B.A. in Geography said,...
"Although GPS's are good tools, they will never be more accurate than wheeling out a course, but wheeling out a course has limitations too. And for this reason, people should understand that the GPS is great for places that you cannot wheel out, but the distance the GPS tells them is probably inaccurate. I know there are a lot of "calculation freaks"out there running (I mean come on, we're runners) but drives me crazy when people start questioning the accuracy of tracks because their GPS said it was not 400m."
I have certified XC courses and can tell you that more often then not, when used properly, the GPS is actually more accurate than the wheel. The best way to measure is the Jones counter which is what I use. I Steel tape a minumum of 800m on the course, then ride the tape with the Jones counter a minimum of 5 times to calibrate the device. Then I ride the course using all tangents. When using the Garming (i use a 305), I will drive the course very very slowly so that all curves are represented. If you walk the course with the GPS on your wrist, your arm swing can throw the measurement off my several meters over an 8k or 10k course. In every case, that I have wheeled and GPS'd a course that I have measured with a Jones counter, the GPS is the more accurate device regardless of elevation changes.