altitude sickness: see decompression sickness.

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.

or mountain sickness

Acute reaction to a change from low altitudes to altitudes above 8,000 ft (2,400 m). Most people gradually adapt, but some have a severe reaction that can be fatal unless they return to low altitude. Normal adaptations to the reduced oxygen at high altitude (e.g., breathlessness, racing heartbeat) are exaggerated; other manifestations include headache, gastrointestinal upsets, and weakness. Pulmonary edema is quickly reversed with oxygen and evacuation to a lower area.

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Two coordinates describing the position of an object above Earth in a coordinate system called the altazimuth, or horizon, system, and used in astronomy, gunnery, navigation, surveying, and other fields. Altitude in this sense is expressed as angle of elevation (up to 90°) above the horizon. Azimuth, in astronomical measurement, is the number of degrees clockwise from due north to the point on the horizon directly below the object.

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