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).
MIG 21 - Aircraft Cabin Altimeter Indicator - 0-600 m