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Running Distance Usually Short?

Stearmandriver
over 3 years ago

Hello, 

New Fenix 6 Pro owner here.  I've noticed that when using the device to record a Run activity, the distance measured is always a little shorter than Strava on my phone records the same run. 

I've set the GPS to record every second, and looking at the track on a satellite map, GPS accuracy seems quite acceptable.  Certainly the line sometimes deviates from the road a bit, but for a watch, accuracy seems darn good.  The GPS course follows the actual route, turns around at the correct places etc. 

So what I don't understand is, with these sorts of small deviations, I would expect that if anything, the watch would be measuring a slightly LONGER distance than Strava (which is using the better antenna in a cell phone, and therefore shows a straighter track with fewer deviations).

So why is the Fenix measuring a shorter distance? 

I've found other threads discussing this but they seem to mostly refer to the "trail run" activity.  I'm seeing this behavior on the regular "run" activity. 

Thanks for any insight! 

  • 0 over 3 years ago in reply to Ɛpsilon

    And even then, I haven't used it much in the last couple of months mainly because one of the beta's gave HORRIBLE battery drain when using it (it's a BT one. not a ANT+ one) - for some reason I was up to 6-8% per hour battery usage when using it so I probably havent used it in about 2 months.

  • 0 over 3 years ago in reply to Ɛpsilon

    I used a Forerunner 245 and Fenix 6 for a month together back in Feb / March and I never saw any obvious distance discrepancy between the two.

    I still have the .FIT files and could pop up some comparsions....

  • 0 over 3 years ago in reply to 5135968
    5135968 said:
    they claim it's measured with a measuring wheel

    All parkruns have to be measured and should be done so, with a calibrated wheel parkrun course design. Since there is no guarantee that the wheel was calibrated it's not possible to be certain the course has been properly measured. Chances are good though that it is pretty close. The two parkruns I've set up here were both measured by an experienced national level athlete and were actually measured following the shortest track around the course (running straight lines where possible, shorter path around bends etc) with 40m, extra added. People come up short and some people come up long but generally well within the acceptable margins of error one would reasonably expect from a wrist worn GPS. And despite many parkruns with my F6 on the same course I cannot see a consistently short result every time; sometimes it's slightly short, sometimes slightly long.

    I would suggest coming up 35 to 70 yds (.02 or .04 *1760) can easily be down to not running exactly the same course as the wheel as much or satellite error or any other of a host of things, including Garmin's machinations with the data. However, the difference is trivial.

  • 0 over 3 years ago in reply to Dr Phil

    Just noticed there is one bad outlier on my list - but I know why. I did an ultra which was 5K out on a path, then back - five times - over a track that went underground in tunnels for 3K of the 5K. REALLY screwed up the GPS tracking on the device (that was on my 5X+) but I have to be honest that was a complete unique issue.

  • 0 over 3 years ago

    No foot pod here.  I have just been correcting the distance in the Garmin Connect software to the Strava distance (which is no doubt more accurate in my case anyway, as verified by measuring the course with GIS tools).  

    My GPS settings are GPS + GLONASS, record every second.  I saw someone mention using GPS Only.  Is there a reason to do that? 

    If this really is just an algorithm issue, it seems a simple fix.  Perhaps a user-defined setting to specify the amount of smoothing applied?

    Based on my experience with other devices, I'm surprised no one has created a hacked / patched firmware yet that allows these sorts of things.  But maybe the "adventure watch" market isn't populated with folks who can do that stuff; it's certainly beyond me ;).

  • 0 over 3 years ago in reply to Stearmandriver
    Stearmandriver said:
    My GPS settings are GPS + GLONASS, record every second.  I saw someone mention using GPS Only.  Is there a reason to do that?

    I only do it for the slight increase in battery life- admittedly it's marginal.

  • 0 over 3 years ago in reply to Ɛpsilon

    I do have a reasonable amount of comparison data using two watches but I also have data from my wife's 945 or Instinct when we run together. I can say, hand on heart, that there is little to tell in the distance recorded between the two. This for example from a lumpy trail run yesterday

    I can show similar for road runs too. So, while I have no doubt that some see problems, my experience is to the contrary. yes there are difference, but they are generally not significant differences. 

  • 0 over 3 years ago in reply to Dr Phil

    Do you have 3D distance on?

    Over a good distance, a degree of elevation change could make a difference. if <a> is just measuring a distance on the map point to point and not allowing for elevation change and <b> is allowing for elevation change - then for some people, whose runs have a degree of climb - they may see a definite difference between apps is (for example) Strava doesn't factor in elevation

    They say "Cons: A flat surface is assumed, and vertical speed from topography is not accounted for. Similar to the above, straight lines connect the GPS points." - when using the strava app on the phone, which may explain what the OP is seeing

    Strava themselves says:

    (https://support.strava.com/hc/en-us/articles/216919487-How-Distance-is-Calculated)

    Methods of Calculating Distance

    There are two main ways to calculate distance for most sports - Ground Speed Distance and GPS-calculated Distance. Ground speed will measure your speed along the surface you are traveling (counting the revolutions of a wheel), and GPS-calculated distance will "connect the dots" between your GPS points and triangulate the distance between the coordinates. Each method of gathering data can and may introduce some inaccuracy.

    • GPS-based, device approach: The Strava mobile apps and many GPS devices will calculate your distance accumulated in "real-time" while the device is recording based on the GPS data.
      • Pros: Refined calculation to gather distance data that is built into the file in the distance stream, measured in meters.
      • Cons: The complicated nature of this "real-time" calculation can lead to stuck points, where no additional distance is recorded from the previous point, which can cause some Strava calculations like Estimated Best Efforts for Run to fail. Since this is a GPS-calculated distance, a flat surface is assumed, and vertical speed from topography is not accounted for. Also, some accumulated distance may be lost as straight lines connect each GPS coordinate, instead of an arc. This method of calculation does not capture variations in the route between GPS points and may vary further when battery-saving features are enabled.
    • GPS-based, Strava post-upload approach: After GPS data is recorded, and uploaded to Strava, the data is parsed into streams of data and analyzed. At this time, a calculation can be run on the GPS coordinates to get distance. This is how Strava determines distance for any uploaded file that does not include a distance stream. You can elect to use this method if you suspect there is a problem with your device's recorded distance (see reverting distance.)
      • Pros: Post-upload GPS-based distance can eliminate problems like stuck points (see above) and create smoother, more accurate distance data than the device equivalent.
      • Cons: A flat surface is assumed, and vertical speed from topography is not accounted for. Similar to the above, straight lines connect the GPS points.

    • Speed/Cadence Sensor Garmin GSC-10 approach: Ground Speed distance is measured by counting the revolutions of the wheel, and then multiplying by the wheel circumference.
      • Pros: A wheel sensor will capture vertical speed and the additional percentage of distance accumulated with changes in elevation. For mountain bikers who gain and lose a lot of elevation gain rapidly, this could become a slightly more significant factor.
      • Cons: Common problems with relying on a wheel sensor include: wheel size is not documented accurately, the device is moved to another bike with a different wheel size and not adjusted, the Auto wheel size is calculated wrong either because of GPS inaccuracies or because the magnet did not count every wheel revolution. See this article for how to ensure you get the most accurate distance data and the correct wheel setting.
  • 0 over 3 years ago in reply to Ɛpsilon

    So - what happens if on a app recording via the strava app, you do the correct distance thing in Strava - does it agree with what Garmin has then recorded>