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tracking that didn't happen

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3 comments

  • DarrenB

    That's called GPS drift. There is inherent error in the successive GPS locations that your device estimates, and those random "movements" are an artifact of that error. The point where you were sitting was probably near the centre of that white circle you drew.

    In GPS tracks, we notice that error most when stationary, but in fact, there are errors in all the points the GPS records. We just don't notice them as much when the points are spread out.

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  • spenceke

    Thanks for your explanation.  Does the fact that my phone is in my pocket make a difference?   I have used this quite often and never seen such large discrepancies.  I think I actually had some 1 bar cell service and so didn't use a downloaded map and wonder if that makes a difference?   I assume there is nothing one can do if these items I mention aren't part of the problem.

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  • DarrenB

    Yes, potentially. Anything that impedes signal reception between the GPS satellite constellation and your device can impact the precision and accuracy of the GPS location estimates. Rough terrain and obstacles between your device and the satellites (e.g., buildings, mountains, forest canopy cover, etc.) will all impact location estimates.

    Under optimal conditions with an iPhone, you can expect most GPS locations to be within a few metres of the actual location. But any of the things I mention above will degrade that accuracy, sometimes to 10s of metres or more.

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