On instrument approach plates, obstacles are measured relative to which altitude reference?

On instrument approach plates, obstacles are measured relative to which altitude reference?

• A. Pressure Altitude
• B. Above Ground Level
• C. Above Mean Sea Level
• D. Mean Sea Level

Answer: (D)

On instrument approach plates, obstacle heights are referenced to mean sea level. This fixed reference matches the altitudes published along the approach (the minima, step-down fixes, and missed-approach altitudes), which are all in feet MSL. Using mean sea level ensures a consistent, universal basis for evaluating vertical clearance regardless of the airport’s elevation or surrounding terrain. If obstacles were measured in height above ground level, the same obstacle could require different clearances at different airports, creating confusion. Pressure altitude is used for flight levels, not for charted obstacle measurements, and above ground level would vary with location, making it unreliable for approach planning.

On instrument approach plates, obstacle heights are referenced to mean sea level. This fixed reference matches the altitudes published along the approach (the minima, step-down fixes, and missed-approach altitudes), which are all in feet MSL. Using mean sea level ensures a consistent, universal basis for evaluating vertical clearance regardless of the airport’s elevation or surrounding terrain. If obstacles were measured in height above ground level, the same obstacle could require different clearances at different airports, creating confusion. Pressure altitude is used for flight levels, not for charted obstacle measurements, and above ground level would vary with location, making it unreliable for approach planning.