What does indicated altitude measure?

Indicated altitude measures the altitude displayed by the aircraft's altimeter when it is set to the current local atmospheric pressure, expressed in feet above a reference point known as the standard datum plane. This measurement directly reflects the pressure changes in the atmosphere.

The altimeter works by measuring the weight of the air above the aircraft through barometric pressure. When set to the local pressure, the altimeter shows the altitude based solely on this pressure information, which is why the correct answer focuses on the altitude indicated by the altimeter. This altitude may not necessarily reflect the actual height above sea level or above the ground due to variations in atmospheric pressure or temperature.

Understanding indicated altitude is crucial for pilots as it is used primarily for navigation and maintaining a safe flying altitude, often indicated on the cockpit instruments in flight.