Good point, justamaniac. If you want accuracy, measure with a calibrated device, like a surveyor's wheel, or a bicycle with a calibrated wheel (how USATF certified race distances are measured). If you delve into the documentation for GPS, you'll find it can be off by several meters at any measurement point. Once upon a time it was 15 meters ( !), now typically 3 to 5 meters. So your corresponding total distance, overall pace, distance from the last "milepost", last mile pace, and particularly "instantaneous" pace, all can be off considerably. The main reason, of course, is that the electronics to measure more accurately won't come close to fitting on your wrist. GPS is great for comparing run-to-run and overall time/distance. Just don't tell me that your 10K race was long because your GPS measured it at 6.38 miles.
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