Dictionary:Alidade

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(al’ ∂ dād,) A ruler equipped with sights, which allows aligning a straight edge in the direction of an object. See Figure A-9. A sighting telescope is usually equipped with vertical circle and stadia (q.v.) cross-hairs. The alidade is used on a plane table; the object to be located is sighted through the telescope and a line is drawn on the plane-table paper along the straight edge to indicate the direction to the object.