What is true altitude?

• A. The vertical distance of the aircraft above sea level.
• B. The vertical distance of the aircraft above the surface.
• C. The vertical distance of the aircraft above the horizon.
• D. The vertical distance of the aircraft above the earth's center.

Answer: (A)

True altitude is the vertical distance of the aircraft above mean sea level. This fixed reference surface lets you know how high you are relative to the global standard sea level, which is essential for obstacle clearance and altitude planning. The distance above the surface describes height above the terrain (ground level), not above mean sea level. Height above the horizon isn’t a defined altitude reference, and distance above the Earth’s center would be a geocentric measure, not used for flight. In practice, you set the altimeter to the local pressure so the reading corresponds to altitude above mean sea level under standard conditions, with small adjustments for nonstandard temperature as needed.