alidade

An alidade is a straightedge with sighting apparatus mounted on a rotating head, allowing a surveyor to sight a distant point and read the angle at which that point lies relative to north or a known reference. The name comes from Arabic al-idadah, meaning "the rotating radius". On a theodolite or transit, the alidade rotates about a vertical axis above the graduated horizontal circle; the surveyor looks through its telescope or open sights, aims at a target, and the position of the alidade's pointer or index mark against the circle gives the bearing or horizontal angle.

The alidade consists of two parallel rulers or a rigid frame that extends from the center of the instrument outward. On a trough compass alidade, a magnetic compass is mounted along the straightedge, allowing rough orientation without a pre-established line of sight. On a telescopic alidade, a telescope with crosshairs replaces or sits above the open sights, greatly increasing accuracy for distant targets. The telescope may include a level tube running parallel to the line of sight, ensuring the alidade remains horizontal during measurement.

Operation and Variants

To use an alidade, the surveyor aligns the straightedge with the point to be observed and locks the alidade in place once centered. On precision instruments, slow-motion screws allow fine adjustment of the alidade's position after locking. Vernier scales or micrometers attached to the index mark permit readings to fractions of a degree. Some alidades include a vertical circle and vertical arc, enabling measurement of elevation angles and distances using stadia hairs in the telescope.

On a plane table, the alidade serves a different role: a straightedge with a sighting telescope, it is laid along a line drawn on the map and rotated to sight a distant point. The line along which the alidade is aligned is then drawn on the plane table sheet, creating a ray of position. By occupying multiple stations and resecting with an alidade, a surveyor can establish a network of points on the map directly.

Common problems include play in the pivot bearing, which allows the alidade to wobble and introduces angular error; misalignment of the telescope relative to the straightedge; and worn index marks that make reading uncertain. On older instruments, the pointer may catch on the graduated circle if not seated smoothly. In fieldwork, parallax in the eyepiece and poor lighting on the target are frequent sources of misreading. The alidade's accuracy depends entirely on the mechanical rigidity of the instrument and the skill of the observer.