The GPS measures what you have traveled. I can go down a series of switchbacks and drop 1,000 feet and it will tell me how far I've gone on the trail - it's more than it would tell me if I had not walked all those switchbacks and just slid down a pole. It leaves breadcrumbs and measures speed as well as time.

I suppose I could do an experiment - throw the GPS off a hill and go get it at the bottom, and see what it tells me, then walk back up the slope and compare the two. But it's not going to short you because the distance is not linear.
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