[al-ti-tood, -tyood]
altitude, vertical distance of an object above some datum plane, such as mean sea level or a reference point on the earth's surface. It is usually measured by the reduction in atmospheric pressure with height, as shown on a barometer or altimeter. In surveying and astronomy, it is the vertical angle of an observed point, such as a star or planet, above the horizon plane. The altitude of a feature of the earth's surface is usually called its elevation. Recent spacecraft instrumentation has also measured vertical distances on the earth and other planets, determining the height of planetary features by means of radar and optical imaging.

In astronomy, altitude is the angular distance of a heavenly body above the astronomical horizon as determined by the angle which a line drawn from the eye of the observer to the heavenly body makes with the plane of the horizon. The reading of the apparent altitude, as determined by a telescope attached to a graduated circle, must be corrected for refraction by the atmosphere and for certain other effects to ascertain the true altitude. The altitude of the north celestial pole, which is approximately that of the star Polaris, is equal to the observer's latitude.

Altitude is the elevation of a point or object from a known level or datum (plural: data). 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. In aviation, altitude is measured in feet. For non-aviation uses, altitude may be measured in other units such as metres or miles.

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. It is the fall in pressure that leads to a shortage of oxygen (hypoxia) in humans on ascent to high altitude.

Altitude in aviation

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.

  • 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)
  • 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, 1013.25 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". 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.
  • Density altitude is the altitude corrected for non-ISA International Standard Atmosphere (ISA) conditions at which the air density is unequal to ISA 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 may be so high as to preclude takeoff, particularly for helicopters or a heavily loaded aircraft.

Altitude in sport

Atmospheric pressure decreases as altitude increases, and as the pressure decreases less oxygen is available for sportsmen to utilise. These are the basis for two contradictory effects of altitude on exercise and sport. For explosive events (sprints up to 400 metres, long jump, triple jump) the reduction in atmospheric pressure means there is less resistance from the atmosphere and the athlete's performance will generally be better at altitude. For endurance events (races of 5000 metres or more) the predominant effect is the reduction in oxygen which generally reduces the athlete's performance at altitude.

Living at altitude causes the body to physiologically adapt to the reduction in available oxygen (a process known as acclimatisation) so that an advantage in oxygen take-up is evidenced when the athlete returns to a lower altitude. These changes are the basis of altitude training which forms an integral part of the training of athletes in a number of endurance sports including track and field, distance running, triathlon, cycling and swimming.

Sports organisations also acknowledge the effects of altitude on performance. The International Association of Athletic Federations (IAAF), for example, have ruled that performances achieved at an altitude greater than 1000 metres will not be approved for record purposes.

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


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