What is AGL (Above Ground Level)?

AGL (Above Ground Level) in Aviation and Drone Operations

AGL, or Above Ground Level, is a foundational concept in aviation and unmanned aircraft (drone) operations. It describes the vertical distance between an object, such as an aircraft, drone, or structure, and the terrain or surface directly beneath it. Unlike altitude measured relative to sea level (MSL), AGL is a local, terrain-dependent measurement that can change rapidly as the underlying landscape changes.

What is AGL?

AGL is defined by the International Civil Aviation Organization (ICAO) as “a height above the known runway or ground elevation.”
(ICAO Doc 9889 )

Key Points

• Local Reference: AGL is always measured from the ground immediately below the object, not from a fixed datum.
• Variable Measurement: As the terrain elevation increases or decreases, AGL changes even if the object’s MSL altitude remains constant.
• Critical for Safety: Used to ensure adequate clearance from terrain and obstacles, especially during takeoff, landing, low-level flight, and drone operations.

Example

A drone hovering 300 feet above a hillside is at 300 feet AGL, regardless of the hill’s elevation above sea level. An aircraft at 2,500 feet MSL over mountains may be only 500 feet AGL, depending on the local terrain elevation.

Where and How is AGL Used?

Aviation Operations

• Takeoff and Landing: Pilots use AGL to judge height above the runway for flaring and touchdown.
• Obstacle Avoidance: Regulations specify minimum clearances above obstacles, typically in AGL.
• Approach Procedures: Precision approaches use Decision Height (DH) in AGL.

Drone (UAS) Operations

• Regulations: Most civil aviation authorities, including the FAA, restrict drones to 400 feet AGL unless operating near a structure (FAA Part 107 ).
• Terrain Following: Modern drones use sensors to maintain a constant AGL over uneven ground.

Meteorology and Weather Reporting

• Cloud Bases: In METARs and TAFs, cloud heights are always given in feet AGL (NWS METAR Guide ).
• Visibility and Weather Phenomena: For pilots to assess VFR/IFR conditions relevant to their proximity to ground features.

Charting and Obstacle Data

• Aeronautical Charts: Obstacle heights are shown with both MSL and AGL values (e.g., “2,000 (350)” = 2,000 feet MSL, 350 feet AGL).

AGL vs. MSL: Understanding the Difference

• MSL (Mean Sea Level): A global, fixed reference used for navigation, airspace structure, and ATC.
• AGL: A local measurement, crucial for operational safety and regulatory compliance.

How is AGL Measured?

1. Radar (Radio) Altimeter

• Reads: Direct height above terrain (AGL).
• Usage: Final approach, landing, low-level operations, obstacle avoidance.
• Range: Effective up to about 2,500 feet AGL.
• ICAO Status: The only sensor required to provide direct terrain clearance measurement (ICAO Doc ).

2. GPS-based Systems

• Reads: Geometric height above the WGS-84 ellipsoid, accurate AGL requires terrain database integration.
• Usage: Modern aircraft, drones, EFBs.
• Caution: Terrain data must be current for reliable AGL.

3. Barometric Altimeter

• Reads: Altitude above MSL, based on atmospheric pressure.
• Limitation: Does not display AGL, requires subtraction of ground elevation.
• Reference: SKYbrary: Altimeter Pressure Settings

Practical Applications and Examples

Takeoff and Landing

• Decision Height (DH): The AGL at which a pilot must commit to landing or initiate a go-around on a precision approach.
• Example: If the DH is 200 feet AGL, the pilot must decide to land or go around at that height above the runway.

Obstacle Avoidance and Lighting

• Regulations: Structures exceeding 200 feet AGL must be marked and/or lit for aviation safety in the U.S. (FAR Part 77 ).
• Minimum Safe Altitudes: 500 feet AGL over uncongested areas, 1,000 feet AGL over congested areas (SKYbrary: Minimum Safe Altitude ).

Drone Operations

• Legal Limit: 400 feet AGL (in the U.S.), unless operating within 400 feet of a structure (FAA Part 107 ).
• Terrain Variability: Pilots must adjust for changing ground elevation to remain compliant.

Reading AGL and MSL on Charts

Rule of Thumb:
Numbers in parentheses = AGL, all others = MSL.

Converting AGL and MSL

Formulas:

• MSL = AGL + Ground Elevation (MSL)
• AGL = MSL - Ground Elevation (MSL)

Example Calculation:

• Airport elevation: 5,280 ft MSL
• Aircraft altitude: 5,700 ft MSL
• AGL: 5,700 - 5,280 = 420 ft AGL

Regulatory Context

• ICAO: Specifies AGL for obstacle clearance and approach procedures.
• FAA: Minimum safe altitudes, drone regulations, and obstacle lighting are based on AGL.
• Weather Reporting: Cloud bases, visibility, and certain meteorological phenomena are always referenced to AGL.

Instrument Comparison Table

*GPS alone gives geometric altitude, AGL requires terrain data.

Common Pitfalls and Best Practices

• Misreading AGL/MSL: Can lead to aircraft flying too low, causing terrain or obstacle collisions.
• Instrument Limitations: Barometric altimeter does not account for terrain changes.
• Operational Recommendations:
  • Always verify ground elevation for accurate AGL/MSL conversion.
  • Use radar altimeter or GPS with updated terrain data for low-level or mountainous flight.
  • Double-check chart symbology, parentheses indicate AGL values.

AGL in Weather Reporting: METAR and TAF

• Cloud Bases: Always reported in feet AGL (e.g., “BKN015” = broken clouds at 1,500 ft AGL).
• Importance: Provides pilots with actionable information for VFR/IFR decisions.
• Reference: NWS METAR Guide

Further Reading and References

• Height above ground level - Wikipedia
• ICAO Doc 9889
• FAA Small UAS Regulations (Part 107)
• SKYbrary: Minimum Safe Altitude
• Pilot Institute: MSL vs. AGL

Summary

AGL (Above Ground Level) is a dynamic, local measurement vital for aviation and drone operations. It ensures safe clearance from terrain and obstacles, supports regulatory compliance, and provides critical data in weather reporting. Understanding the distinction between AGL and MSL, how to measure and interpret them, and their respective regulatory contexts is essential for pilots, drone operators, and all aviation professionals.