An altimeter is the instrument used to measure altitude. It works by determining the height above a reference level, often sea level, based on atmospheric pressure. In aviation, altimeters are crucial for pilots to maintain safe flight altitude and navigate effectively. Typically, altimeters can be barometric, relying on pressure changes, or GPS-based, using satellite signals to calculate height. Accurate altitude readings are essential for aviation safety, mountaineering, and various outdoor activities. In addition to aviation, altimeters are used in numerous fields including meteorology, hiking, and various scientific research applications.