AGL (Above Ground Level) measures how high you are above the terrain directly beneath you, while MSL (Mean Sea Level) measures your altitude relative to the global average sea level. For drone pilots, the distinction matters every flight — FAA regulations set limits in AGL, airspace boundaries are defined in MSL, and confusing the two can put you in controlled airspace without realizing it.
What Is AGL (Above Ground Level)?
AGL is a dynamic measurement — it tells you how far your drone is from the ground directly below it, whatever terrain that happens to be. If you hold a steady altitude while flying from a valley toward a hill, your AGL decreases even though your drone has not climbed. AGL is measured in real time using radar altimeters, LiDAR sensors, or onboard proximity sensors built into drone parts designed for low-altitude operations.
When to Use AGL
Use AGL whenever your primary concern is proximity to the ground or obstacles below you:
- FAA Part 107 altitude limit: 400 feet AGL in Class G uncontrolled airspace
- Low-altitude operations: Inspection, agricultural, and terrain-following missions where ground clearance is what matters
- Obstacle avoidance: Staying clear of trees, buildings, and structures
- Airspace boundaries: Class G and Class E transition airspace are set at 700 ft AGL and 1,200 ft AGL, respectively
What Is MSL (Mean Sea Level)?
MSL is a fixed, universal reference — the average height of the ocean’s surface used as a global standard for expressing altitude. Unlike AGL, MSL does not change based on the terrain below: a drone at 1,000 feet MSL over a mountain and one at 1,000 feet MSL over the ocean are at the exact same absolute altitude. Most GPS/GNSS modules report elevation in MSL by default, making it the standard altitude reference for navigation and airspace compliance.
When to Use MSL
Use MSL whenever you need a fixed, consistent altitude value:
- Aeronautical charts and sectional maps: All terrain elevations, obstacle heights, and airport elevations are in MSL
- Controlled airspace boundaries: Class B, C, and D airspace floors and ceilings are defined in MSL
- Mountainous terrain: MSL gives a stable reference that does not fluctuate with changing ground below
- Weather reports (METARs) and GPS altitude readings: Both use MSL as the reference
What Is the Key Difference Between AGL and MSL?
The simplest way to understand it: AGL tells you how far you are from the ground below you; MSL tells you how far you are from sea level.
| AGL | MSL | |
| Reference point | Ground directly below | Global mean sea level |
| Dynamic or fixed? | Dynamic — changes with terrain | Fixed universal reference |
| Primary use | Drone limits, terrain clearance | Airspace boundaries, charts |
| FAA drone limit | 400 ft AGL (Class G) | Varies by airspace class |
The practical consequence is that a drone can be within its 400-foot AGL limit and simultaneously above an MSL-defined airspace floor. Neither measurement alone tells the full story — a complete picture requires both.
How Do You Convert Between AGL and MSL?
The conversion is straightforward once you know the terrain elevation at your location:
MSL = AGL + Terrain Elevation
AGL = MSL − Terrain Elevation
A concrete example: you are flying at 300 feet AGL over terrain that sits at 600 feet MSL. Your drone’s MSL altitude is 900 feet. If there is a Class B airspace floor at 800 feet MSL in your area, you have just entered controlled airspace — even though your drone is only 300 feet above the ground.
- AGL to MSL: 350 ft AGL + 800 ft terrain elevation = 1,150 ft MSL
- MSL to AGL: 1,150 ft MSL − 800 ft terrain elevation = 350 ft AGL
Terrain elevation data is available from topographic maps, sectional charts, and apps like Google Earth. Most drone flight planning tools incorporate it automatically and display both AGL and MSL values in real time.
Why Does This Matter for Drone Pilots?
Several common situations make the AGL/MSL distinction operationally critical:
Flying in Mountainous or Elevated Terrain
If you launch from a hilltop at 1,500 feet MSL and fly at 400 feet AGL, your drone is at 1,900 feet MSL — potentially conflicting with manned aircraft in the area. A properly configured flight controller that integrates both GPS altitude and barometric data is essential for maintaining accurate MSL awareness in elevated terrain, not just height above the launch point.
Airspace Class Boundaries
Class B, C, and D airspace floors are always expressed in MSL. If the floor of Class B airspace is 1,500 feet MSL and your local terrain sits at 1,200 feet MSL, you only have 300 feet of AGL clearance before entering controlled airspace — not the standard 400 feet. Misreading this is one of the most common regulatory errors drone pilots make.
BVLOS and Long-Range Operations
BVLOS missions — pipeline inspection, survey, agricultural runs — cross terrain with significant elevation changes. A drone maintaining fixed MSL altitude will have variable AGL clearance across the route, creating collision risk in lower terrain sections. The FAA’s Part 107 regulations require operators to maintain awareness of both AGL and MSL altitude throughout any BVLOS operation, and flight planning software that accounts for terrain changes along the entire route is considered best practice.
How Your Drone Reports Altitude
Most consumer drones report altitude relative to the home point — neither true AGL nor MSL. This is fine for simple flat-ground flights, but misleading when flying from an elevated position or over terrain that drops below the launch elevation. Knowing what your aircraft is actually measuring is an important part of pre-flight planning.
Frequently Asked Questions
Which altitude does the FAA 400-foot drone limit use?
The FAA Part 107 limit of 400 feet applies in AGL — specifically, 400 feet above the ground surface or the tallest structure within 400 feet horizontally of the drone.
Does my drone’s altimeter measure AGL or MSL?
Most consumer drones measure altitude relative to the home point, which approximates AGL on flat ground. GPS altitude readings reflect MSL. Check your aircraft’s documentation to confirm what each readout represents.
Are airspace class boundaries AGL or MSL?
Class B, C, and D airspace floors and ceilings are defined in MSL. Class G and Class E transition airspace boundaries are defined in AGL.
Why does my drone show a different altitude than my GPS app?
Your drone reports height above the home point; your GPS app typically reports MSL altitude from satellite data. The difference equals the terrain elevation at your launch location.
Does AGL change if I fly over a building?
Yes. AGL is always measured from the surface directly below the drone. Flying over a building shifts the ground reference to the rooftop, so your AGL drops accordingly.
Conclusion
AGL tells you how far you are from the ground below; MSL tells you where you sit in the global airspace system. Knowing which one applies — and how to convert between them — is a foundational skill for any drone pilot operating beyond flat, open terrain. YAHREE manufactures the GPS modules, motors, and flight control components that give professional UAV operators the positional accuracy they need to fly confidently across varied environments — contact us to learn more.
