A theodolite () is a precision optical instrument for measuring angles between designated visible points in the horizontal and vertical planes. The traditional use has been for land surveying, but it is also used extensively for building and infrastructure construction, and some specialized applications such as meteorology and rocket launching. It consists of a moveable telescope mounted so it can rotate around horizontal and vertical axes and provide angular readouts. These indicate the orientation of the telescope, and are used to relate the first point sighted through the telescope to subsequent sightings of other points from the same theodolite position. These angles can be measured with accuracies down to microradians or seconds of arc. From these readings a plan can be drawn, or objects can be positioned in accordance with an existing plan. The modern theodolite has evolved into what is known as a total station where angles and distances are measured electronically, and are read directly to computer memory. In a transit theodolite, the telescope is short enough to rotate about the trunnion axis, turning the telescope through the vertical plane through the zenith; for non-transit instruments vertical rotation is restricted to a limited arc. The optical level is sometimes mistaken for a theodolite, but it does not measure vertical angles, and is used only for leveling on a horizontal plane (though often combined with medium accuracy horizontal range and direction measurements). Principles of operation Preparation for making sightings {Main|Temporary adjustments of theodolites} Temporary adjustments are a set of operations necessary in order to make a theodolite ready for taking observations at a station. These include its setting up, entering, leveling up and elimination of parallax, and are achieved in four steps these are given:- 1:- Setting up: fixing the theodolite onto a tripod along with approximate leveling and centering over the station mark. 2:- Centering: bringing the vertical axis of theodolite immediately over station mark using a centering plate also known as a tribrach. 3:- Leveling: leveling of the base of the instrument to make the vertical axis vertical usually with an in-built bubble-level. 4:- Focusing: removing parallax error by proper focusing of objective and eye-piece. The eye-piece only requires adjustment once at a station. The objective will be re-focused for each subsequent sighting from this station because of the different distances to the target. Sightings Sightings are taken by the surveyor, who adjusts the telescope's vertical and