Any way to "calibrate" distances on Fenix 5S Plus

Hi - I went out for a run today and of course the watch reported the distance based on GPS.

I prefer to use the distance that google maps reports when the course is drawn out there. Is there an adjustment I can make that scales/adjusts the GPS distance used by the watch so it reports the google maps distance? Something like the scaling factor in Stryd, for example?

Thanks!

  • No there is not. What makes you believe the distance you have on the route from Google Maps is better than the GPS?

  • Thanks, philipshambrook.

    Google maps distances match distances from one or two other running sites (map-o-meter is one iirc). It may be those others all simply use google maps data, and therefore it's only n=1 without basis for choosing GPS vs. google maps. I have no idea TBH! 

    I went back and checked my Stryd ... it seems to match GPS distances with "calibration = 100". So, if the various running sites that match google map distances are actually simply using google maps distances, then maybe GPS *is* more accurate (Stryd and GPS concordant, google maps being the odd man out, under that circumstance)!

    It's a difference of 2.7%. I really should get to a track and calibrate the "right" way, but I guess at this point when I'm not training for a soon-upcoming race, I don't feel a pressing need.

  • ‘Calibrating’ GPS around a running track is likely to leave you disappointed for a number of reasons. If you want to check your GPS distance then best to check against a known distance. Accurately measure out a km or so with a calibrated measuring wheel in a straight line in open space without any overhanging trees or building. Check your GPS against that.

  • Hmm, what exactly is a "known distance"? Do I trust highway markers? Any other suggestions?

    "Calibrated measuring wheel in a straight line without overhanging trees or buildings for a Km or so …". Not sure that is going to happen, sounds a little too hard to make that happen for me, unfortunately … 

    Thanks, @philipshambrook .

  • Hi - Following up on the above -

    Does anyone know whether GPS is more accurate than google maps, or vice versa?

    Thanks!

    Notne

  • Former Member
    0 Former Member over 2 years ago in reply to Notne
    "known distance"? Do I trust highway markers? Any other suggestions

    As Phil said, use a calibrate measuring wheel. You can then walk off any known distance you want to have to conduct your test. The wheels aren't very expensive if you don't own one or know of anyone who'll let you borrow theirs. 

  • Former Member
    0 Former Member over 2 years ago in reply to Notne

    The problem with using Google maps is distance is measured using line of sight. It tracks your route but not the contour of your route. As you know, a mile is 5280 feet.  If you were to run a "Google" mile, you might not necessarily be running 5280 feet. Even the slightest accents or descents will add footage to that run. So will any deviation from running a perfectly straight line from point A to point B.  If your Stryd is calibrated correctly, using it to track distance is your best bet. 

  • I assume you’re right, but do we actually know for sure that the measuring tool in Google Maps does not account for contours? I mean, the information is there in their maps..

    Totally agree on the Stryd!

  • In any case, the influence from slopes is usually overestimated.

    A 1% slope will cause an error of 0.005%.
    A 5% slope will cause an error of 0.12%.

    A 10% slope will cause an error of 0.5%


    It will usually be pretty easy to find a test route with no slopes exceeding 5%.

    In any case, the question was about the difference between GPS and Google Maps. A GPS distance measurement on a Garmin watch will be 2D unless you enable 3D distance - which can't really be recommended unless you climb really, really steep slopes.

    So if there is an error from measuring in 2D, it will hit GPS and Google Maps equally.

  • And since we can, or should if we are being realistic, expect GPS errors to occur we should not be surprised when they do.

    Sadly, some people still expect a degree of precision and accuracy from a wrist worn GPS that is totally unrealistic and impossible to achieve. - GPS sources of errors. I recall some years back discussions about the maps and the comparison between the GPS track and trail on the map. We tend to take as 'gospel' the the trail on the map is in the right place when there are also associated cartographic errors - online mapping errors

    Hence why I suggest the best you can do is compare to a known straight-line measure. And even then, if you do it repeatedly you'll likely end up with a range of results due to satellite positions etc. at the time of measure. If you do measure repeatedly you can end up with a mean and standard deviation and a 95% confidence interval that will at least give you some solace for the measured distance. However, how much that is relevant to running on narrow single track under dense tree cover is another matter entirely.