Altitude in aviation and in spaceflight
Vertical Distance Comparison
In aviation, the altitude can be measured and shown in several different ways. The height is either from the average sea level (also called Mean Sea Level or MSL), or from the ground (also called Above Ground Level, or AGL).
Airplanes can find their altitude by using air pressure. The air gets thinner the higher it is (and it has less pressure, because there's less air pushing down on it from above). This pressure can be measured and this can be converted into a height above sea level. The instrument for measuring altitude is the
altimeter (from altitude and meter). The common pressure altimeter is a type of barometer with a dial showing distance (feet or metres) instead of atmospheric pressure.
There are several types of aviation altitude.
In simple terms, these types of altitude can be explained as ways of measuring the altitude:
- Indicated altitude -- What the
altimeter indicates (shows) for the height above sea level. It is usually pretty good, but can be fooled a bit sometimes, it needs to be adjusted to use the local barometric pressure.
- True altitude -- Altitude as the distance above sea level.
- Absolute altitude -- Altitude as the distance above the terrain (the ground) directly below it (Above Ground Level).
- Height -- Altitude in terms of the distance above some point. This can also be called the terrain elevation. Radio altimeters measure this height.
- Pressure altitude -- Altitude measured using air pressure. The air pressure at sea level is normally 1013.25 millibars or 29.92" Hg, when the air temperature is 15 °C (59 °F). Pressure altitude and indicated altitude are the same when the altimeter is adjusted to use this standard level.
- Density altitude -- Altitude measured using the density of the air. This depends on atmospheric conditions (mainly heat and humidity). There is a International Standard Atmosphere chart that converts density to altitude.
The density of the air also affects how well an aircraft works. Density altitude is affected by barometric pressure, humidity and temperature. On a very hot day, density altitude at an airport (especially one at a high elevation) may be so high that aircraft can not take-off. This most often affects helicopters or an aircraft that is carrying a large amount of cargo.