I’ve been reading about stories of GPS inaccuracies on Strava, and this post by Suffolk cyclist got me thinking. How could two riders, ascending a hill together end up recording quite different times?
I decided that some more experimentation was required. Evidence is a wonderful thing, after all, except if you’re a creationist.
There may be variation in the way different GPS units record tracks,which results in spurious timings when the data is uploaded to sites such as Strava, which allows the comparison of specific segments of a route. This would essentially undermine the whole principle of Strava leaderboards and make the coveted KOMs rather meaningless.
To determine if there is consistency between different GPS tracking devices.
To determine the reliability of segment timings as calculated by Strava.
One Sunday club run with the fabulous Rhyl Cycling Club.
4 GPS equipped riders, as follows:
|Paul, iPhone 4S, strava app.|
|Tim, Garmin 500|
| Craig, Strava app on
|Me, Garmin edge 800, plus (for this test only) android app MyTracks running on
Samsung Galaxy S2
A nice hill called Glascoed road, which is exactly 2 miles long, and gains 405ft, average gradient 3.7%. The strava segment is called Craig’s Favourite.
Yes, named after the Craig in our group, as he doesn’t like that particular hill very much.
For the test hill, the gps riders stayed together in a tight group, so that their segment times should all be within a few seconds of each other.
One rider (myself) ran two GPS logs – one from a Garmin 800, and one from an android phone app.
The data was uploaded to Strava and compared.
iPhone 4S (running Strava app)
Strangely, although the ride did register many of the segments on the route, this particular segment did not register. On closer inspection, the actual track is all over the place, and I thought the issue was so significant that it deserved its own post.
HTC Sensation (running Strava app)
This phone crashed at some point in the day, and the data was lost.
Galaxy S2 (running Google App MyTracks)
This phone/app combo produced a very accurate track which matched the Garmin Edge 800 exactly.
The two Garmin devices in the test both performed impeccably, creating accurate tracks.
Here is the data we did manage to record and review.
|MyTracks app on Galaxy S2||13:09||Alan|
|Strava app on HTC Sensation||n/a||Craig|
|Strava app on iPhone||n/a||Paul|
Paul contacted Strava about the missing segment, and somehow they tweaked it so that it did register, 36 hours later. The time it gave was 13:06, the same as Tim and his Garmin 500. Paul and I did the exact same ride as we live on the same street. Over 58 miles, it was a surprise to discover that the elevation data were very close – only 31 metres difference, in over 1200m of climbing.
I’ve left the iPhone timing data out of the table because it shouldn’t be necessary to have to email Strava to put your data right. The quality of the iPhone track in this experiment was very poor. See the screenshots.
The Garmin 800 and MyTracks andoid app which were on the same rider came out with identical scores.
The Garmin 500 recorded 3 seconds faster, as did the adjusted iPhone data, but there may have been genuine slight variation as it is impossible to know exactly where the segment begins and ends. A more scientific approach would be for one rider to carry all the GPS units. But this was a sunday club ride, not a laboratory controlled experiment, so take it with a pinch of salt.
We didn’t witness any wild variation in timings, as others have reported. It would appear that there is consistency between the Garmin 500 and 800, and it was very reassuring to see the Garmin 800 and the MyTracks app (both carried by the same rider) produce exactly the same times for the segment.
It would be fair to say, from the evidence gathered, that the quality of the iPhone track was inconsistent. It was so poor at times that the recorded track departed significantly from the actual road, which caused the segment to not be recognised by Strava. In contrast, the Garmin tracks from the 500 and the 800 were consistently following the road with no deviation at any time.
I wonder how many iPhone users have stolen KOMs from people simply because their hardware does not track adequately?