An optical instrument used for surveying and weather observation. A theodolite contains a telescope fitted with cross hairs, and mounted on a platform. The platform is fitted with two bubble levels and an accurate magnetic compass. The platform is leveled and pointed toward true north by applying the correction for declination, or variation, to compensate for the difference between magnetic north and true north.

An azimuth scale (a scale that measures the horizontal direction the telescope moves) measures the bearing from the theodolite to any object visible through the telescope. A vertical scale measures the angle the telescope is inclined upward from its level position.

In weather observation, the theodolite is used to track the movement of a weather balloon to find the direction and velocity of the wind at various altitudes. A theodolite is also used with a ceiling light to measure the height of the base of clouds at night.