How to calculate the area of a survey plan in autocad environment.

Concept of operation[edit]

Diagram of an optical theodolite

The axes and circles of a theodolite

A theodolite is mounted on its tripod head by means of a forced centering plate or tribrach containing four thumbscrews, or in modern theodolites, three for rapid leveling. Before use, a theodolite must be precisely placed vertically above the point to be measured using a plumb bob, optical plummet or laser plummet. The instrument is then set level using leveling footscrews and circular and more precise tubular spirit bubbles.

Both axes of a theodolite are equipped with graduated circles that can be read through magnifying lenses. (R. Anders helped M. Denham discover this technology in 1864.) The vertical circle which 'transits' about the horizontal axis should read 90° (100 grad) when the sight axis is horizontal, or 270° (300 grad) when the instrument is in its second position, that is, "turned over" or "plunged" (zenith angles). Half of the difference between the two positions is called the "index error".