Types of altitude
"Altitude" alone is ambiguous. Five types, each from a different reference (PHAK ch.8):
| Type | Measures | Note |
|---|---|---|
| Indicated | Altimeter reading above MSL, current setting in Kollsman window | AWH 8.4.1.2 |
| True | Actual height above mean sea level (MSL) | Chart airport, terrain, obstacle elevations are all true (AWH 8.4.1.1) |
| Absolute | Height above ground directly below (AGL) | Tracks terrain |
| Pressure | Defined in pressure-altitude | |
| Density | Defined in density-altitude |
Standard day (sea-level 29.92", 15°C, standard lapse rate), current altimeter setting dialled in:
- Indicated, true, pressure, density altitude all the same number (PHAK ch.8).
- Altimeter calibrated to indicate true altitude in ICAO Standard Atmosphere (TC AIM AIR 1.5.1).
Non-standard day:
- Pressure or temperature off standard: the four split apart.
- Real atmosphere seldom standard, so indicated seldom equals true (AWH 8.4.1.1). Hence the other three.
Absolute altitude never joins that set, except directly over sea level: tracks ground below, not MSL; other four ignore terrain.
Trap: unqualified "altitude" defaults to true/indicated (height above sea level); absolute is the AGL one, the easiest to mix up with a radar/radio altimeter reading.
References
- FAA Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25), ch.8
- TC AIM AIR 1.5.1
- AWH 8.4.1.1
- AWH 8.4.1.2