<P> The aneroid altimeter is calibrated to show the pressure directly as an altitude above mean sea level, in accordance with a mathematical model atmosphere defined by the International Standard Atmosphere (ISA). Older aircraft used a simple aneroid barometer where the needle made less than one revolution around the face from zero to full scale . This design evolved to altimeters with a primary needle and one or more secondary needles that show the number of revolutions, similar to a clock face . In other words, each needle points to a different digit of the current altitude measurement . However this design has fallen out of favor due to the risk of misreading in stressful situations . The design evolved further to drum - type altimeters, the final step in analogue instrumentation, where each revolution of a single needle accounted for 1,000 feet, with thousand foot increments recorded on a numerical odometer - type drum . To determine altitude, a pilot had first to read the drum to determine the thousands of feet, then look at the needle for the hundreds of feet . Modern analogue altimeters in transport aircraft are typically drum - type . The latest development in clarity is an Electronic flight instrument system with integrated digital altimeter displays . This technology has trickled down from airliners and military planes until it is now standard in many general aviation aircraft . </P> <P> Modern aircraft use a "sensitive altimeter". On a sensitive altimeter, the sea - level reference pressure can be adjusted with a setting knob . The reference pressure, in inches of mercury in Canada and the United States, and hectopascals (previously millibars) elsewhere, is displayed in the small Kollsman window, on the face of the aircraft altimeter . This is necessary, since sea level reference atmospheric pressure at a given location varies over time with temperature and the movement of pressure systems in the atmosphere . </P> <P> In aviation terminology, the regional or local air pressure at mean sea level (MSL) is called the QNH or "altimeter setting", and the pressure that will calibrate the altimeter to show the height above ground at a given airfield is called the QFE of the field . An altimeter cannot, however, be adjusted for variations in air temperature . Differences in temperature from the ISA model will accordingly cause errors in indicated altitude . </P> <P> In aerospace, the mechanical stand - alone altimeters which are based on diaphragm bellows were replaced by integrated measurement systems which are called air data computers (ADC). This module measures altitude, speed of flight and outside temperature to provide more precise output data allowing automatic flight control and flight level division . Multiple altimeters can be used to design a pressure reference system to provide information about the airplane's position angles to further support inertial navigation system calculations . </P>

Air pressure is measured with an instrument called a