Altitude

Altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The exact definition and reference datum varies according to the context (e.g., aviation, geometry, geographical survey, sport, or atmospheric pressure). 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.

For the usage in astronomy, see Altitude angle. For other uses, see Altitude (disambiguation).

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

Indicated altitude is the reading on the altimeter when it is set to the . 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)^[2]

local barometric pressure at mean sea level

Absolute altitude is the vertical distance of the aircraft above the terrain over which it is flying.^: ii It can be measured using a radar altimeter (or "absolute altimeter").^[3] Also referred to as "radar height" or feet/metres above ground level (AGL).

[3]

True altitude is the actual elevation above .^[3]^: ii It is indicated altitude corrected for non-standard temperature and pressure.

mean sea level

Height is the vertical distance above a 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 )^[2]

QFE

Pressure altitude is the elevation above a standard datum air-pressure plane (typically, 1013.25 millibars or 29.92" Hg). Pressure altitude is used to indicate "flight level" which is the standard for altitude reporting in the U.S. in Class A airspace (above roughly 18,000 feet). Pressure altitude and indicated altitude are the same when the altimeter setting is 29.92" Hg or 1013.25 millibars.

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.

Density altitude

The term altitude can have several meanings, and is always qualified by 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.

Aviation altitude is measured using either mean sea level (MSL) or local ground level (above ground level, or AGL) as the reference datum.

Pressure altitude divided by 100 feet (30 m) is the flight level, and is used above the transition altitude (18,000 feet (5,500 m) in the US, but may be as low as 3,000 feet (910 m) in other jurisdictions). So when the altimeter reads the country-specific flight level on the standard pressure setting the aircraft is said to be at "Flight level XXX/100" (where XXX is the transition altitude). When flying at a flight level, the altimeter is always set to standard pressure (29.92 inHg or 1013.25 hPa).

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 metres) instead of atmospheric pressure.

There are several types of altitude in aviation:

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

: surface to 8,000 metres (5.0 mi) at the poles, 18,000 metres (11 mi) at the Equator, ending at the Tropopause

Troposphere

: Troposphere to 50 kilometres (31 mi)

Stratosphere

: Stratosphere to 85 kilometres (53 mi)

Mesosphere

: Mesosphere to 675 kilometres (419 mi)

Thermosphere

: Thermosphere to 10,000 kilometres (6,200 mi)

Exosphere

Atmosphere of Earth

At higher altitudes, the air density is lower than at sea level. At a certain altitude it is very difficult to keep an airplane in stable flight.

Coffin corner (aerodynamics)

Geocentric altitude

Near space

. Apex (altitude physiology expeditions). Retrieved 8 August 2006.

"Altitude pressure calculator"

. U.S. Centennial of Flight Commission. Archived from the original on 9 March 2006. Retrieved 25 January 2006.