Key Takeaways
- AGL measures altitude above the ground directly below you; MSL measures altitude from average sea level, a fixed reference point.
- The FAA’s Part 107 400-foot altitude limit is measured in AGL, not MSL.
- ATC assigns altitudes and airspace boundaries, including Class B, C, and D, in MSL so all aircraft share a common reference.
- Class G airspace and the Class E transition layer typically begin at 700 ft or 1,200 ft AGL, with airspace below those floors remaining Class G unless otherwise designated.
- Most drone flight controllers display altitude in AGL relative to the takeoff or home point (varies by system), while sectional charts, topographic maps, and apps like Google Earth typically reference MSL or terrain elevation.
- Understanding which altitude reference you’re using helps prevent airspace violations and improves situational awareness.
What Is AGL (Above Ground Level)?
AGL, or Above Ground Level, is the height of an aircraft or drone measured directly above the terrain beneath it. Because AGL is tied to the ground rather than a fixed reference point, it changes dynamically as terrain rises and falls. A drone flying at 400 ft AGL over a valley is physically higher in the sky than the same drone at 400 ft AGL over a hilltop, even though the instrument reading is identical. This distinction matters for every drone pilot: the FAA’s Part 107 400-foot altitude limit is measured in AGL, not MSL, so your legal ceiling moves with the terrain below you rather than staying fixed at one altitude.
What Is MSL (Mean Sea Level)?
MSL, or Mean Sea Level, is altitude measured from the average surface of the Earth’s oceans, a consistent baseline used across all of aviation. Unlike AGL, MSL doesn’t change based on what’s underneath the aircraft; it provides a fixed reference point regardless of terrain, buildings, or bodies of water below. That consistency is exactly why aviation relies on it: ATC assigns altitudes in MSL so that every aircraft in the sky, no matter what’s beneath them, can share a common language and maintain safe separation. Airport elevations, chart altitudes, and most published aviation data are also expressed in MSL for the same reason.
AGL vs MSL — How They Work Together (Practical Example)
The two measurements work together any time you need both your legal altitude and your position on a chart. Classic example: launch a drone from a hilltop at 5,000 ft MSL and fly up to 200 ft AGL, and your drone is actually at 5,200 ft MSL, even though your controller still reads 200 feet.
Terrain changes the math further. Flying at 300 ft AGL over flat desert keeps you at a steady altitude the whole flight; the same 300 ft AGL over rolling hills means constantly climbing and descending to stay at that height above the ground. AGL is what keeps you legal and safe in both cases, while MSL is what shows up on a chart.
One more distinction worth knowing: Google Earth and most flight planning apps typically display MSL, while your drone controller’s altitude readout is AGL.
AGL vs MSL on Sectional Charts
Sectional charts follow one consistent rule: any altitude number NOT in parentheses is MSL, and any number IN parentheses is AGL. Obstruction towers are the clearest example: a tower marked “1049 (1036)” stands 1,049 ft MSL and 1,036 ft AGL, meaning the terrain beneath it sits at roughly 13 ft MSL.
Airspace boundaries follow the same MSL convention. The floors and ceilings of Class B, Class C, and Class D airspace are published in Mean Sea Level (MSL) so pilots and air traffic control can reference the same altitude system. One important exception is the Class E transition area, which often begins at 700 feet AGL or 1,200 feet AGL, with the airspace below remaining Class G unless otherwise designated. On sectional charts, these Class E transition areas are depicted with fuzzy magenta shading for 700-foot AGL floors and fuzzy blue shading for 1,200-foot AGL floors. Understanding the difference between AGL and MSL, and knowing when each is used, is a concept that appears frequently on the Part 107 knowledge test, making it well worth committing to memory before exam day.
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