Changes

{| class="wikitable"

! Device

| {{Garmin 205}}

| 10.0

| 10.0

|-

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

| 7.4

| 9.0

|-

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

| 5.8

| 7.9

|-

| iPhone 4s

| 4.4

| 7.2

|-

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

|-

| {{Polar RC3 GPS}}

| 5.7

| 6.5

|}

=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.

! Condition

! Device

! Trueness (Average Distance Error)

! 95% within

|-

Normal

| {{Garmin 205}}

| 779

| 0.063 (331.2 Ft/Mile 62.7 m/Km)

|-

| iPhone 4s

| 668

| 0.070 (372.0 Ft/Mile 70.5 m/Km)

|-

| {{Polar RC3 GPS}}

| 983

| 0.082 (434.7 Ft/Mile 82.3 m/Km)

|-

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

|-

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

|-

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

| 514

| 0.059 (311.5 Ft/Mile 59.0 m/Km)

|-

| All

| style="background-color: #FFEB84;"|

|-

| {{Garmin 205}}

| 66

| style="background-color: #FA8671;"|

0.074 (390.2 Ft/Mile 73.9 m/Km)

|-

| iPhone 4s

| 59

| 0.067 (356.1 Ft/Mile 67.4 m/Km)

|-

| {{Polar RC3 GPS}}

| 86

| style="background-color: #F97A6F;"|

0.078 (412.7 Ft/Mile 78.2 m/Km)

|-

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

| 222

| 0.098 (520.0 Ft/Mile 98.5 m/Km)

|-

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

|-

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

| 43

| style="background-color: #F8696B;"|

0.084 (444.5 Ft/Mile 84.2 m/Km)

|-

| All

|-

Post Bridge

| {{Garmin 205}}

| 66

| 0.048 (250.9 Ft/Mile 47.5 m/Km)

|-

| iPhone 4s

| 54

| 0.060 (314.9 Ft/Mile 59.6 m/Km)

|-

| {{Polar RC3 GPS}}

| 84

| 0.100 (526.4 Ft/Mile 99.7 m/Km)

|-

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

| 188

| 0.075 (394.8 Ft/Mile 74.8 m/Km)

|-

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

|-

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

| 40

| 0.043 (226.4 Ft/Mile 42.9 m/Km)

|-

| All

|-

Turn Around

| {{Garmin 205}}

| 72

| 0.052 (273.9 Ft/Mile 51.9 m/Km)

|-

| iPhone 4s

| 62

| 0.089 (471.3 Ft/Mile 89.3 m/Km)

|-

| {{Polar RC3 GPS}}

| 92

| style="background-color: #FA7D6F;"|

0.077 (407.3 Ft/Mile 77.1 m/Km)

|-

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

| 205

| 0.083 (439.2 Ft/Mile 83.2 m/Km)

|-

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

|-

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

| 43

| 0.051 (270.7 Ft/Mile 51.3 m/Km)

|-

| All

|-

Power Line

| {{Garmin 205}}

| 127

| 0.046 (245.1 Ft/Mile 46.4 m/Km)

|-

| iPhone 4s

| 110

| 0.058 (305.3 Ft/Mile 57.8 m/Km)

|-

| {{Polar RC3 GPS}}

| 167

| 0.072 (377.7 Ft/Mile 71.5 m/Km)

|-

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

| 515

| 0.069 (366.5 Ft/Mile 69.4 m/Km)

|-

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

|-

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

| 90

| 0.048 (254.4 Ft/Mile 48.2 m/Km)

|-

| All

|-

Mixed

| {{Garmin 205}}

| 1110

| 0.070 (371.2 Ft/Mile 70.3 m/Km)

|-

| iPhone 4s

| 953

| 0.073 (384.8 Ft/Mile 72.9 m/Km)

|-

| {{Polar RC3 GPS}}

| 1412

| style="background-color: #FDC57D;"|

0.051 (269.9 Ft/Mile 51.1 m/Km)

|-

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

|-

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

| style="background-color: #FFDA81;"|

|-

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

| 730

| 0.071 (377.2 Ft/Mile 71.4 m/Km)

|-

| All

|}

==Accuracy by Device then Condition==

! Device

! Condition

! Trueness (Average Distance Error)

