I've started writing a primer on altitude/airspeed using pressure sensors. It is complete by now, peer-reviewed (thanks ArduPilot Mega team!) and should be helpful and correct.
If you however see something unclear or wrong, please let me know or directly change it by registering to the Wiki:
General feedback (e.g. wether it is too complex, etc.) is also very welcome.
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My only comment is that you should add something in about diurnal or weather related variation in barometric pressure. This can cause significant errors over surprisingly short periods of time. Corrections from a base station barometer might be useful.