You mean calibrate to an absolute altitude value? Depends on the app I guess, but honestly I usually don't care about the absolute values, only the elevation changes.

Besides, even with a "calibrated" Garmin, a their barometeric altimeters have issues. I did a ride on Sunday with 5 other guys, me with an iPhone 6 and them with various Garmins, and our elevation changes varied from 623ft to 1565ft. My phone read 1319ft (utilizing the barometer) and Strava's "corrected" value was 1236ft. Who knows which one was right?

EDIT: Updated the numbers from Strava rather than memory. BTW 623ft was definitely not right!