Altitude fusing GPS and barometer

I've wondered the same, having the need for accurate elevation profiles for running-workout courses. From my recent testing, it's clear that CoreLocation is pure GPS - and it's very inaccurate (sometimes > 5 metres different from the actual absolute or relative elevation). CMAltimeter provides consistently accurate data (+/- a few centimetres), with very little "noise" - thus making it easy to programmatically identify slope segments.

Being barometer based, CMAltimeter is prone to rapidly changing weather events such as a thunderstorm or a passing front, so that any longer period workout (eg marathon) could see spurious altitude values.

Regards,

Michaela