[Footpod]]

|-

| {{Garmin 910XT}} with [[Footpod]]

|-

|-

| {{Polar RC3 GPS}}

| 92

|-

| {{Garmin 310XT}} with [[Footpod]]

|-

| {{Garmin 310XT}} no [[Footpod]]

|-

| {{Garmin 910XT}} with [[Footpod]]

|-

|-

| {{Polar RC3 GPS}}

| 167

0.036 (188.8 Ft/Mile 35.8 m/Km)

|-

| {{Garmin 310XT}} with [[Footpod]]

|-

| {{Garmin 310XT}} no [[Footpod]]

|-

| {{Garmin 910XT}} with [[Footpod]]

| 90

| style="background-color: #63BE7B;"|

|-

|}

What do these statistics mean? This is my interpretation:

* The turnarounds are even less accurate than going under a bridge, but Power Lines do not seem to impact accuracy noticeably.

* Without the Footpod, the 310XT has similar problems to the RC3 on the turn around, but the RC3 does quite a bit worse under the bridge.

=Recommendations=

* GPS watches are accurate enough for most general running.

* 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.

|[[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=

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]].

** I will evaluate the [http://www.amazon.com/Garmin-010-01128-30-Forerunner-620-Bundle/dp/B00FBYYC90 Garmin 620] when it becomes available as it has some promising functionality.

* Add in some 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.

* Look at how accurate the GPS watches are for measuring elevation, and compare with barometric data.

* Check for any correlation between accuracy and running pace.

* Write up general GPS accuracy.

* List the sources of potential error.

* Evaluate the accuracy of the [[Footpod]] alone, both trueness and precision.