Description
The theodolite is a relatively simple tool used for measuring angles, both horizontal and vertical. It works using the same principles as a protractor, the ‘point A’ is located and the angle noted, and then the telescope is pointed at ‘point B’ and the second angle is taken.

Although primarily used in surveying, the theodolite can be applied to both meteorology and navigation.

Gemma Frisius proposed the idea behind the theodolite in 1533. At the time, new methods of surveying were being used, and by combining an Alidade, a magnetic compass, and the degree scale on the back of an Astrolabe, the calculations made by modern theodolites could be observed. Unfortunately, this method was not practical due to the combination of instruments. The best of the attempts to simplify the process was the ‘theodolitus’, first described in print by Leonard Digges in 1571.

However, this instrument could only take measurements in the horizontal plane. Despite this, it was still thought of as the ‘common’ theodolite up to the late 18th Century.

During the 19th Century, the altazimuth theodolite was considered the most useful theodolite, as it could measure on the horizontal and vertical planes. Three notable types of altazimuth theodolite were developed: The Everest theodolite, the plain theodolite and the transit theodolite. It is the transit theodolite that is still used today.