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Garmin Forerunner 220: watch distance vs map distance (Read 77 times)

James Logan


    I've been uploading my garmin data to running ahead for a few months now, and I've noticed that when I look at the map of my run after I've uploaded it, the map will say that I've run farther than my watch will say that I have.  For example, my watch will say I've run 6 miles, but the map of the run that I uploaded says I've run 6.2.  I know this isn't a significant difference, but I would like to know which one is more accurate.  Thanks!


    an amazing likeness

       

      The difference between Garmin and RA's distances should be about 2%.  Most of the GPSes if not all use a simpler formula to calculate distance between two points on a sphere.  The earth is not a sphere.  RA's formula accounts for that and tend to provide a slightly different number.  One can argue that either  is accurate.

       

      eric Smile

       

      The device records data points.

      The software on the device then calculates distance from those data points.

      The RA map also works through the same data points and calculates total distance.

      End result is two different numbers.

      RA has move computational power available, and an algorithm that doesn't have to fit in the firmware of the Forerunner.

      I'd bet on RA.

      Acceptable at a dance, invaluable in a shipwreck.

      Fredford66


      Waltons ThreadLord

        I wonder whether either method takes into account vertical distance as well.  Is distance straight line point to point at sea level, or does it truly measure the number of miles of pavement you've traveled up and down hills?

        5k 23:48.45 (3/22); 4M 31:26 (2/22); 5M 38:55 (11/23); 10k 49:24 (10/22); 
        10M 1:29:33 (2/24); Half 1:48:32 (10/22); Marathon 4:29:58 (11/23)

        Upcoming races: RunAPalooza (Asbury Park) HM, 4/6; Clinton Country Run 15k, 4/27

         


        an amazing likeness

          Excerpt from an excellent explanation...

           

          "GPS-based distance assumes a flat surface and cannot account for vertical speed, or the 3D velocity vector that would take into account the increase in distance with topography. However, the effect on topography for GPS-calculated distance is minimal - for a 10% grade, distance would only increase by 0.5%."

          Acceptable at a dance, invaluable in a shipwreck.

          pedaling fool


            I would measure a course and run it and see what the difference is and if you have access to a track that may be a good thing too.

            RichardR


              Can you link this explanation? There are probably other good things in it l'd be interested in. Thanks

               

              Excerpt from an excellent explanation...

               

              "GPS-based distance assumes a flat surface and cannot account for vertical speed, or the 3D velocity vector that would take into account the increase in distance with topography. However, the effect on topography for GPS-calculated distance is minimal - for a 10% grade, distance would only increase by 0.5%."

              roadrunner54502


                I've noticed the same thing with my Garmin Fenix 5 (and a Tom Tom device I trialled Before going for the Garmin) and my experience was that the GPS watch recorded a distance that was nearly always shorter than the mapped version, and rarely if ever added miles to a pre-map-measured route. One of my routes though - set over fairly rough, undulating and forested woodland in the relative wilds of Sweden (i.e. terrain the relevant variable here) - has been measured on both RA.com mapping feature and on the cateye mitty 2 bike computer when I rode the trail on my MTB (years ago). The result was that the mapping feature of RA.com matched perfectly (spookily) the distance of the MTB ride and bearing in mind that the bike computer is measuring wheel revolutions (by the length of the wheel circumference) it is measuring actual ground covered with no (or very small) error created by algorithm / data point calculations. Also, I run park run when I can and my Garmin always drops about 4-8 meters, and these are pre-measured routes.

                 

                Sorry for the detail - a numbers geek! BUT, I'd go with the distance of the RA.com mapping feature over the GPS data Smile - hope this helps!

                 

                BTW - whilst here, can the data from Garmin Fenix be uploaded to RA.com like the Forerunner series - anyone? cheers...

                RichardR


                  I've noticed the same thing with my Garmin Fenix 5 (and a Tom Tom device I trialled Before going for the Garmin) and my experience was that the GPS watch recorded a distance that was nearly always shorter than the mapped version, and rarely if ever added miles to a pre-map-measured route. One of my routes though - set over fairly rough, undulating and forested woodland in the relative wilds of Sweden (i.e. terrain the relevant variable here) - has been measured on both RA.com mapping feature and on the cateye mitty 2 bike computer when I rode the trail on my MTB (years ago). The result was that the mapping feature of RA.com matched perfectly (spookily) the distance of the MTB ride and bearing in mind that the bike computer is measuring wheel revolutions (by the length of the wheel circumference) it is measuring actual ground covered with no (or very small) error created by algorithm / data point calculations. Also, I run park run when I can and my Garmin always drops about 4-8 meters, and these are pre-measured routes.

                   

                  Sorry for the detail - a numbers geek! BUT, I'd go with the distance of the RA.com mapping feature over the GPS data Smile - hope this helps!

                   

                  BTW - whilst here, can the data from Garmin Fenix be uploaded to RA.com like the Forerunner series - anyone? cheers...

                   

                  Fenix 5....  Try this link:

                  https://www8.garmin.com/manuals/webhelp/fenix5/EN-US/GUID-9A740EA8-1467-4033-891D-1B98F08876D9.html

                   

                  What really happens in all cases is the Garmin watches upload to Garmin Connect, and Garmin Connect pushes it to RA.

                   

                  Good luck