For other uses see Altitude (disambiguation)

Altitude is the elevation of a point or object from a known level or datum (plural: data). The elevation of a Geographic location is its height above a fixed reference point often the mean sea level. This article describes a concept from Surveying and Geodesy. For other meanings see Datum (disambiguation. Common data are mean sea level, local ground level (Above Ground Level, or AGL), or the surface of the WGS-84 geoid, used by GPS. Mean sea level (MSL is the average (mean height of the Sea, with reference to a suitable reference surface The World Geodetic System defines a reference frame for the earth for use in Geodesy and Navigation. The geoid is that Equipotential surface which would coincide exactly with the mean ocean surface of the Earth if the oceans were in equilibrium at rest and extended through Basic concept of GPS operation A GPS receiver calculates its position by carefully timing the signals sent by the constellation of GPS Satellites high above the Earth In aviation, altitude is measured in feet. Aviation refers to activities involving man-made flying devices ( Aircraft) including the people organizations and regulatory bodies involved with them A foot (plural feet or foot; symbol or abbreviation ft or sometimes &prime – the prime symbol) is a non-SI unit For non-aviation uses, altitude may be measured in other units such as metres or miles. The metre or meter is a unit of Length. It is the basic unit of Length in the Metric system and in the International

Atmospheric pressure decreases as altitude increases. This principle is the basis of operation of the pressure altimeter, which is an aneroid barometer calibrated to indicate altitude instead of pressure. An altimeter is an instrument used to measure the Altitude of an object above a fixed level History The first barometer is thought to have been built unintentionally by Gasparo Berti, sometime between 1640 and 1643 It is the fall in pressure that leads to a shortage of oxygen (hypoxia) in humans on ascent to high altitude. Chronic Hypoxia is a pathological condition in which the body as a whole ( generalized hypoxia) or region of the body ( tissue hypoxia) is deprived of adequate

## Altitude in aviation

Vertical distances.

In aviation, the term altitude can have several meanings, and is therefore qualified by either explicitly adding a modifier (e. g. "true altitude"), or implicitly through the context of the communication. Parties exchanging information concerning this topic must be clear which definition is being used. [1]

• True altitude is the elevation above mean sea level. Mean sea level (MSL is the average (mean height of the Sea, with reference to a suitable reference surface 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]
• height is the elevation above a ground reference point, commonly the terrain elevation. QNH is a Q code. It is a pressure setting used by pilots, Air traffic control (ATC and Low frequency Weather beacons to refer to Height is the measurement of vertical Distance, but has two meanings in common use 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)[2]
• Absolute altitude is the height of the aircraft above the terrain over which it is flying.
• Indicated altitude is the reading on the altimeter.
• Pressure altitude is the elevation above a standard datum plane (typically, 1013. In Aviation, pressure altitude is the indicated altitude when an Altimeter is set to an agreed baseline pressure setting 25 millibars or 29. 92" Hg and 15°C). 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 3,000ft in other countries); so when the altimeter reads 18,000 ft on the standard pressure setting the aircraft is said to be at "Flight level 180". A Flight Level ( FL) is a standard nominal Altitude of an Aircraft, in hundreds of Feet. Transition altitude is the maximum Altitude at which an aircraft should use an altimeter pressure setting indicating height above mean sea level (QNH. When flying at a Flight Level, the altimeter is always set to standard pressure (29. 92 / 1013. 2). Below the transition level, altitudes are read in thousands, pronounced "one three thousand" for 13,000, "seven thousand" for 7,000 etc. Transition altitude is the maximum Altitude at which an aircraft should use an altimeter pressure setting indicating height above mean sea level (QNH.
• Density altitude is the altitude corrected for non-ISA International Standard Atmosphere (ISA) conditions at which the air density is unequal to ISA conditions. Density altitude is the Altitude in the International Standard Atmosphere at which the air density would be equal to the actual air density at the The International Standard Atmosphere (ISA is an atmospheric model of how the Pressure, Temperature, Density, and Viscosity 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 may be so high as to preclude takeoff, particularly for helicopters or a heavily loaded aircraft.

## 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. Geography (from Greek γεωγραφία - geografia) is the study of the Earth and its lands features inhabitants and phenomena

Mountain medicine recognizes three altitude regions:[3]

• 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). Altitude sickness, also known as acute mountain sickness ( AMS) altitude illness, or soroche, is a pathological condition that is caused by acute High altitude pulmonary edema (HAPE is a life-threatening form of non-cardiogenic Pulmonary edema that occurs in otherwise healthy Mountaineers at altitudes above High altitude pulmonary edema (HAPE is a life-threatening form of non-cardiogenic Pulmonary edema that occurs in otherwise healthy Mountaineers at altitudes above High altitude cerebral edema (or HACE) is a severe (frequently fatal form of Altitude sickness. High altitude cerebral edema (or HACE) is a severe (frequently fatal form of Altitude sickness. These conditions are caused by the profound hypoxia associated with travel to high altitudes.

The Earth's atmosphere is divided into several altitude regions:[4]

• Troposphere — surface to 5 miles (8 km) at poles – 11 miles (18 km) at equator, ending at the Tropopause. Temperature and layers The temperature of the Earth's atmosphere varies with altitude the mathematical relationship between temperature and altitude varies among five The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 75% of the atmosphere's mass and almost all of its Water vapor and The equator (sometimes referred to colloquially as "the Line") is the intersection of the Earth 's surface with the plane perpendicular to the
• Stratosphere — Tropopause to 31 miles (50 km)
• Mesosphere — Stratopause to 53 miles (85 km)
• Thermosphere — Mesopause to 420 miles (675 km)
• Exosphere — Thermopause to 6200 miles (10,000 km)

## References

1. ^ (1 December 1989) Air Navigation. The stratosphere is the second major layer of Earth's atmosphere, just above the Troposphere, and below the Mesosphere. This article is about the atmospheric mesosphere for the Earth's mantle see Mesosphere (mantle. The thermosphere is the layer of the Earth's atmosphere directly above the Mesosphere and directly below the Exosphere. The exosphere is the uppermost layer of the atmosphere. On Earth, its lower boundary at the edge of the Thermosphere is estimated to be 500 km to Department of the Air Force. AFM 51-40.
2. ^ a b (1 January 1995) Radiotelephony Manual. UK Civil Aviation Authority. CAP413.
3. ^ Non-Physician Altitude Tutorial. International Society for Mountain Medicine. Retrieved on 22 December, 2005.
4. ^ Layers of the Atmosphere. JetStream, the National Weather Service Online Weather School. National Weather Service. Retrieved on 22 December, 2005.