Don't laugh, folks. I score/time 5K/10K/triathlons, up to marathons. Current course certification, in NC, anyway, is with a Jones counter, and it uses a piece just like that. You get a laser-measured 1000 meter course, ride it twice to calibrate the counter, then use the counter to measure your courses by bike. They have yet to accept GPS measuring, even with .00 units. I bought a Cateye with .00 units, as well, and while I thought it worked fine, and was accurate, and was willing to calibrate it on the 1000-meter course, it was not acceptable. The funny thing is, you have to add .1 of 1% anyway, so a perfectly accurate 10K course will still be 160' longer than 10K.

Experienced runners can tell if a course is long or short, anyway. You run enough 10K's and 5K's, you know if the course is long or short (if it's relatively flat).