What instrument is used to measure altitude?

The altimeter is the instrument specifically designed to measure altitude. It operates by comparing the atmospheric pressure at a given altitude to a pre-measured standard air pressure at sea level. As an aircraft climbs, the air pressure outside decreases, which the altimeter translates into altitude. This makes it essential for pilots to maintain awareness of their altitude during flight, especially for avoiding obstacles and during landings.

The other instruments serve different purposes: a speedometer measures the aircraft's speed, a vertical speed indicator shows the rate of climb or descent, and a heading indicator displays the aircraft's current heading relative to magnetic north. While all these instruments are crucial for safe and effective flying, the altimeter is uniquely focused on determining how high the aircraft is flying.