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Airspace and charts
MSL vs AGL: altitude math on a sectional chart
Part 107 limits altitude above ground level (AGL), but charts print most altitudes in feet MSL. Altitude MSL equals ground elevation plus height AGL, so a drone 400 ft above a 400 ft field is at 800 ft MSL.
What is the difference between MSL and AGL?
MSL is height above mean sea level and AGL is height above the ground directly beneath. On the chart, airspace shelves and Class D ceilings use MSL, airport elevations are MSL, and obstruction labels give both. The 400 ft limit in 14 CFR 107.51(b) is AGL.
How do I read an obstruction label?
The big number is the top of the obstruction in feet MSL and the number in parentheses is its height above ground, so 1049 (980) is 1,049 ft MSL and 980 ft AGL. The difference, 69 ft, is the ground elevation at the base. Within a 400 ft radius of a structure you may fly up to 400 ft above its top.
How do I work out whether I am inside a shelf?
| Step | Example |
|---|---|
| Find the ground elevation at the site | Airport at 400 ft MSL |
| Add the planned height AGL | 400 + 400 = 800 ft MSL |
| Compare with the airspace floor and ceiling in MSL | Class D ceiling [29] is 2,900 ft MSL, so 800 ft is inside |
| Decide | Inside Class D: prior ATC authorization required |
What is a maximum elevation figure?
Each quadrangle on a sectional, bounded by ticked latitude and longitude lines 30 minutes on each side, shows a maximum elevation figure (MEF) in thousands and hundreds of feet MSL for the highest terrain or obstacle in that area, with a safety allowance added. It is a quick check, not a planning substitute.
Sources (checked 2026-11-12): FAA Aeronautical Chart Users' Guide.
What about mountains and uneven ground?
The 400 ft limit follows the ground beneath the aircraft, so AGL changes as terrain rises and falls. Always convert using the local ground elevation, not just the airport's.
Free practice questions on this topic
6 original questions from the Sectional Cue bank. Try each one, then open the answer.
Class C ceiling calculation
1. A Class C primary airport sits at 1,100 ft MSL and its airspace follows the usual design. About what ceiling would the chart show?
- 40, meaning 4,000 ft MSL
- 25, meaning 2,500 ft MSL
- 51, meaning 5,100 ft MSL
Show answer and explanation
C. 51, meaning 5,100 ft MSL
Class C normally extends to 4,000 ft above the airport elevation, so 1,100 plus 4,000 gives 5,100 ft MSL, printed as 51. A label of 40 would ignore the airport elevation. A ceiling of 2,500 ft above the airport is the usual Class D design, not Class C.
Source: AIM 3-2-4; FAA Aeronautical Chart Users' Guide (July 2026)
MEF quadrangles
2. On a sectional chart, what area does each maximum elevation figure cover?
- A circle of 10 NM radius around the figure
- A quadrangle bounded by ticked lines of latitude and longitude, 30 minutes on each side
- The whole county in which the figure appears
Show answer and explanation
B. A quadrangle bounded by ticked lines of latitude and longitude, 30 minutes on each side
Each maximum elevation figure applies to the quadrangle bounded by ticked latitude and longitude lines, 30 minutes by 30 minutes on a sectional. It does not describe a 10 NM circle. Counties are not used for chart elevation figures.
Source: FAA Aeronautical Chart Users' Guide (July 2026)
Obstruction height
3. In Figure 20, north of area 5 and northeast of Harbinger, a lighted obstruction is labeled 1049 with (1041) below it. How tall is it above the ground? (uses Figure 20, open the free chart lesson)
- 1,041 ft AGL, with its top at 1,049 ft MSL
- 1,049 ft AGL, with its top at 1,041 ft MSL
- 8 ft AGL, the difference between the two numbers
Show answer and explanation
A. 1,041 ft AGL, with its top at 1,049 ft MSL
On a sectional the upper number is the top of the obstruction in feet MSL and the number in parentheses is its height above ground. So the structure is 1,041 ft AGL with its top at 1,049 ft MSL. Swapping the two numbers is a common misreading, and the 8 ft difference is the ground elevation at its base, not its height.
Source: FAA-CT-8080-2H, Figure 20; Legend 1; 14 CFR 107.51
Ground elevation from obstruction
4. In Figure 26, the obstruction just south of area 3 reads 1843 (299). What is the ground elevation at its base? (uses Figure 26, open the free chart lesson)
- 1,843 ft MSL
- 2,142 ft MSL
- 1,544 ft MSL
Show answer and explanation
C. 1,544 ft MSL
The bold figure is the top in MSL and the figure in parentheses is the height above ground, so the base is 1,843 minus 299, or 1,544 ft MSL. 1,843 ft MSL is the top of the structure. Adding the two numbers, 2,142 ft, has no meaning.
Source: FAA-CT-8080-2H, Figure 26; Chart Users' Guide
Obstruction clearance
5. In Figure 69, the obstruction just east of area 4 reads 291 (250). If a drone passes over it at 400 ft AGL, about how much vertical clearance does it have above the top? (uses Figure 69, open the free chart lesson)
- About 150 ft
- About 109 ft
- About 400 ft
Show answer and explanation
A. About 150 ft
The figure in parentheses, 250, is the height above ground, so a drone at 400 ft AGL clears the top by about 150 ft. Using 291 as an AGL height gives 109 ft, but 291 is the top in feet MSL. 400 ft ignores the structure entirely. Guy wires may extend well beyond the tower, so lateral clearance matters too.
Source: FAA-CT-8080-2H, Figure 69; Chart Users' Guide; 14 CFR 107.51
Obstruction height
6. In Figure 69, an obstruction near Sinton reads 1048 (1000). How tall is it above the ground? (uses Figure 69, open the free chart lesson)
- 1,000 ft AGL
- 1,048 ft AGL
- 48 ft AGL
Show answer and explanation
A. 1,000 ft AGL
The number in parentheses is the height above ground, so the structure is 1,000 ft AGL. 1,048 is the elevation of its top in feet MSL. 48 ft is the ground elevation at its base (1,048 minus 1,000), not the height of the structure.
Source: FAA-CT-8080-2H, Figure 69; Chart Users' Guide; 14 CFR 107.51
Related guides
- How to read airspace classes on a sectional chart
- Part 107 operating rules: altitude, speed, visibility and clouds
- Sectional chart symbols a remote pilot must know
- All Part 107 guides
Study the charts on your phone
Sectional Cue has 1150+ original questions with explanations, 27 FAA figures you can tap to read, and timed 60-question rehearsals. About 152 questions and three chart lessons are free.
Sources
- FAA Aeronautical Chart Users' Guide, checked 2026-11-12
- 14 CFR 107.51 (eCFR), checked 2026-11-12
How this page was made: written by Certloom Learning from the sources above, with the rules checked against the current regulation text. Practice items are original questions from the app's reviewed bank. Spot an error? certloomlearning@gmail.com.