Altimeter

An altimeter measures altitude above a reference level. Common types infer height from atmospheric pressure or measure distance using radio waves.

Atmospheric pressure: The force per unit area exerted by the weight of air above a surface. A barometric altimeter estimates altitude from the way air pressure decreases with height.
Altitude: The vertical distance of a point above a specified reference level. An altimeter reports altitude only after a reference level is chosen.
Aircraft altimeter: An aircraft instrument that indicates altitude, typically from atmospheric pressure. Pilots use its reading to maintain assigned altitudes and separation.
Absolute altitude: The vertical distance between an aircraft and the terrain directly below it. A radio altimeter measures this, unlike a barometric altimeter's pressure-derived altitude.
Barometric altimeter: An altimeter that estimates altitude from measured atmospheric pressure. This is the common pressure-based design used in aircraft and hiking instruments.
Sea level: The average height of the ocean surface, used as a reference for elevations and altitudes. Pressure altimeters can be adjusted to report height relative to mean sea level.
Flight level: A standard pressure-altitude surface expressed in hundreds of feet using a common pressure setting. Aircraft above the transition altitude use standardized altimeter settings to coordinate vertical separation.
True altitude: The actual vertical distance above mean sea level. It can differ from the altitude indicated by a pressure altimeter.
Radio altimeter: An instrument that measures height above terrain by timing reflected radio signals. It provides direct height above the surface beneath an aircraft.
Pressure altitude: Altitude indicated when an altimeter is set to the standard pressure of 29.92 inches of mercury or 1013.25 hectopascals. This standardized reading supports aircraft performance and flight-level calculations.

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