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)..

For more information about the topic Altimeter, read the full article at Wikipedia.org, or see the following related articles:

Altitude — Altitude is the elevation of an object from a known level or datum. Common datums are mean sea level and the surface of the WGS-84 geoid, used by ...  > read more

Sea level — Mean sea level (MSL) is the average (mean) height of the sea, with reference to a suitable reference surface. Defining the reference level, however, ...  > read more

Boiling — Boiling is the rapid vaporization of a liquid, which typically occurs when a liquid is heated to a temperature such that its vapor pressure is above ...  > read more

Pressure — Pressure (symbol: p) is the force per unit area acting on a surface in a direction perpendicular to that surface. Pressure is transmitted to solid ...  > read more

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