GPS

I ran a local 5k last weekend (4th overall, 2nd AG) with several friends and afterwards we were talking about the course. Since everyone these days wears a GPS, we were comparing distances. They all ranged from 3.2 to 3.4 miles. About two miles of the course format was out and back which gave me a good chance to see what my GPS measured.
I uploaded my run and switched the route view to satellite. The scale in the picture is one into to 200 feet. The course is on a paved bike path so the maximum width of the course is eight feet. You can clearly see in two areas where the out and back don't overlay. I try to run the tangents and since going out, I was near the front, there wasn't any problems cutting corners. You can also see where it looks like I'm running off the concrete path and through a field and a parking lot.
A GPS is a great tool for mapping runs and estimating distances but it's only as accurate as it's reception. While this path had very few trees and only on building over two stories, the GPS wasn't perfect and I don't think it's right to expect it to be. It's worn on the wrist so it's pretty small with a small antenna. It has a rechargable battery so it's not very powerful. So if you have GPS, enjoy your ability to measure your runs anywhere you go but remember, it's not perfect.