An alidade is a device that allows one to sight a distant object and use the line of sight to perform a task. This task can be, for example, to draw a line on a plane table

Plane table

A plane table is a device used in surveying and related disciplines to provide a solid and level surface on which to make field drawings, charts and maps...

 in the direction of the object or to measure the angle to the object from some reference point. Angles measured can be horizontal, vertical or in any chosen plane. The alidade's primary use is for creating maps in the horizontal plane.

The alidade was originally a part of many types of scientific and astronomical instrument.