An altimeter is an instrument used to measure altitude, which is the height of an object above a reference point, usually sea level. It is commonly found in aircraft, where it helps pilots determine their elevation during flight. Altimeters can be barometric, using air pressure to calculate altitude, or GPS-based, using satellite signals for more precise measurements. In addition to aviation, altimeters are also used in various outdoor activities like hiking and mountaineering. They help adventurers track their elevation gain and navigate mountainous terrain. Devices like smartwatches and fitness trackers often include altimeter features for this purpose.