Accuracy of different devices when measuring runs?

So I've just been out for a run and recorded it on all the different platforms. My Fitbit charge 2 says 5.88 miles, my garmin 220 says 6.2 miles. On my iPhone 6s Strava says 6.3 miles and MapMyRun says 6.47 miles! Who on earth am I meant to believe??!! ???? I'd hope that my garmin is the most accurate as it's a dedicated GPS tracker but when hubby and I run together his garmin 205 never gives the same distance as mine! Is there anyway of finding out exactly how far a distance is or working out which device is most accurate? Thanks ???? Lucy


  • Hi, Sorry cant give much advice as im trying to sort the same thing out myself! I have my phone with strava on as well as a garmin GPS watch. The difference between them gets bigger throughout the run and by the end on Saturday the difference was over half a mile (Garmin more than Strava). When ive plotted the route on map my run before the run it just about matches up with strava on the phone so im going with that and using the extra 1/2 mile for a cool down as im aiming to hit distances at the moment. Also tempted to suggest my phone is more accurate given how advanced phones are....
  • senidMsenidM ✭✭✭
    Ah,  the problems of the modern runner.

    I believe races are still measured by a bloke walking them with a wheel, so they are accurate. Why not run a measured 10K race, with all your various tech devices and see which is the most accurate, if any.

    and very good luck with that.
  • None of them are entirely accurate, at the end of the day.  You could take them all to a track, run 10 laps and odds are none of them will read 4.0 km.  

    For a start, they're not necessarily getting the same signals - the garmin may use Glonass as well as GPS, and any device may pick up different satellites.  Any reading of position has a margin of error - locating you to within 10m may be accurate enough for knowing where you are in the world, but when you're using those readings to calculate speed and distance the errors add up.  Most devices use some sort of smoothing to eliminate those errors, but garmin and strava process the raw data in different ways.  

    If you've got a traditional bike computer that uses a magnet to count wheel revolutions then that can give you a better idea of distance travelled for comparison, but of course that needs to be set up properly as well.

    Or, you could not worry about it too much - if you stick to the same device you won't go far wrong.  An hour's hard run is a hard run no matter how far it is. 

  • Ive started running quite a bit past whatever distance im aiming for...

    I ran my PB 10k by 3 minutes a few weeks back. According to my fenix 3 i stopped the clock at 10.1k but when it sent to strava it only recorded at 9.9k and missed out on my PB. Don't know how that one happened being as it was my fenix 3 that told strava the distance!
  • Strava reprocesses the raw data that is transferred from other devices and in doing so may calculate a different distance. For display purposes Strava also rounds down the distance to 1 decimal place so 9.99km is rounded down to 9.9km.
  • This is the problem I'm finding - just coming back to running and have an Apple Watch 2 which I thought would be fine, but apparently all GPS watches vary
  • Lucy why would you use so many devices ? Ignore the two that are furthest out and your GPS watch is in the middle.  I find my garmin to be almost bob on for race distances.  Good enough for me. 
  • JT141JT141 ✭✭✭
    Occasionally wear my Garmin FR220 watch on my bike. The bike has a wheel turn bike computer. The Garmin always comes up short, anything 25miles+ can be half a mile less. It's a running watch so not ideal for biking which may account for the difference. Still, even if it's not entirely accurate the Garmin is pretty consistent in it's measurement.
  • MacMac ✭✭✭
    edited August 2017
    I have an FR210 and noticed on a recent run with a friend (who has a FR620) that there was some "drift" between our watches. We both pressed start at precisely the same time and the first mile marker bleeped within a second or two of each other, mine first then almost immediately after his. However on the second mile his bleeped first and mine a fair few seconds larer. Mine started to lag behind more with each mile and after 7 miles was quiet a bit behind on the mile markers. Strange thing is though that on the total distance at the end we were within a couple of tenths of a mile out from each other! Not a problem but strange.
  • I use the wahoo ticker app to record my runs. When I upload them to Strava the numbers change slightly. HR 1 beat less, distance maybe .2k longer. I don't worry about it... just use the same device all the time and you'll see the trends and whether you're getting slower, faster, fitter, etc.
  • I find that readings on Strava are inevitably higher for calories and elevation than on Garmin Connect. Distance run is much the same, which is the most important for me.
  • I think its down to the accuracy of the time in the GPS devices.. sounds a bit daft but the calculation is in relation to signals transmitted at speed of light and therefore unless you have accuracy of an atomic clock - the 10 nano seconds or so of watch or phone accuracy can mean 3m+ difference and account for "drift" ... apparently :-) 

    Looks like the 205 is most accurate of your devices.
  • Fairly new here, but this has been bothering me for a while too. I have a number of routes that I planned out using MapMyFitness and the distance measured by TomTom Runner Cardio is always out by a bit. I know GPS are not 100% accurate (the TomTom often tracks me running down the middle of a river), but I also don't trust the maps 100% as I know many places were paths are incorrectly plotted.
    On the one hand I shouldn't be bothered, as long as I'm happy with my times and the GPS is consistent in its in/accuracy,which it is.
    But its particularly irritating if I'm training for a race (have only done a few 10k's) or want to compare times from different routes.
    The difference can be .2 of a mile over 3.5 miles for example, which might not seem much, but when you look at the pace it can mean a 20-30 seconds a mile difference which is significant.
    nigelfar - you will get to know your own GPS and how it tends to measure distances.   I have a Garmin and if I look in the instructions it does say that it can have a tendency to over measure by maybe 1% or 2%.   My Garmin will measure a marathon distance at about 26.4 miles, any less then I would suspect a short course.

    Your Tom Tom seems to over measure by a bit more so if you know what % that is then that's what you need to take into account when predicting your race time.
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