Aviation altitude

Aviation altitude is the vertical distance of an aircraft above a specified reference surface. Its value depends on whether the reference is sea level, terrain, or another pressure surface.

Mean sea level: The average level of the sea over a long period, used as a standard vertical datum. Altitude above mean sea level is the basis for most charted elevations and flight levels.
Barometric altimeter: An instrument that estimates altitude from measured atmospheric pressure. It is the standard cockpit instrument for determining pressure-based altitude.
Flight level: A pressure altitude expressed in hundreds of feet using the standard altimeter setting. Above a transition altitude, aircraft use flight levels for standardized vertical separation.
Elevation: The vertical distance of a point on or near Earth’s surface above a specified datum, usually mean sea level. Elevation describes terrain or structures, while aviation altitude commonly describes an aircraft.
Altimeter setting: The pressure value entered into an altimeter so its readings correspond to a chosen vertical reference. Incorrect settings can produce misleading altitude indications and reduce separation margins.
Height above ground level: The vertical distance between an object and the local ground beneath it. It differs from altitude above sea level whenever terrain is not at sea level.
Atmospheric pressure: The force per unit area exerted by the weight of air above a surface. A barometric altimeter converts pressure changes into an altitude estimate.
Transition altitude: The altitude at or below which aircraft set altimeters to local pressure settings. It marks the change from local altitude readings to standard-pressure flight levels.
Flight height: The vertical distance of an aircraft above the ground or another local surface. It is surface-relative, unlike altitude referenced to mean sea level.
Transition level: The lowest flight level available for use above a transition altitude. It completes the local boundary between altitude and flight-level procedures.

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