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The alidade is possibly one of the oldest known mapping instruments (Low, 1952). Its current function, however, is largely incorporated into modern surveying instruments (Ahmed and Almond, 1983) (see also Surveying, Electronic; Surveying, General). The term alidade (from the Arabic) originally referred only to a rule or straightedge but now commonly identifies the entire instrument, which incorporates simple telescopic or other sighting devices with index reading or recording accessories (Breed and Hosmer, 1945; Lahee, 1961; Bates and Jackson, 1980). The alidade is used for determining the directions of objects and is commonly deployed in detailed survey (q.v.). especially plane table, mapping (q.v.). Modern telescopic alidades, like the one shown in Fig. 1, include a focusing telescope, a striding level, and vernier scale for measuring vertical angles; a magnetic needle, stadia hairs in the eyepiece for determining distances read on a stadia rod; and commonly a Beaman arc for