altiméter

An altimeter is an instrument that measures altitude, the height above a reference level. The most common type is a barometric or pressure altimeter, which uses a sealed aneroid pressure capsule connected to a mechanical or electronic display. As ambient air pressure decreases with altitude, the capsule expands or contracts, moving the display to indicate altitude. The instrument is calibrated to a reference sea-level pressure, known in aviation as QNH when displaying altitude above mean sea level, QFE to display height above the reference field elevation, or a standard pressure of 1013.25 hPa for pressure altitude and flight levels.

Radio or radar altimeters determine altitude by transmitting radio waves toward the ground and measuring the time of flight; these provide height above terrain, especially during approach, but rely on the terrain to produce a suitable reflection.

GPS-based altimeters derive altitude from satellite positioning, yielding ellipsoidal height, which may differ from mean sea level; in practice, GPS altitudes are combined with barometric data to produce a reliable height reading.

In aviation, accurate altitude measurement is essential for safe separation and navigation. Altimeters must be calibrated and corrected for temperature, pressure changes, and instrument error. Nonstandard temperatures can cause density altitude to differ from true altitude, affecting performance and flight planning.

In consumer devices, barometric sensors enable personal altimeters in smartphones and watches, often supplemented by GPS. These devices provide relative altitude changes useful for hiking and fitness activities.