A theodolite is a measuring instrument that gives a precise angle between two points. Theodolites are small telescopes mounted on tripods so that they can swivel both horizontally and vertically. The lens of a theodolite has crosshairs and angle scales that the viewer uses to determine angles.
Theodolites are commonly used for surveying and triangulation in road building, tunnel alignment and other civil engineering projects. They come in several varieties suited to different types of projects. A transit theodolite is built so that it can completely reverse directions. A phototheolodite is equipped with a camera, and is commonly used in map making.