The device used to measure altitude is called an altimeter. Altimeters are commonly used in aviation to help pilots determine their height above sea level. There are two main types of altimeters: barometric altimeters, which measure altitude based on atmospheric pressure, and GPS altimeters, which determine altitude using satellite signals. Barometric altimeters function by detecting changes in air pressure; as altitude increases, air pressure decreases. In contrast, GPS altimeters rely on signals from Global Positioning System satellites to provide accurate altitude readings. Both types of altimeters are crucial for navigation and ensuring safety during flights and other activities in varying terrains.