I use mapmyrun to select routes for running, and of course it gives distance too, down to the hundredth of a mile. I’ve noticed lately that my S9P has been, on occasions, as much as a half of a mile off from the mapmyrun numbers. Is there a margin of error for the GPS?
Also, with it getting colder, i’m wondering if having a jacket on that covers the watchface will affect the accuracy of the GPS reading?
Depending on your surrounding (tall buildings, trees, mountains, etc.) and the weather (clouds, rain, snow), GPS accuracy may be really good or just terrible and useless. That’s in the nature of GPS (and GNSS in general). Generally, I found S9P to be pretty good and accurate. But you can’t expect 100% accuracy - and any model or make.
If you don’t use navigation, the Snap-to-Route feature can probably help. It should give nearly perfect GPS data. I haven’t tested that myself, though.
A jacket shouldn’t do much harm… unless you’re wearing a radiation suite :'-D