I’m curious what other riders experience when comparing mountain bike distance traveled for a GPS app (like MapMyRide or Strava) vs. a cycle computer. For me, I’ve found that for road biking the difference is negligible – it seems that GPS is quite accurate. However, for mountain bike riding, I typically see that GPS short changes the distance anywhere between 10 and 20%. I’m confident I’ve got my two cycle computers calibrated to correct wheel circumference, as I don’t see the same gap if I ride the same bikes on the road or bike path.
Not sure if this is due to the short dips and roles found on the average off-road path, or perhaps trees and other obstacles are causing momentary miscalcs for the GPS, or some other reason. So I’m curious what others are experiencing?
There are a number of factors that introduce errors into distance calculations for off road rides, but it seems the most significant is due to the discreet sampling interval used by GPS units.
Around the same time of the test above I used the cyclocomputer to measure a local trail that’s incredibly dense and curvy. My GPS reports this trail is about a mile long, but the cyclocomputer says it’s 2 miles long. That means the GPS is short changing by 50%! (I know from our track test that the cyclocomputer is calibrated correctly, within 2% or so.)
The takeaway is the GPS will generally read shorter distances than a cyclocomputer, unless you’re riding a straight line. On the road (or so a track) the two will be pretty close. On a spaghetti-like trail, GPS distance can be off by 50% (or maybe more). So it’s not surprising that you’re seeing 10-20% difference on what I assume is a reasonable trail with a normal amount of twists and turns. Every trail is different, so the error rate will not be constant.
Thanks Jeff. I actually found and read this article before my post, but your follow-on statement about your subsequent test on a “dense and curvy” trail really speaks to my question, so thanks! I totally agree it will depend on any particular trail, as opposed to a road or bike path that is wide open with relative “straightness.” Thanks for confirming! Still curious about what kinds of ranges other people are seeing on their trails?
On the trails I ride I typically see a distance variation of about 10-20%. I have also noticed that sometimes the GPS signal can be off based on location and/or time in terms of the trail. This can be particularly noticeable if I do multiple laps of a trail but the lap tracks don’t overlap very well. Keep in mind that aside from the issues that Jeff mentioned the GPS can be hindered by the tree canopy so I attribute some of the variation to a suboptimal signal. Now, if you really want to drive yourself crazy try to figure out how accurate your elevation gain is. Lol.
I don’t use a cyclocomputer anymore, but I do track my rides on Strava. At the places and routes I ride repeatedly a fair amount, I’ve noticed the distances (and elevation gains) can vary quite a bit more at the places that have very twisty and hilly trails versus the flatter trails with few twists and turns. Last week, a regular loop I ride came in at about half a mile less than it usually does…and I added a small section. LOL
It’s also interesting to see the differences in logged data when you ride with someone else who also logs their ride and you both ride the exact same route with no deviation.
I imagine the most precise phone GPS is accurate to about 20 feet or so, so when you’re riding on trails that turn around on themselves in multiple 3-foot-arc hairpin turns, the GPS track is bound to have many inaccuracies.
I use a Garmin GPS watch and I am surprised by some of the weird things that Strava will tell me after the rides.
First – on my ride to work I cross over the highway, which is in a natural depression. It counts my elevation as if I drop 20ft and go right back up. It is all the elevation gain on my ride.
Second – there is a park in a valley with various parallel trails that have been entered into Strava. If I made my rides public, I would have the KOM for a super sketchy eroded decent, with drops and switchbacks. But I am actually riding bike path next to the park but either GPS or margin of error in Strava maps me on to the trails.