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I disagree with the part of the definition about "measuring angles." Decades ago I had occasion to use a plane table and alidade in a surveying class. The alidade was basically the telescope part of a surveyor's transit attached to a scaled metal ruler. The plane table was similar to a horizontal draftboard mounted on legs. There was paper or semi-transparent medium affixed to the plane table and a pin represented the position of this equipment. One member of the team would move a stadia rod from place to place over the area to be mapped. The alidade was pointed at the stadia rod, and having established the true elevation of the alidade the elevation of the selected point could be read; then using the upper and lower of three crosshairs in the alidade the distance between the alidade and the stadia rod could be read. Finally, using the scaled ruler, the point could be plotted on the plane table and the elevation noted. There was never an angle "measurement" since the direction to the stadia rod was the same as the alignment of the scaled ruler. While a theodolite and a surveyor's transit have horizontal and vertical vernier rings for measuring angles, the much simpler alidade has no such features. Dick Kimball 15:56, 1 May 2008 (UTC)