An altimeter is a device used to measure altitude, or how high something is above a certain reference point, usually sea level. It works by detecting changes in air pressure; as altitude increases, air pressure decreases. There are two main types of altimeters: barometric altimeters, which use air pressure, and GPS altimeters, which rely on signals from satellites. Altimeters are commonly used in aviation, allowing pilots to know their altitude during flight. They are also found in outdoor activities like hiking and climbing, helping adventurers track their elevation changes. Accurate altitude readings are essential for safety and navigation in these activities.