Plain Alidade

Given figure shows a typical plain adidate. At each end of the ruler a sight vane is provided. The vane with narrow slit serves as eye vane and the other having wide slit and also a thin wire at its centre serves as object vane. The two vanes are provided having hinges at the ends of ruler so that on the ruler when not in use they can be folded. Plain alidade is not appropriate in surveying hilly areas as the inclination of line of sight in this case is limited.

                          Plane alidade