Hold MENU, and select Settings > Sensors & Accessories > Altimeter.

Calibrate

Allows you to manually calibrate the altimeter sensor.

Auto Cal.

Allows the altimeter to self-calibrate each time you use satellite systems.

Sensor Mode

Sets the mode for the sensor. The Auto option uses both the altimeter and barometer according to your movement. You can use the Altimeter Only option when your activity involves changes in altitude, or the Barometer Only option when your activity does not involve changes in altitude.

Elevation

Sets the units of measure for elevation.