! 95% within

|-

| rowspan="6"|

| Normal

| 779

| 0.063 (331.2 Ft/Mile 62.7 m/Km)

|-

| Under Bridge

| 66

| style="background-color: #FA8571;"|

0.074 (390.2 Ft/Mile 73.9 m/Km)

|-

| Post Bridge

| 66

| 0.048 (250.9 Ft/Mile 47.5 m/Km)

|-

| Turn Around

| 72

| 0.052 (273.9 Ft/Mile 51.9 m/Km)

|-

| Power Line

| 127

| 0.046 (245.1 Ft/Mile 46.4 m/Km)

|-

| Mixed

| 1110

| 0.070 (371.2 Ft/Mile 70.3 m/Km)

|-

| rowspan="6"|

| Normal

| 668

| 0.070 (372.0 Ft/Mile 70.5 m/Km)

|-

| Under Bridge

| 59

| 0.067 (356.1 Ft/Mile 67.4 m/Km)

|-

| Post Bridge

| 54

| 0.060 (314.9 Ft/Mile 59.6 m/Km)

|-

| Turn Around

| 62

| 0.089 (471.3 Ft/Mile 89.3 m/Km)

|-

| Power Line

| 110

| 0.058 (305.3 Ft/Mile 57.8 m/Km)

|-

| Mixed

| 953

| style="background-color: #FFEB84;"|

0.036 (192.4 Ft/Mile 36.4 m/Km)

|-

| rowspan="6"|

| Normal

| 983

| 0.082 (434.7 Ft/Mile 82.3 m/Km)

|-

| Under Bridge

| 86

| style="background-color: #F97A6F;"|

0.078 (412.7 Ft/Mile 78.2 m/Km)

|-

| Post Bridge

| 84

| 0.100 (526.4 Ft/Mile 99.7 m/Km)

|-

| Turn Around

| 92

| style="background-color: #FA7D6F;"|

0.077 (407.3 Ft/Mile 77.1 m/Km)

|-

| Power Line

| 167

| 0.072 (377.7 Ft/Mile 71.5 m/Km)

|-

| Mixed

| 1412

| 0.102 (539.9 Ft/Mile 102.3 m/Km)

|-

| rowspan="6"|

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

| Normal

|-

| Under Bridge

| 222

| 0.098 (520.0 Ft/Mile 98.5 m/Km)

|-

| Post Bridge

| 188

| 0.075 (394.8 Ft/Mile 74.8 m/Km)

|-

| Turn Around

| 205

| style="background-color: #FFDD82;"|

0.042 (219.6 Ft/Mile 41.6 m/Km)

|-

| Power Line

| 515

| 0.069 (366.5 Ft/Mile 69.4 m/Km)

|-

| Mixed

|-

| rowspan="6"|

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

| Normal

|-

| Under Bridge

|-

| Post Bridge

|-

| Turn Around

|-

| Power Line

| style="background-color: #F8E983;"|

|-

| Mixed

| style="background-color: #FFE383;"|

|-

| rowspan="6"|

| Normal

| 514

| 0.059 (311.5 Ft/Mile 59.0 m/Km)

|-

| Under Bridge

| 43

| style="background-color: #F8696B;"|

0.084 (444.5 Ft/Mile 84.2 m/Km)

|-

| Post Bridge

| 40

| 0.043 (226.4 Ft/Mile 42.9 m/Km)

|-

| Turn Around

| 43

| 0.051 (270.7 Ft/Mile 51.3 m/Km)

|-

| Power Line

| 90

| 0.048 (254.4 Ft/Mile 48.2 m/Km)

|-

| Mixed

| 730

| 0.071 (377.2 Ft/Mile 71.4 m/Km)

|}

What do these statistics mean? This is my interpretation:

* Not surprisingly, accuracy drops going under the bridge.

* More interestingly the trueness just after the bridge is even lower, suggesting that the GPS watches are struggling to reacquire the satellites.

** Note that I'm intentionally using an uncalibrated Footpod (factor = 1.000) to gather data for a comparison of Foodpod and GPS.

* The Garmin 205 does remarkably well, especially for an older device.

=Recommendations=

Here are some recommendations for GPS watches.

=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.