I rode a time trial this past weekend that was multiple laps around a 10-km course. I was using the Cyclemeter app to track my performance. Cyclemeter will use the barometric sensor on my iPhone, but I was on the course long enough that barometric pressure changed. It's interesting to see how this affected the altitude readings. Multiple laps of the same route make it clear. This screenshot is taken from Cyclometer's web-based "explorer."

I knew beforehand that the course has 97 meters of climbing per lap, and I could see as I was riding it that even over the course of a single lap, it wasn't recording my altitude very accurately. This is not a big surprise.

Ride with GPS can correct the altitude of a track after the fact—I uploaded this route to RwGPS, applied altitude correction, and it looks correct there now. AFAICT, Cyclemeter cannot perform post-hoc altitude corrections.



Altitude in Cyclemeter