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alidade
An alidade (archaic forms include alhidade, alhidad, alidad) or a turning board is a device that allows one to sight a distant object and use the line of sight to perform a task. This task can be, for example, to draw a line on a plane table in the direction of the object or to measure the angle to the object from some reference point. Angles measured can be horizontal, vertical or in any chosen plane.
The alidade was originally a part of many types of scientific and astronomical instrument. At one time, some alidades, particularly those used on graduated circles as on astrolabes, were also called ''diopters''.〔"diopter", ''The Compact Edition of the Oxford English Dictionary'', Oxford University Press, 1971〕 With modern technology, the name is applied to complete instruments such as the plane table alidade.
== Origins ==
The word in Arabic (العضادة ''al-idhâdah'', "the ruler"), signifies the same device. In Greek and Latin, it is respectively called ''διοπτρα'', "dioptra", and ''linea fiduciae'', "fiducial line".
The earliest alidades consisted of a bar, rod or similar component with a vane on each end. Each vane (also called a pinnule or pinule) has a hole, slot or other indicator through which one can view a distant object. There may also be a pointer or pointers on the alidade to indicate a position on a scale. Alidades have been made of wood, ivory, brass and other materials.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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