FWIW: Aircraft use baro pressure to tell altitude. They have a knob to adjust for local barometric pressure as reported by nearby ground stations. Not perfect, but works acceptably well for that purpose.

http://en.wikipedia.org/wiki/Altimeter

There're not a lot of options. Radar altimeters measure distance above the ground, which never changes for a road bike. GPS altitude is relatively imprecise in that it's not good for small changes and rates of change as needed for cycling. So you're left with baro.