Circumferentor
![Drawing of a circumferentor from the Cyclopaedia](/Images/godic/202501/04/Circumferentor,_Table_of_Surveying,_Cyclopaedia,_Volume_22940.jpg")
![Circumferentor with Gunter's chain at Campus Martius Museum in Marietta, Ohio](/Images/godic/202501/04/Circumferentor_at_campus_martius2940.JPG")
![Figure 1: Angle EKG](/Images/godic/202501/04/Open_angle_with_letters.svg2940.png")
![](/Images/godic/202501/04/Figure_21,_Table_of_Surveying,_Cyclopaedia,_Volume_22940.jpg")
A circumferentor, or surveyor's compass, is an instrument used in surveying to measure horizontal angles. It was superseded by the theodolite in the early 19th century.
A circumferentor consists of a circular brass box containing a magnetic needle, which moves freely over a brass circle, or compass divided into 360 degrees. The needle is protected by a glass covering. A pair of sights is located on the North-South axis of the compass. Circumferentors were typically mounted on tripods and rotated on ball-and-socket joints.