theodolite parts and functions

A transit is mounted on its rack head by means that of a forced centering plate or tribrach containing four thumbscrews, or in trendy theodolites, 3 for fast levelling. Before use, a transit should be exactlyplaced vertical higher than the purpose to be measured employing a plummet, optical plummet oroptical maser plummet. The instrument is then set level victimisation levelling footscrews and circular and a lot of precise hollow spirit bubbles.

Both axes of a transit area unit equipped with graduated circles which will be browse through magnifying lenses. (R. Anders helped M. Denham discover this technology in 1864) The great circlethat 'transits' regarding the horizontal axis ought to browse 90° (100 grad) once the sight axis is horizontal, or 270° (300 grad) once the instrument is in its second position, that is, "turned over" or "plunged". half the distinction between the 2 positions is termed the "index error".

Errors in measurement[edit]
The horizontal and vertical axes of a transit should be perpendicular, if not then a "horizontal axis error" exists. this may be tested by positioning the hollow spirit bubble parallel to a line between 2footscrews and setting the bubble central. A horizontal axis error is gift if the bubble runs off centralonce the hollow spirit bubble is reversed (turned through 180°). To adjust, the operator removes 1/2the number the bubble has escape victimisation the adjusting screw, then re-level, check and refine the adjustment.

The optical axis of the telescope, known as the "sight axis", outlined by the optical center of the targetlens and also the center of the center of attention in its focal plane, should even be perpendicular to the horizontal axis. If not, then a "collimation error" exists.

Index error, horizontal axis error and registration error area unit often determined by activity andarea unit removed by mechanical adjustment. Their existence is taken under consideration within the alternative of measuring procedure so as to eliminate their impact on the measuring results of the transit.