Mean Sea Level is used as the reference for altitudes.

• A. Maximum Sea Level
• B. Local Sea Level
• C. Mean Low Level
• D. Mean Sea Level

Answer: (D)

The concept hinges on using a universal, stable reference plane for elevations. Mean Sea Level provides that baseline because it’s the long-term average height of the sea surface, smoothed to remove daily tides and other short-term fluctuations. Local sea level varies with location and time, so it wouldn’t give a consistent standard for all aviation charts, airports, and obstacles. By publishing elevations in feet above this mean level, pilots have a common reference across the globe, which is essential for planning, navigation, and ensuring obstacle clearance.

In practice, when you’re flying, altimeter settings (the QNH) are used so the instrument shows height above mean sea level. If you set a local QNH, the altitude readout corresponds to AMSL. That’s why Mean Sea Level is the reference used for altitudes. The other options aren’t standard aviation references: Maximum Sea Level and Mean Low Level aren’t used as global altitude baselines, and Local Sea Level isn’t a fixed, universal datum.