Pendulum Self-leveling Alidade
This is a telescopic alidade in which a pendulum device, or compensator replaces the conventional bubble for establishing a horizontal reference line from which vertical angles may be measured. Alidades were the primary mapping instrument in the U.S. from about 1865 to the 1980s, when they were replaced by the total station. This alidade was manufactured by Keuffel & Esser Company, New York & New Jersey.
Object ID: USGS-000160