Quote Originally Posted by OakLeaf View Post
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.
There is usually agreement among the cyclists as to what the grade is. If there is a variation, the cyclists will say something like 6-7%, and it is sufficient to know how to tackle the hill, knowing that it is 6-7% versus 12% or 16%. But usually everyone gets the same reading and it is known that such and such hill is one mile long and mostly 18% and tapering down to 13% toward the top, and just knowing it is that steep and at what mile the hill is will give the cyclist the ability to plan on how to cycle the hill, or even walk it. When I get to the hills, my GPS tells me the same grade is what the other cylists said it was, so if there is an inaccuracy it is common among all of us. If there is a slight inaccuracy for ALL cyclists, then it isn't critical because the training is on how to climb the different grades, because different grade ranges take a different kind of gear.

I am not a particularly fast cyclist. However I really improved on the hill climbing by focusing on the grades, and learning specific techniques for each grade range relative to the length. My improvement has been tremendous. Even last week I went up a hill with a 11-13% grade, one where I shifted into my granny at the bottom in prior years, and now I go up the hill in my middle cog and my speed when I crest the hill isn't below 9 mph.