The Triangulation gallery contains images depicting Coast and Geodetic Survey activities in determining distances, latitude and longitude, state plane coordinates, and directions between points by the method of triangulation. Furthermore, these images show observers and a variety of instruments used for observing angles of triangles and other polygons; heliotropes used to reflect sunlight or electric lamps for showing lights at night; the various apparatus used to measure distances prior to the development of electronic distance measuring devices; early electronic distance measuring instruments; and the great towers that were part of the American landscape for close to 150 years that allowed the geodetic surveyors to measure angles between points tens of miles apart.