Altimeters

An altimeter is an instrument that measures altitude—the vertical distance above a reference level, typically mean sea level. The most common type in aviation is a barometric altimeter, which infers altitude from ambient air pressure. Because atmospheric pressure decreases with altitude, a sealed aneroid capsule inside the instrument expands or contracts as pressure changes, and a linkage translates that movement into a numeric indication of altitude.

Altitude readings depend on the pressure setting applied to the instrument. Indicated altitude is the number shown after the local pressure setting is applied. When the altimeter is set to standard pressure of 1013.25 hPa, the reading is pressure altitude. True altitude is indicated altitude corrected for nonstandard temperature and pressure; density altitude is true altitude adjusted for temperature for aircraft performance. In aviation, common settings are QNH (local mean sea level pressure, so indicated altitude equals altitude above mean sea level), QFE (station pressure, so indicated altitude equals height above the aerodrome), and QNE (1013.25 hPa, standard).

Other altimeter types include radio altimeters, which measure altitude above ground using radar, and GNSS-based altimeters that compute height from satellite data. Pilots often rely on multiple sources for vertical guidance and redundancy. Accuracy varies with weather, temperature deviations, calibration state, and instrument age; barometric readings are particularly sensitive to nonstandard temperatures, which can cause height errors if uncorrected. Regular calibration and correct setting are essential for safe operation, especially during instrument meteorological conditions or near mountainous terrain.