Thursday, May 11, 2006

Running in Circles

While my Garmin Forerunner 205 is definitely more accurate than its predecessor, the Forerunner 201, it still has problems measuring distance at the track.

On the surface it sounds like it should be easy. Open space, flat surface and 400 meters all the way around in the middle of lane one. No problem, right?

Wrong. I don't know the exact technical reason, but I think the GPS inaccuracy is due to the tightness of a track's turns, and the time it takes to bounce a signal from the Forerunner to satellite and back again. Don't quote me on it.

My run yesterday took me to the local high school. I wanted to run on a soft, flat surface and the playing field inside of the track is one of those newer, fake grass fields. Less pounding on the body.

I ran 8 laps or around 3200 meters, which is about 14 meters short of 2 miles. The picture above was generated by MotionBased using the GPS waypoints from the 205 and Google Earth.

If you click on the picture and look close enough, it seems like I ran most of the laps in the middle and outer lanes, and even went up in the stands and on top of the announcer's booth! In actuality, I spent most of the time on the fake grass infield trying to hug the curb running along lane 1. I think I went out to lane 4 on the turns for a couple of laps in order to counter running mostly on the inside of lane 1 on the infield.

FYI, the typical 400 meter track is measured from the middle of lane 1. For each lane you can add roughly 6-7 meters for one lap. So if you run a lap in the middle of lane 2, then you ran about 406-7 meters, in lane 3 - 413-14 meters, etc.

So after the 8 laps, my 205 said I ran 2.10 miles. SportTracks calculated the distance at 2.09 miles. I logged it as 2 miles and moved on.

If you are only going to run on a track then there really is no need to wear a GPS device like the Forerunner anyway. You know the distance of each lap and a stopwatch should suffice. You just might have to brush up on your math skills to calculate pace....

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