I’ve recently started doing more trail running and my GPS almost always shows the route to be short compared to marked trail distances, measured race courses, etc. On the road I know it usually measures long due to tangents, and margin of error, …although it’s usually within a 1/4 mile over a marathon for me. Do other trail runners on here find this to often be the case? I guess I should note that the trails are very twisty here so perhaps it’s missing sections and cutting switchbacks or something. Today I ran the course for an upcoming race and the distance was suppose to be 18 miles. At the end of the run I was just a hair under 16 miles on the GPS. I’m confident I made all the correct turns and stayed on the singletrack trail but was shocked by the difference. (I was however happy to be back at the car much sooner than I anticipated)
Just looking for some confirmation of this with other trail runners.
Maybe your gps is not accounting for elevation? Try uploading your run to some site (garmin, strava or whatever) to see if then it considers elevation and gives you a more accurate result.
I’m always a bit short with a 310 set on 1s recording. I did a 10k and a few of is were right at 5.9 miles. Uploaded to Strava: bam, 6.2.
My bro has a 610 that reads shorter than mine w/ 1s recording on as well. Switchbacks in hilly redwoods don’t play nice with GPS.
I would love to see the new Fenix2 use the accelerometer to override the GPS data when the signal is weak. Makes it tough if you are looking for a turn at mile 3.5 and you run right past it because you are trusting the watch.
My 910 is always short when I trail run. I have had 50ks come up in the 27-28 mile range. Its just the nature of running on twisty trails in the woods.
Also, trail races are notorious for being poorly measured. Don’t just assume the RD has accurately measured the course with a roller.
+1 on the footpod, especially on trails. And the new Fenix 2 looks very cool – I wonder if they’ll go to half second tracking or if that’s even possible (it probably is).
Smart recording is usually good, but when you have a lot of movement, side to side and turns, it might miss a bit and it will just connect a straight line from your last data point. The 1-second recording should help. There might also be thick cover from all the tree branches and such that breaks up the signal a bit.
Garmin did just come out with the Fenix2 that has a Trail Running category, I just haven’t tested it out yet.
Same here. My Timex GPS watch measure perfectly on the road but when I run on the tight twisty trails near my house I figure it to be .08 off per mile. I got it this fall right before all the snow piled up this winter. When I ran my old established road routes, it was right on. Now that I can get back into the trails it’s off. I checked it with the mountain bike with computer. I think it straightens some of those tight turns.
Thanks for all the feedback. I was able to find some Strava entries from other runners who ran the same route and they were all short as well. I think it has to do with all the turns on the course and the topography rather than it being measured short. (One of the trails was named Small Intestine for a reason) My theory is that it cut off a lot of small switchbacks and also lost signal as I dropped into some gullies or into thick wooded areas and then drew a straight line once it regained a signal. Like it does with tunnels.
I was able to compare some mile markers on the course map with my GPS track showing splits and it was actually almost spot on through the first 5-miles and then it started jumping ahead as I got into the climbs and twisty sections. I did discover that I made one small deviation from the route but it shouldn’t have shortened the route by more than 1/4 mile.
The elevation is probably also a factor. The GPS showed 1900+’ of climbing but the race course data claims only 1500+. Interestingly, the error of more elevation gain should add distance to the route, abet small.
I’m not too worried about it, I was just really curious as I’ve never had my GPS show a known route as short. Thanks again.
The good thing is that it will make me run harder because I don’t want to look so slow!
I’m a decent runner and have been eating a steady diet of humble pie since I started trail running. It’s a totally different animal. I was thinking the same thing when I was running yesterday and looking at my pace, time and distance. On a long easy road run I’ll average 7:45-8:00/mile and yesterday I was just hoping to keep the average under 10:00/mile. I thought I missed that goal at first but I guess I succeeded given the distance error. I didn’t check my current pace too often but when I did, most of the time I kept saying to myself “why are there 4 digits!?” and then I’d try to pick it up.
I’ve never had my GPS show a **known **route as short.
That is the trick. For a certified road race, they use a calibrated bicycle to measure it out. On a trail race, it was likely an average of a few watch files. If you are looking at trailhead signs…those are often the park ranger looking at a topo map and drawing/measuring a line. So, use the tricks mentioned here to get as close as you can, and don’t bother about the rest. The true distance of a covered, hilly, curvy path will always be unknown.
My 910 always seems pretty accurate even in steep, mountainous terrain where I’ll be scrambling and gaining 2-3k feet in a mile. It just messes up the elevation gain.
Doesn’t GPS become less accurate with more turns? I would think that switchbacks and loops throw it off. If the trails are hard, distance is irrelevant anyway.