Telescopic Alidade

This alidade has a telescope with level mounted below, and a vertical arc that is silvered, graduated to degrees, and read by vernier to single minutes. The base has six round holes, two short level vials; the beveled edge has a 12-inch scale divided to sixteenths of an inch. The U.S. Geological Survey transferred this example to the Smithsonian in 1907, describing it as a "preliminary type" of alidade devised for the Survey about 1890.
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 U. S. 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.”
Ref: 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 Libraries.
E. Hergesheimer, "A Treatise on the Plane-Table and Its Use in Topographical Surveying.” Annual Report of the Superintendent of the United States Coast and Geodetic Survey (1880), Appendix 13, pp. 172-200.
Currently not on view
base: 18 in; 45.72 cm
telescope: 9 in; 22.86 cm
overall: 8 1/8 in x 18 in x 4 1/4 in; 20.6375 cm x 45.72 cm x 10.795 cm
ID Number
catalog number
accession number
Credit Line
U.S. Geological Survey
See more items in
Medicine and Science: Physical Sciences
Measuring & Mapping
Surveying and Geodesy
Data Source
National Museum of American History, Kenneth E. Behring Center


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