Theodolite and total station

A theodolite measures horizontal and vertical angles precisely using a telescope that rotates about vertical and horizontal axes over graduated circles. A total station adds electronic distance measurement (EDM) and a computer to an electronic theodolite, so a single pointing gives angle, distance and therefore coordinates of the target. It is the workhorse of engineering surveying: control, topographic detail, setting out and monitoring.

Setting up involves centring over the station mark with an optical or laser plummet and levelling with the plate bubble or electronic tilt sensor. Angles are measured on both faces (face left and face right) and averaged, which cancels collimation, trunnion axis and index errors. EDM works by measuring the phase of a modulated infrared or laser signal reflected by a prism, or, in reflectorless mode, by the target surface itself. Atmospheric corrections for temperature and pressure, and a prism constant, must be set.

With coordinates of the station and a backsight point, the total station computes new coordinates directly. For setting out, it guides the operator to where a design point lies by showing the offset to move. Robotic total stations follow the prism automatically and allow one-person work and continuous deformation monitoring. Accuracy is specified as an angle (for example 1 or 5 arc-seconds) and as a distance accuracy of a few millimetres plus a few parts per million.