Altitude can be determined by the measurement of atmospheric pressure. The greater the altitude, the lower the pressure. When a barometer is supplied with a nonlinear calibration so as to indicate altitude, the instrument is a type of altimeter called a pressure altimeter or barometric altimeter. A pressure altimeter is the altimeter found in most aircraft, and skydivers use wrist-mounted versions for similar purposes. Hikers and mountain climbers use wrist-mounted or hand-held altimeters, in addition to other navigational tools such as maps, magnetic compasses, and receivers for global navigation satellite systems (GNSS), such as GPS.
Calibration The calibration of an altimeter follows the equation
z = c T log ( P o / P ) , {\displaystyle z=c\;T\;\log(P_{o}/P),}
where c is a constant, T is the absolute temperature, P is the pressure at altitude z, and Po is the pressure at sea level. The constant c depends on the acceleration of gravity and the molar mass of the air. However, one must be aware that this type of altimeter relies on "density altitude" and its readings can vary by hundreds of feet owing to a sudden change in air pressure, such as from a cold front, without any actual change in altitude. The most common unit of measurement used for altimeter calibration worldwide is hectopascals (hPa), except in North America and Japan, where inches of mercury (inHg) are used. To obtain an accurate altitude reading a pressure altimeter must be calibrated to local barometric pressure by use of the barometric formula.
History The scientific principles behind the pressure altimeter were first written by Rev. Alexander Bryce, a Scottish minister and astronomer who in 1772 realised that the principles of a barometer could be adjusted to measure height.
Applications
Use in hiking, climbing and skiing A barometric altimeter, used along with a topographic map, can help to verify one's location. It is more reliable, and often more accurate, than a global navigation satellite systems (GNSS) receiver (such as a GPS receiver) for measuring altitude; the GNSS signals may be unavailable, for example, when one is deep in a canyon, or it may give wildly inaccurate altitudes when all available satellites are near the horizon. Because barometric pressure changes with the weather, hikers must periodically re-calibrate their altimeters when they reach a known altitude, such as a trail junction or peak marked on a topographical map.
Skydiving
An altimeter is the most important piece of skydiving equipment, after the parachute itself. Altitude awareness is crucial at all times during the jump, and determines the appropriate response to maintain safety. Since altitude awareness is so important in skydiving, there is a wide variety of altimeter designs made specifically for use in the sport, and a non-student skydiver will typically use two or more altimeters in a single jump:
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