An altimeter is the geographical instrument used to measure altitude, or the height of an object above a reference level. In geography and surveying, altimeters help determine elevations and changes in terrain height.
The most familiar type is the barometric altimeter, which estimates altitude from air pressure. Atmospheric pressure generally decreases as elevation increases, so the instrument converts the measured pressure into a height reading. Because weather systems also change air pressure, pilots must regularly set a barometric altimeter to a known reference pressure.
Aircraft may also use radar altimeters. Instead of estimating height from atmospheric pressure, a radar altimeter sends radio waves toward the ground and measures the time taken for the signal to return. This gives the aircraft’s height above the terrain directly beneath it, rather than its elevation above mean sea level.
An altimeter is related to, but not identical with, a GPS receiver or a surveying level. GPS can calculate elevation from satellite signals, while a level compares height differences between points. “Altitude” often describes height above sea level or another datum; “height above ground level” is a separate reference used especially in aviation.