Obstacle heights: MSL vs AGL
Updated 2026-10-16. Independent guide, not affiliated with the FAA.
Every charted obstacle shows two numbers. The bold one is the height of the top above mean sea level (MSL). The number in parentheses is its height above the ground (AGL). The difference is the ground elevation at the base.
Example: 1749 (1102) means the top is at 1,749 ft MSL and the tower is 1,102 ft tall; the ground is about 647 ft MSL.
The 400 ft structure rule
Under 14 CFR 107.51(b), you may fly higher than 400 ft AGL only while within a 400 ft radius of a structure and no higher than 400 ft above its uppermost limit. For a 1,102 ft AGL tower that is 1,502 ft AGL. Adding 400 to the MSL value is the classic trap.
Airspace still applies: a tall tower inside a Class E surface area or under a Class B shelf can put you in controlled airspace.
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- Which obstacle number is AGL?
- The number in parentheses. The bold number above it is MSL.
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