A measurement of an aircraft’s height directly above the terrain beneath it. Unlike MSL (Mean Sea Level), which references a fixed global baseline, AGL represents the aircraft’s true altitude relative to the surface at its current position.
Key characteristics:
- Local reference: AGL changes constantly as terrain elevation rises or falls beneath the aircraft.
- Measured by radar altimeter: Many aircraft use a radio (radar) altimeter to display precise AGL values at low altitude, especially below 2,500 ft.
- Critical for safety: Used for obstacle clearance, terrain avoidance, helicopter operations, and approach minima.
- Approach usage: Decision Height (DH) on precision approaches is expressed in AGL, not MSL.
- Low-level flight: Pilots flying nap-of-the-earth or CAS missions rely heavily on AGL to avoid terrain or structures.
Application in DCS World
- DCS models both AGL and MSL: Aircraft with radar altimeters (A-10C, F-16C, F/A-18C, helicopters) can display real-time height above terrain.
- Terrain-induced fluctuations occur: Rapid elevation changes can make AGL values jump quickly, simulating the real challenges of low-level flight.
Cadets should train to use radar altimeter data when flying low-level or on short final, and learn to distinguish when to reference AGL (terrain) versus MSL (charts and ATC).