Altimeter

An altimeter is an active instrument used to measure the altitude of an object above a fixed level. The traditional altimeter found in most aircraft works by measuring the air pressure from a static port in the airplane. 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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  Kollsman-type barometric aircraft altimeter set at 75 ft (23 m) showing a pressure of 29.87 in (1011.5 hPa). This altimeter is mounted in a console at an airfield radio station.
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  World War 1 Altimeter (metric)
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  Wrist-altimeter
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  MIG 21 - Aircraft Cabin Altimeter Indicator - 0-600 m

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  Drawing of an altimeter
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  Altimeter
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  Altimeter