I calibrate my barometer manually, from a topo or known height like at a saddle or summit. Garmin can auto calibrate them once at the start of an activity from GPS or continually nudge it towards agreement, but 99% of the time it works very well.

When knowing my elevation is important, I have a data screen on my Garmin that shows the altitude according to both sources. And I can work it out from a topographic map.