What does an altimeter measure?

An altimeter measures the altitude of the aircraft above sea level by detecting changes in atmospheric pressure. As an aircraft ascends, the external air pressure decreases, and this change is translated into altitude by the altimeter, allowing pilots to determine their elevation relative to sea level. This measurement is crucial for navigation and ensuring safe separation from terrain and other aircraft. Understanding this function is vital for flight operations, particularly when maintaining altitude and during takeoff and landing phases. The other options do not accurately describe the role of an altimeter, which is solely focused on measuring altitude based on pressure variations.