Definition
Altitude measured as the vertical distance between the aircraft and the terrain directly below it. Because the ground elevation changes as the aircraft moves, AGL altitude changes with the terrain even when the aircraft holds a constant altitude above sea level.
Plain English
How high the airplane is above the ground right under it. If the aircraft flies over a hill, the height above the ground gets smaller, even if the aircraft hasn't climbed or descended.
Context Anchor
Pilots see AGL when a height is being compared to the ground, such as runway height, obstacle height, cloud height, or low-altitude flying.
Derivation
AGL stands for 'Above Ground Level.' The phrase is literal: it specifies that the altitude is measured from the ground beneath the aircraft, not from sea level. This distinction matters because most aircraft instruments display altitude above sea level, not above the actual terrain.
Why Pilots Care
Ensures safe clearance from terrain and obstacles during flight near the ground.
Grounding Statement
If the airplane stays level while the ground rises underneath it, its altitude (AGL) gets smaller.
Intuition Check
Do not assume altitude always means height above sea level. With altitude (AGL), the reference is the ground below, so the number can change as the terrain rises or falls.
Example Sentence 1
The standard traffic pattern is flown at 1,000 feet AGL, so over an airport with a field elevation of 500 feet, the altimeter would read about 1,500 feet.
Example Sentence 2
AGL altitude helps confirm safe clearance over rising terrain during a low approach.