GPS Accuracy
I evaluated the real world accuracy of GPS watches while running over 1,500 miles/2,800Km and recording over 7,000 data points. Under good conditions the watches are all remarkably good, but when things get a little tough the differences become more apparent.
Contents
1 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 Jones counter to provide an accurate measure of distance. The Jones counter is the device used to certify courses, and I followed the 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.
2 Accuracy, Trueness and Precision
For this evaluation I'll use the definition of 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.
3 Accuracy
I've divided the laps into those that go under the bridge, those that are immediately after a lap that goes under the bridge, those that go next to or under the power lines, those that turn around and then all the rest (normal).
| Device | Condition | Count | Trueness (Average) | Precision (Standard Deviation) | 95% within |
|---|---|---|---|---|---|
| Polar RC3 GPS |
Normal |
897 |
0.994% (33.1 Ft/Mile 6.3 m/Km) |
0.037 (195.6 Ft/Mile 37.0 m/Km) |
0.074 (391.2 Ft/Mile 74.1 m/Km) |
| Garmin 310XT with Footpod | 2453 |
1.000% (1.9 Ft/Mile 0.4 m/Km) |
0.031 (163.0 Ft/Mile 30.9 m/Km) |
0.062 (326.0 Ft/Mile 61.8 m/Km) | |
| Garmin 310XT no Footpod | 594 |
0.996% (18.8 Ft/Mile 3.6 m/Km) |
0.042 (221.9 Ft/Mile 42.0 m/Km) |
0.084 (443.8 Ft/Mile 84.1 m/Km) | |
| Garmin 910XT with Footpod | 481 |
0.994% (33.7 Ft/Mile 6.4 m/Km) |
0.027 (145.1 Ft/Mile 27.5 m/Km) |
0.055 (290.2 Ft/Mile 55.0 m/Km) | |
| All | 4425 |
0.998% (12.4 Ft/Mile 2.3 m/Km) |
0.034 (178.4 Ft/Mile 33.8 m/Km) |
0.068 (356.7 Ft/Mile 67.6 m/Km) | |
| Polar RC3 GPS |
Under Bridge |
85 |
0.952% (252.8 Ft/Mile 47.9 m/Km) |
0.078 (411.0 Ft/Mile 77.8 m/Km) |
0.156 (821.9 Ft/Mile 155.7 m/Km) |
| Garmin 310XT with Footpod | 212 |
0.985% (81.0 Ft/Mile 15.3 m/Km) |
0.035 (186.7 Ft/Mile 35.4 m/Km) |
0.071 (373.3 Ft/Mile 70.7 m/Km) | |
| Garmin 310XT no Footpod | 58 |
1.018% (96.8 Ft/Mile 18.3 m/Km) |
0.047 (250.3 Ft/Mile 47.4 m/Km) |
0.095 (500.5 Ft/Mile 94.8 m/Km) | |
| Garmin 910XT with Footpod | 42 |
0.998% (8.4 Ft/Mile 1.6 m/Km) |
0.031 (164.8 Ft/Mile 31.2 m/Km) |
0.062 (329.7 Ft/Mile 62.4 m/Km) | |
| All | 397 |
0.984% (84.7 Ft/Mile 16.0 m/Km) |
0.053 (281.5 Ft/Mile 53.3 m/Km) |
0.107 (563.0 Ft/Mile 106.6 m/Km) | |
| Polar RC3 GPS |
Post Bridge |
166 |
0.956% (231.2 Ft/Mile 43.8 m/Km) |
0.066 (348.8 Ft/Mile 66.1 m/Km) |
0.132 (697.6 Ft/Mile 132.1 m/Km) |
| Garmin 310XT with Footpod | 365 |
0.980% (107.0 Ft/Mile 20.3 m/Km) |
0.033 (172.0 Ft/Mile 32.6 m/Km) |
0.065 (344.0 Ft/Mile 65.2 m/Km) | |
| Garmin 310XT no Footpod | 110 |
0.973% (144.3 Ft/Mile 27.3 m/Km) |
0.038 (199.3 Ft/Mile 37.7 m/Km) |
0.075 (398.6 Ft/Mile 75.5 m/Km) | |
| Garmin 910XT with Footpod | 86 |
0.969% (166.3 Ft/Mile 31.5 m/Km) |
0.026 (134.7 Ft/Mile 25.5 m/Km) |
0.051 (269.4 Ft/Mile 51.0 m/Km) | |
| All | 727 |
0.972% (148.1 Ft/Mile 28.0 m/Km) |
0.044 (231.0 Ft/Mile 43.7 m/Km) |
0.087 (462.0 Ft/Mile 87.5 m/Km) | |
| Polar RC3 GPS |
Turn Around |
92 |
0.943% (301.9 Ft/Mile 57.2 m/Km) |
0.078 (411.1 Ft/Mile 77.9 m/Km) |
0.156 (822.2 Ft/Mile 155.7 m/Km) |
| Garmin 310XT with Footpod | 244 |
0.962% (200.2 Ft/Mile 37.9 m/Km) |
0.040 (209.8 Ft/Mile 39.7 m/Km) |
0.079 (419.7 Ft/Mile 79.5 m/Km) | |
