Made by Polar, Garmin, Nike and Timex, global positioning system watches track the distance you have run and your pace, including your average pace and your instantaneous pace. They beep at intervals, like every mile, if you want to train by doing some segments of your course at a faster pace. And when you are finished running, you can download all your data onto your computer.
But after a while, I noticed something disconcerting. My watch might record my run as, say, six miles, but according to Google Maps, the actual distance was more like 6.5 miles.
Flickr photo by J R