On yesterday's run, all 3 of our GPS devices kept telling us the mile markers were short. By the time we were done running the marked course of 20 miles, our GPS devices only had 19 miles and small change. At first, everyone else thought the markers were wrong, but I put my trust in the markers.
Why would GPS devices be wrong? Well, we were running in fairly dense woods, along curvy bicycle trails. The trees have a tendency to interfere somewhat with the GPS signals, so I figure the devices did not *always* know exactly where we were.
So, suppose you have run from point A to point B, on a non-straight path. Next suppose the device has lost track of where you were between A & B.
How does it imagine you got from A to B? I figure it draws a straight line, thus shortening the actual distance covered.
I believe my GPS
sometimes has to guess.