A decision height (DH) or decision altitude (DA) is a specified height or altitude point/level used as a reference. This altitude specifically state to which type of altitude a pilot is referring to. The term altitude is normally used to refer to the altitude above sea level, which is used to depict obstacles and airspace, as well as to separate air traffic. Pilots are mainly concerned with five types of altitude:
- Indicated altitude, this is the altitude that is read directly from the altimeter when it is set to the current altimeter.
- True altitude, the vertical distance from the aircraft above sea level, the actual altitude.
- Absolute altitude, the vertical distance of an aircraft above the terrain or ground level.
- Pressure altitude, the altitude indicated when the altimeter setting window is adjusted to 29.92.
- Density altitude, pressure altitude corrected for variations from standard temperature.