The term alidade can refer to the sighting mechanism of any instrument used for surveying or navigation. In this catalog the term refers to the sighting mechanism used with a plane table for topographical work—that is, for mapping the surface features of the earth. Early alidades were simple bars with open sights at either end. Telescopic alidades came into use in Europe in the early 1800s, and were soon introduced to American practice. In 1865 the United States Coast Survey stated that the plane table with telescopic alidade was the “principal instrument for mapping the topographical features of the country,” and noted that it was “universally recognized as the most efficient and accurate means for that purpose.”
A. M. Harrison, "On the Plane-Table and Its Use in Topographical Surveying," Annual Report of the Superintendent of the United States Coast Survey (1865), Appendix 22, pp. 203-231. For more historical information see Harrison's manuscript of the same title, in the Dibner Library of the History of Science and Technology, Smithsonian Institution Libaries.
E. Hergesheimer, "A Treatise on the Plane-Table and Its Use in Topographical Surveying,&qupt; Annual Report of the Superintendent of the United States Coast and Geodetic Survey (1880), Appendix 13, pp. 172-200.