An altimeter is an active instrument used to measure the altitude of an object above a fixed level.
The measurement of altitude is called altimetry, which is related to the term bathymetry, the measurement of depth underwater. The traditional altimeter found in most aircraft works by measuring the air pressure from a static port.
Air pressure decreases with an increase of altitude - about one millibar (0.03 inches of mercury) per 27 feet (8.23 m) close to sea level.
The altimeter is calibrated to show the pressure directly as altitudes, in accordance with a mathematical model defined by the International Standard Atmosphere (ISA).
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