| Garmin 310XT no Footpod | 64 |
0.965% (184.7 Ft/Mile 35.0 m/Km) |
0.075 (395.2 Ft/Mile 74.9 m/Km) |
0.150 (790.5 Ft/Mile 149.7 m/Km) | |
| Garmin 910XT with Footpod | 48 |
0.953% (249.8 Ft/Mile 47.3 m/Km) |
0.025 (132.8 Ft/Mile 25.2 m/Km) |
0.050 (265.7 Ft/Mile 50.3 m/Km) | |
| All | 448 |
0.957% (228.2 Ft/Mile 43.2 m/Km) |
0.058 (304.1 Ft/Mile 57.6 m/Km) |
0.115 (608.1 Ft/Mile 115.2 m/Km) | |
| Polar RC3 GPS |
Power Line |
167 |
0.987% (66.4 Ft/Mile 12.6 m/Km) |
0.036 (188.8 Ft/Mile 35.8 m/Km) |
0.072 (377.5 Ft/Mile 71.5 m/Km) |
| Garmin 310XT with Footpod | 457 |
0.994% (29.5 Ft/Mile 5.6 m/Km) |
0.028 (149.5 Ft/Mile 28.3 m/Km) |
0.057 (299.1 Ft/Mile 56.6 m/Km) | |
| Garmin 310XT no Footpod | 115 |
0.988% (60.9 Ft/Mile 11.5 m/Km) |
0.039 (207.4 Ft/Mile 39.3 m/Km) |
0.079 (414.7 Ft/Mile 78.5 m/Km) | |
| Garmin 910XT with Footpod | 90 |
0.984% (83.5 Ft/Mile 15.8 m/Km) |
0.022 (113.8 Ft/Mile 21.6 m/Km) |
0.043 (227.6 Ft/Mile 43.1 m/Km) | |
| All | 829 |
0.991% (47.2 Ft/Mile 8.9 m/Km) |
0.031 (165.2 Ft/Mile 31.3 m/Km) |
'0.063 (330.5 Ft/Mile 62.6 m/Km) ' |
4 Interpretation
What do these statistics mean? This is my interpretation:
- Under normal conditions the GPS accuracy is quite good for all devices, though the 310XT/910XT with a Footpod have a slight advantage.
- Not surprisingly, the 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.
- The turnarounds are even less accurate than going under a bridge, but Power Lines do not seem to impact accuracy noticeably.
- The Footpod improves the accuracy of the 310XT and the 910XT, especially on the turn around and under the bridge.
- 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.
5 Recommendations
This data leads me to a few conclusions.
- GPS watches are accurate enough for most general running.
- Adding a Footpod to the Garmin 310XT and the Garmin 910XT improves their accuracy.
- 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 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 interval training. Instead, use a track or measure out the distance using some other mechanism.
6 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.
 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. |
 The Garmin 910XT on the same run having no problems, and only the standard, expected level of inaccuracy. |
 Two days later and it's the turn of the Garmin 310XT to have a good day. |
 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. |
7 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 GPS Running Watches.
- I will evaluate the Garmin 620 when it becomes available as it has some promising functionality.
- I am gathering data on an older Garmin 205. It's not a watch I'd recommend, but it will be interesting to see if the technology has improved over time.
- I will soon start gathering data on an iPhone 4S to see how it compares with dedicated devices.
- Add in some graphs of the distribution of accuracy, and possibly a Q-Q plot (which shows reasonably 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.
- Look for any correlation between accuracy and the use of heart rate monitor. The radio signal from the heart rate monitor could interfere with accuracy.
- Write up general GPS accuracy.
- List the sources of potential error.
- Evaluate the accuracy of the Footpod alone, both trueness and precision.