5 Types of Altitude in Aviation
Pilots use the word altitude for several different ideas. The easiest way to keep them straight is to ask one question: “Altitude relative to what?” The altimeter, mean sea level, the ground, standard pressure, and standard density are five different references.
The quickest comparison
| Type | Reference | Think of it as | Common pilot use |
|---|---|---|---|
| Indicated | Altimeter + current setting | What the instrument shows | Normal altitude reference in flight when properly set |
| True | Mean sea level | Actual height above MSL | Terrain/obstacle relationship and altitude concepts |
| Absolute | Ground below | Actual height AGL | Height above terrain |
| Pressure | Standard datum / 29.92 inHg | Pressure expressed as altitude | Performance inputs and flight levels |
| Density | Standard-atmosphere density | Air density expressed as altitude | Understanding aircraft/engine/propeller performance |
A mental model that actually works
Pressure and temperature describe the air around the airplane.
Ask: “At what ISA altitude would this pressure normally exist?”
Ask: “At what ISA altitude would this air density normally exist?”
That is why pressure altitude and density altitude are useful performance concepts: they translate invisible atmospheric properties into an altitude scale pilots and performance charts can work with.
Indicated altitude
Indicated altitude is what you read on the altimeter after setting the current altimeter setting. It is the cockpit number pilots normally use for altitude references below the flight levels when the instrument and setting are appropriate.
Indicated altitude is not automatically identical to true altitude. Nonstandard atmospheric conditions and instrument/system error can create differences, which is why altimetry procedures matter.
True altitude
True altitude is the aircraft's actual vertical distance above mean sea level. Airport elevations, terrain elevations, and obstacle elevations are generally published relative to MSL, so true altitude is the physical MSL relationship we ultimately care about for terrain and obstacle clearance.
Do not confuse “true altitude” with “true airspeed.” They are unrelated uses of the word true.
Absolute altitude
Absolute altitude is the aircraft's height above the terrain directly below. In everyday pilot language, this is the AGL idea. If you are at 5,500 feet MSL over terrain at 4,000 feet MSL, your height above that terrain is roughly 1,500 feet AGL.
Terrain changes, so absolute altitude can change even while an airplane holds the same MSL altitude.
Pressure altitude
Pressure altitude is pressure expressed as an altitude in the standard atmosphere. Set an altimeter to 29.92 inHg and the indicated value is pressure altitude. A common training estimate is:
Performance charts often use pressure altitude because it provides a standard pressure reference independent of the day's local altimeter setting.
Density altitude
Density altitude is air density expressed as an equivalent altitude in the standard atmosphere. A common training description is pressure altitude corrected for nonstandard temperature.
High density altitude means the air is less dense. That can reduce engine power, propeller effectiveness, lift for a given true airspeed, and climb performance. The exact performance effect is aircraft-specific, so density altitude is a warning/context number—not a substitute for the approved performance charts.
Why pressure altitude and density altitude can be different
Pressure altitude answers a pressure question. Density altitude answers a density question. Temperature changes density even when pressure altitude stays the same, so a hot day can make density altitude much higher than pressure altitude.
Under standard-atmosphere conditions, pressure altitude and density altitude line up. Move away from standard temperature and they separate.
A 5,000-foot airport on a hot day
Suppose an airport is 5,000 feet MSL, the altimeter is 30.12 inHg, and OAT is 30°C. The quick pressure-altitude estimate is about 4,800 feet. ISA temperature near 4,800 feet is roughly 5°C, so the day is about ISA +25. The common 120-ft/°C shortcut gives a density altitude around 7,800 feet.
So one location can simultaneously have:
- Field elevation: 5,000 ft MSL
- Pressure altitude: about 4,800 ft
- Density altitude: about 7,800 ft
Those are not contradictory numbers. They are answering different questions.
Indicated = instrument.
True = above mean sea level.
Absolute = above the ground.
Pressure = pressure translated into altitude.
Density = air density translated into altitude.
Common altitude mistakes
Do not use field elevation when a performance chart asks for pressure altitude. Do not treat density altitude as your actual height above terrain. Do not assume an indicated altitude is automatically the same as true altitude under every atmospheric condition. And do not use density altitude by itself to claim a takeoff or climb capability.
Source check
The FAA Pilot's Handbook of Aeronautical Knowledge defines pressure and density altitude in its aircraft-performance discussion and explains the standard-atmosphere relationship. See the current FAA Pilot's Handbook of Aeronautical Knowledge, the current AIM, and PilotDesk Sources & Methods.