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
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."
Smithsonian National Museum of American History