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Issue of Accuracy of Course Distances

I ran a previously mapped course today. However due to prior flooding and path diversions, my previously hand mapped course following the old map of the park did not correspond exactly to the actual run route. I noticed today that there was an option to map the run I did to create a new course, so I did so. Interestingly, using a Garmin 910XT with a foot pod, my Garmin Connect analysis of today's run indicated 12.16k, presumably from the GPS data, not the footpod data. However the course I created directly from the same recorded run I did today, following the GC instructions, only added up to a distance listing of 12.01k. I made sure the box "stay on roads" was NOT checked.

The difference in distance is not hugely important to me, but it irks me that something is happening that I don't understand.

What gives?
  • 1. Satellite triangulation errors - every point plotted has an error of at least 3 metres radius from reality. Those add up

    2. Not following the course you plotted exactly (suppose you run 0.1m to the left of the course the whole way). That adds up

    3. Map accuracy - sometimes maps have to be inaccurate in order to fit important detail in.