Height above a reference point sounds simple until you ask which reference. Altitude can mean height above sea level, above the ground, or above a standard pressure surface. Aviation uses all three depending on the situation.

True altitude is the actual height above mean sea level. Absolute altitude is height above the terrain directly below. Indicated altitude is what the altimeter shows. Pressure altitude uses a standard datum and matters for aircraft performance calculations.

Why it matters

• Engine performance drops as air thins with altitude
• True airspeed rises with altitude for the same indicated speed
• Terrain clearance depends on accurate absolute altitude
• Traffic separation relies on a common pressure reference

Mountain flying punishes altitude errors. A 500-foot mistake over flat terrain is an inconvenience. Over a ridge in cloud, it can be fatal.