How a Theodolite Works

Written by paul favors | 13/05/2017

A theodolite is an instrument used for surveying. It is used mostly for geodetic engineering. This instrument consists of a pair of graduated circles positioned in right angles to each other. A theodolite is primarily used to measure vertical and horizontal angles. However, these instruments measure horizontal angles more accurately than vertical angles as this is the main function in surveying.


Theodolites have been in existence for a long time, dating back to the Table Alidades, a device that provides a graphical map of a terrain. It consists of a plane table and telescope placed on top of the alidade, which is a piece of equipment characterised by a fork-like appearance. In 1571, a surveying textbook called "Pantometria" provided the first description of a theodolite. This was credited to Thomas Digges, who was also associated with the invention of this equipment.


A theodolite instrument is developed in three forms: the Y pattern, the transit and the Everest. All these forms have a stand that is made circular in sections with the legs composed of steel. For the Y theodolite, the telescope support is low. The telescope itself rests on a cradle and supports.

The Everest theodolite has low support as well and the telescope does not have the ability to be transited. The Everest is similar to the Y theodolite but it differs in the structure of the vertical circle, which has two arcs. Lastly, transit theodolites do not have the eye end of the telescope plus they employ the use of a prism.

How to Use

A theodolite must be mounted on a level tripod or stand from where the user can observe. The observer can take a reading by looking through the telescope and adjusting the vertical crosshairs to match the extreme left of the survey target. The horizontal circle is then rotated and locked in place. The direction specified in the circle is recorded by the observer. After this, the second target is surveyed by rotating the circle counterclockwise, which is repeated until all targets are read.


In the field of geodetics, concepts from trigonometry are utilised to measure angles and obtain the direction of a given target. One of the concepts in trigonometry implemented in using a theodolite is triangulation. This type of surveying method, heavily based on the measurement of angles and distances, develops a series of linked triangles wherein plane coordinates are derived.

Modern Development

Compared to the traditional use of theodolites, the modern method for using theodolites utilises digital or electronic reading. A rotary encoder does the reading for the observer. The development of sensors has allowed inventors to add it into the focal plane, providing the observer the means to target automatically and measure instantly using the embedded software.


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