easterbird, I believe the VDO has a barometric altimeter. This is the best for determining actually climbing gains/losses. See my notes above about GPS-only units (such as the Edge 205 and the Forerunners as opposed to an Edge 305). As an example, yellow used a Forerunner 305 for the Little Red Riding Hood which reported about 2,500' climbing. My Ciclo reported 1,089 for the same course. Since this course is very, very flat, the 1,089 is the more accurate number.

Barometic altimeters work using air pressue so as weather patterns change the pressure changes. Just during the course of a day pressure will change. I think that 20-50' is pretty normal.

Just a note though for anyone thinking about GPS based computers, the GPS in yellow's Forerunner is far more accurate for distance and speed than the cycling computers based on wheelcircumference.