AGL vs MSL for Part 107: Convert Chart Altitudes

To convert AGL to MSL, add the ground elevation at the flight location: altitude MSL = ground elevation MSL + height AGL. To convert MSL to AGL, subtract: height AGL = altitude MSL − ground elevation MSL. Use the same location and units in both calculations. AGL is measured above ground; MSL is measured from mean sea level (FAA AIM, Pilot/Controller Glossary, “Altitude”).

This guide uses original learning examples, not FAA test questions. The numbers in the teaching diagram are hypothetical; FAA sources establish the altitude references and chart notation. For the full legend and other symbols, use the Part 107 sectional chart guide.

How do you convert AGL to MSL on a Part 107 chart question?

Add the height AGL to the ground elevation MSL directly beneath the aircraft. First identify the flight location, then check that the ground elevation and the requested height use feet. For example, over ground at 900 feet MSL, a drone at 200 feet AGL is at 1,100 feet MSL: 900 + 200 = 1,100. These references follow the FAA AIM glossary definition of altitude.

In reverse, a drone at 1,100 feet MSL over the same ground is 200 feet AGL: 1,100 − 900 = 200. If the question does not establish the ground elevation at the flight location, you cannot calculate an exact AGL height from an MSL altitude alone.

Write the reference beside the result. A bare answer of “1,100 feet” leaves out whether the question asks for height above ground or altitude above sea level.

What happens to AGL height when the ground elevation changes?

At a constant MSL altitude, the aircraft’s AGL height decreases over higher ground and increases over lower ground. AGL uses the ground at each location, so an elevation from the previous location cannot be carried into the next calculation (FAA AIM, Pilot/Controller Glossary, “Altitude”).

Original teaching diagram, not a navigation chart. Locations A and B and all elevations are hypothetical. Select the image to enlarge it.

In the diagram, a drone stays at 1,100 feet MSL. At location A, the ground is 900 feet MSL, giving 200 feet AGL. At location B, the ground is 1,000 feet MSL, giving 100 feet AGL. Keeping the same altitude above sea level does not keep the same height above terrain.

Which sectional chart altitudes use AGL and which use MSL?

Class B, C and D altitude numbers use MSL; the common Class E floors of 700 or 1,200 feet use AGL. Obstacle tops and airport elevations use MSL, while the height in parentheses beside an obstacle uses AGL. Read each number’s notation instead of assuming all chart altitudes have the same reference (FAA Chart Users’ Guide, pp. 11, 15–17).

The table follows Chart Users’ Guide, pp. 11–17. Some Class E floors are labeled with other values, including MSL labels. A dashed magenta boundary depicts Class E beginning at the surface, not at 700 feet AGL (pp. 17, 27).

How do you convert a Class E floor of 700 feet AGL to MSL?

Add 700 feet to the local ground elevation MSL. Over ground at 900 feet MSL, that Class E floor is 1,600 feet MSL: 900 + 700 = 1,600. Over ground at 1,000 feet MSL, the same 700-foot AGL floor is 1,700 feet MSL. These are original calculations using the Class E floor reference defined in the FAA Chart Users’ Guide, p. 17.

Do not apply the omitted-zero rule for a Class B or C number to magenta shading. Shading that represents 700 feet AGL describes a height above the terrain; it does not establish a fixed 700-foot MSL altitude. A Class E floor above the surface also differs from an airport-designated Class E surface area. The Part 107 airspace guide explains those boundaries and § 107.41 authorization conditions.

Can you use the nearest airport elevation to convert AGL to MSL?

Not automatically: a nearby airport’s elevation does not establish the ground elevation beneath the drone. The FAA defines airport elevation as the highest point on the usable portion of the landing areas. Surrounding terrain may have a different elevation (FAA Chart Users’ Guide, p. 15).

Use the ground elevation that the question provides or establishes for its flight location. If a learning example explicitly says the ground there has the same elevation as the airport, that stated condition allows the substitution. Merely being near the airport does not.

For example, if an airport is 900 feet MSL but the question gives the drone’s ground location as 1,000 feet MSL, a drone at 1,100 feet MSL is 100 feet AGL, not 200 feet AGL. The location-specific value controls the subtraction. Chart contours show lines of equal elevation, but the FAA’s standard sectional contour spacing does not supply an exact value for every point between them (Chart Users’ Guide, p. 10).

Can an obstacle label or MEF give local ground elevation?

An obstacle’s two altitude numbers can give its own base elevation; MEF cannot give the ground elevation at an arbitrary flight location. An obstacle’s MSL top minus its parenthesized AGL height gives the MSL elevation of that obstacle’s base. That does not establish the terrain beneath a drone elsewhere (FAA Chart Users’ Guide, p. 11).

For the FAA legend’s 2049 (1149) obstacle, the base is 900 feet MSL: 2,049 − 1,149 = 900. The upper number is its top elevation, not its height above ground (FAA-CT-8080-2H, Legend 1). The sectional chart guide’s obstacle diagram shows those references.

An MEF represents a 30-minute latitude by 30-minute longitude quadrant’s highest terrain or vertical obstacle, with calculation allowances and upward rounding. For example, an MEF of 2,800 feet MSL does not say the ground beneath the drone is 2,800 feet MSL. Subtracting it from the drone’s altitude does not calculate local AGL height (FAA Chart Users’ Guide, pp. 12–13).

Does an AGL/MSL conversion establish permission to fly?

No. Converting altitude establishes a height reference; it does not establish airspace authorization or satisfy every Part 107 operating rule. Under § 107.51(b), the aircraft generally must remain at or below 400 feet AGL. The structure exception requires it to remain within a 400-foot radius of a structure and no higher than 400 feet above the structure’s immediate uppermost limit.

An MSL altitude below a Class B or C shelf says only that the drone is below that shelf. Identify the airspace at the actual location and altitude: Class B, C or D, or the lateral boundaries of a Class E surface area designated for an airport, require prior ATC authorization under § 107.41. For chart calculations and the other limits, continue with the sectional chart guide and Part 107 rules.

Free AGL/MSL practice with worked answers

These original checks practice adding local ground elevation to AGL, subtracting it from MSL, and recognizing an AGL airspace floor. All numbers are hypothetical. The formulas use the FAA AIM altitude definitions; the Class E reference comes from the Chart Users’ Guide, p. 17.

Apply the units to real, dated FAA excerpts in the free sectional chart practice. For airport frequency questions, use the CTAF and UNICOM guide.