Hold UP, and select Settings > Sensors > Altimeter.

Auto Cal.

Allows the altimeter to self-calibrate each time you turn on GPS tracking.

Barometer

Allows you to set the barometric pressure manually, or select a value from the nearest METAR station through Garmin Connect™ Mobile.

O2 Alert

Sets an alarm to vibrate when you reach a low-oxygen altitude.

Sync to Elevation

Allows you to synchronize the device to your current elevation manually.