Apache AH-64E ASE Practice Test

Altitude in aviation is typically referenced to mean sea level. This provides a stable, universal vertical reference for navigation, obstacle clearance, and sensor geometry, regardless of the terrain beneath. If altitude were shown above ground level, it would change with every hill and valley you fly over, making it hard to maintain consistent situation awareness and perform precise calculations for the detecting platform. GPS altitude exists, but cockpit displays and flight planning conventions rely on barometric altitude referenced to mean sea level. So the detecting platform’s altitude is shown in MSL (mean sea level). AMSL basically means the same thing, but MSL is the common term used here.