What is true altitude in aviation?

• A. The altitude above ground level (AGL).
• B. The vertical distance of the aircraft above sea level, often expressed as feet above mean sea level (MSL).
• C. The indicated altitude corrected for temperature.
• D. The altitude at which the aircraft would level off if the pressure were standard.

Answer: (B)

True altitude is the height of the aircraft above mean sea level. This is the reference used for obstacle clearance and IFR planning, regardless of what the ground underneath looks like. It differs from altitude above the ground (AGL), which is measured relative to the terrain directly beneath the aircraft, not sea level. It also differs from the altitude shown on your altimeter (indicated altitude), which can be affected by local pressure and temperature; true altitude can be higher or lower than indicated depending on atmospheric conditions. In colder-than-standard air, true altitude tends to be lower than indicated; in warmer-than-standard air, it tends to be higher. The option describing the altitude you would reach if the pressure were standard refers to pressure altitude, not true altitude.