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An azimuth () (from Arabic ''al-sumūt'', meaning "the directions") is an angular measurement in a spherical coordinate system. The vector from an observer (origin) to a point of interest is projected perpendicularly onto a reference plane; the angle between the projected vector and a reference vector on the reference plane is called the azimuth. An example is the position of a star in the sky. The star is the point of interest, the reference plane is the horizon or the surface of the sea, and the reference vector points north. The azimuth is the angle between the north vector and the perpendicular projection of the star down onto the horizon.〔http://dictionary.reference.com/browse/azimuth〕 Azimuth is usually measured in degrees (°). The concept is used in navigation, astronomy, engineering, mapping, mining and artillery. ==Navigation== In land navigation, azimuth is usually denoted ''alpha'', , and defined as a horizontal angle measured clockwise from a north base line or ''meridian''.〔U.S. Army, ''Map Reading and Land Navigation'', FM 21–26, Headquarters, Dept. of the Army, Washington, D.C. (7 May 1993), ch. 6, p. 2〕〔U.S. Army, ''Map Reading and Land Navigation'', FM 21–26, Headquarters, Dept. of the Army, Washington, D.C. (28 March 1956), ch. 3, p. 63〕 ''Azimuth'' has also been more generally defined as a horizontal angle measured clockwise from any fixed reference plane or easily established base direction line.〔U.S. Army, ch. 6 p. 2〕〔U.S. Army, ''Advanced Map and Aerial Photograph Reading'', Headquarters, War Department, Washington, D.C. (17 September 1941), pp. 24–25〕〔U.S. Army, ''Advanced Map and Aerial Photograph Reading'', Headquarters, War Department, Washington, D.C. (23 December 1944), p. 15〕 Today the reference plane for an azimuth is typically true north, measured as a 0° azimuth, though other angular units (grad, mil) can be used. Moving clockwise on a 360 degree circle, east has azimuth 90°, south 180°, and west 270°. There are exceptions: some navigation systems use south as the reference vector. Any direction can be the reference vector, as long as it is clearly defined. Quite commonly, azimuths or compass bearings are stated in a system in which either north or south can be the zero, and the angle may be measured clockwise or anticlockwise from the zero. For example, a bearing might be described as "(from) south, (turn) thirty degrees (toward the) east" (the words in brackets are usually omitted), abbreviated "S30°E", which is the bearing 30 degrees in the eastward direction from south, i.e. the bearing 150 degrees clockwise from north. The reference direction, stated first, is always north or south, and the turning direction, stated last, is east or west. The directions are chosen so that the angle, stated between them, is positive, between zero and 90 degrees. If the bearing happens to be exactly in the direction of one of the cardinal points, a different notation, e.g. "due east", is used instead. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「azimuth」の詳細全文を読む スポンサード リンク
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