A theodolite is an optical instrument used by surveyors for measuring horizontal and vertical angles. Using this instrument the map-makers can measure distances and directions to relate different places on Earth’s surface to each other. It is also used for precise angular measurements in the construction of roads, tunnel alignment, and other civil engineering works. Also the astronomers can measure the angle. It is called the system of triangulation. From these angles they can determine the distance of the Moon from the Earth. Now it is desirable that we should know about this useful instrument.
This instrument was developed in the 16th century by the English mathematician Leonard Digges. Over the years many modifications have been made in it. In its modern form, it consists of a telescope mounted to swivel both horizontally and vertically. Accurate horizontal levelling is done with the help of a spirit level. The cross hairs in the telescope eyepiece permit accurate alignment with the object sighted. When the telescope is adjusted precisely, the two accompanying scales — vertical and horizontal, are ready to give accurate readings of the angles. Then using the laws and theorems of geometry of triangles, the distances and other measurements are calculated.
Theodolite is mounted on a tripod to obtain precise measurements. The transit is a variety of theodolite that has the telescope mounted in such a manner that it can be completely reversed or transited.
The phototheodolite is a combination of camera and theodolite mounted on the same tripod, used in terrestrial photogrammetry for map-making and other purposes.