Measurements by Optical Means

The telescope of the angle measuring instrument called theodolite In this system, is provided with 2 additional cross hairs at a and b which are at distance 'i' . instrument is set at P to measure distance 'D' between two point P and Q and a graduated staff is held vertically at Q and vertical intercept AB is recorded. Then distance D can be measured as described below:

                                 Optical principle of linear measurement

But from the law of optics,

Multiplying throughout by uf, we get

If the distance between objective lence at O and centre of telescope is 'd' then

constants for a given instrument and therefore can be found once for all. Thus by measuring vertical intercept's', distance between P and Q can be found. So distance is easily measured by optical means. It is called Tacheometric measurement. But this measurement is not that precise as obtained by measuring with tape or chain. Reader has to refer specialised books on 'surveying and levelling' for details of this method of survey.