http://ohioline.osu.edu/aex-fact/0560.html

One thing all fitness GPS have in common is their small size. I'm sure it would be a great workout to carry aircraft or agricultural-grade GPS (plus its power supply) on a bike, but I rarely see horizontal DOP better than 3 meters on mine, and it's usually worse. Multiply that by 1.5 (many sources say multiply by 2 to 3) for vertical DOP, consider the relatively low speed of a bicycle, and you see the problem. Assuming you're climbing a hill at 10 feet per second (around 7 mph) and think about what a 15 foot error in elevation is going to do to your grade readout.