A few of my running friends and I have read the articles circulating about the inaccuracies of GPS watches. It is not uncommon to run a marathon and log 26.4 miles or a ten miler in 10.3 or a 5K at 3.2 miles. Runners World wrote about this issue and here is a second article. The basic idea article to the Hampton Rockfest piece is:
"A GPS measurement is often not nearly as accurate as a wheel measured course. The normal wrist-held or recreational devices are accurate to anywhere from 3 meters to 10 meters 95% of the time causing them to often report longer distances on an accurate measured course. There are many other issues that prevent a GPS measurement to be as accurate."
For my running, that means that often my GPS is close, but not 100% accurate. When I started running, GPS watches were just starting to catch on with runners. I remember early long runs training with a particular group because my friend Dave had GPS technology. I'd ask how fast each mile was. Eventually, I invested and bought my first GPS watch, which did help my times since I was able to measure time, distance, and pace. Remarkably, I only have had two GPS watches: the Garmin Forerunner 205 and Garmin Forerunner 405. The 405 is likely on its last legs, but until I need a new one, I'm fine with it.
I did start to rely too much on the technology. In the Columbus Marathon, I ran through an urban canyon (tall buildings) and lost signal so it displayed a slower pace. I picked up the pace and since it was early in the race, that likely cost me later as I would have been better off trusting myself. Now, I only use my Garmin for long runs, speed work, races, or runs where I don't know the route. When I have measured several routes through online mapping tools so I know the distance and can simply run without worrying about pace. At the Chicago Marathon, the same thing happened, but I knew better this time from my experience.
Knowing how the watches really work, for my next marathon scheduled for Sunday at Houston, I won't get discouraged when I see the mile record on my watch meters before the marker on the course. Previously, I assumed I wasn't running the tangents well and was adding too much by going to get water or deviating. Now that I know, I can relax and trust my pace and my training. Remember, with regard to the GPS: it's more of a guideline than a rule.