Alidade
The term alidade can refer to the sighting mechanism of any instrument used for surveying or navigation. Early alidades were simple bars with open sights at either end. Telescopic alidades came into use in Europe in the early 1800s. 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.”
Gurley began offering telescopic alidades in 1884, explaining that "this instrument, which has been so largely employed abroad in topography and map drawing, is now fast coming into use in our own country, especially in colleges and schools where the study of surveying is pursued."
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