Altitude

Altitude has one use on the context in which it is used (aviation, geometry, geographical survey, sport, and more). As a general definition, altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The reference datum also often varies according to the context.

Vertical distance measurements in the "down" direction are commonly referred to as depth.

Altitude in aviation
In aviation, the term altitude can have several meanings, and is always qualified by either explicitly adding a modifier (e.g. "true altitude"), or implicitly through the context of the communication. Parties exchanging altitude information must be clear which definition is being used. Air Navigation

Aviation altitude is measured using either Mean Sea Level (MSL) or local ground level (Above Ground Level, or AGL) as the reference datum.

With the exception of a few countries whose aviation authorities use metres (e.g. Russia), altitudes are stated in feet.

Pressure altitude divided by 100 feet is referred to as the flight level, and is used above the transition altitude (18,000 feet in the US, but may be as low as 3,000 feet in other jurisdictions); so when the altimeter reads 18,000 ft on the standard pressure setting the aircraft is said to be at "Flight level 180". When flying at a Flight Level, the altimeter is always set to standard pressure (29.92 / 1013.25).

On the flight deck, the definitive instrument for measuring altitude is the pressure altimeter, which is an aneroid barometer with a front face indicating distance (feet or meters) instead of atmospheric pressure.

There are several types of aviation altitude:


 * Indicated altitude is the reading on the altimeter
 * Absolute altitude is the height of the aircraft above the terrain over which it is flying. Also referred to feet/metres Above Ground Level (AGL).
 * True altitude is the elevation above Mean sea level. In UK aviation radiotelephony usage, the vertical distance of a level, a point or an object considered as a point, measured from mean sea level; this is referred to over the radio as altitude.(see QNH)
 * Height is the elevation above a ground reference point, commonly the terrain elevation. In UK aviation radiotelephony usage, the vertical distance of a level, a point or an object considered as a point, measured from a specified datum; this is referred to over the radio as height, where the specified datum is the airfield elevation (see QFE)
 * Pressure altitude is the elevation above a standard datum air-pressure plane (typically, 1013.25 millibars or 29.92" Hg and 15°C). Pressure altitude and indicated altitude are the same when the altimeter is set to 29.92" Hg or 1013.25 millibars.
 * Density altitude is the altitude corrected for non-ISA International Standard Atmosphere atmospheric conditions. Aircraft performance depends on density altitude, which 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 as to preclude takeoff, particularly for helicopters or a heavily loaded aircraft.

These types of altitude can be explained more simply as various ways of measuring the altitude:


 * Indicated altitude -- what the altimeter says
 * Absolute altitude -- altitude in terms of the distance above the ground directly below it
 * True altitude -- altitude in terms of elevation above sea level
 * Height -- altitude in terms of the distance above a certain point
 * Pressure altitude -- altitude in terms of the air pressure
 * Density altitude -- altitude in terms of the density of the air

Altitude regions
Although the term altitude is commonly used to mean the height above sea level of a location, in Geography the term Elevation is often preferred for this usage.

Mountain medicine recognizes three altitude regions:


 * High altitude = 1500 m – 3500 m (5000 – 11,500 ft)
 * Very High altitude = 3500 m – 5500 m (11,500 – 18,000 ft)
 * Extreme altitude = 5500 m – above

Travel to high altitudes can lead to medical problems, from the mild symptoms of acute mountain sickness to the potentially fatal high altitude pulmonary oedema (HAPE) and high altitude cerebral oedema (HACE). These conditions are caused by the profound hypoxia associated with travel to high altitudes.

The Earth's atmosphere is divided into several altitude regions:


 * Troposphere — surface to 8000 m / 5 miles at poles – 18,000 m / 11 miles at Equator, ending at the Tropopause.
 * Stratosphere — Tropopause to 50 km /31 miles
 * Mesosphere — Stratopause to 85 km /53 miles
 * Thermosphere — Mesopause to 675 km / 420 miles
 * Exosphere — Thermopause to 10,000 km /6200 miles