Here's an odd thing. My GPS watch and my RB's are off. By a lot. I got my GPS as a gift for Christmas several years ago. It's a Times with a separate GPS receiver that I wear on my arm. I've used it for years and never felt it was off, except when it was obviously off, usually because of tree coverage or running between tall buildings.  My RB has a newer unit -- A Garmin forerunner of some sort, which has the GPS receiver on the unit, wrapped around his wrist.

But as we run, these two GPS units get further off on the distance we've run. In fact, during the long runs we were doing prior to the Marine Corp Marathon, they were as much as a half mile off, which is shocking.  We haven'tfigured out why. Or even which is accurate, or which is more accurate in what conditions. I read a recent Runners World article comparing GPS units and commenting on the accuracy of the Forerunners. Which is expected from a GPS captain like Garmin (full disclosure, I own shares of Garmin). But my sense has been that the Timex is more often more accurate. At least, it usually comports with similar route tracing I've done at http://www.usatf.org/routes. My model is so much older though that I would think the Forerunner is the more accurate one.

But how can they be off by a half mile? Makes me wonder about the chartplotter on my boat. I use that heavily for navigation.