Optical Square

Optical Square It is more accurate than the cross staff and it can be used for locating objects situated at larger distances. It is small and compact hand instrument (Fig.8) and works on the principle of reflection. Generally it is a round brass box about 5 cm in diameter and 1.25 cm deep. There is also a metal cover to protect it from dust, moisture etc. As shown in fig. 8, it consists of horizontal mirror (H) and index mirror (1) placed at an angle of 450 to each other. The mirror H is half silvered and the upper half is plain while the mirror I is fully silvered. There are three openings a, b and c on the sides. Let AB is the chain line and it is required to locate an object O during the process of surveying. The optical square is held in such a manner that a ray of light from object O passes through slot c, strikes the mirror, gets reflected and strikes the silvered portion of the mirror H. After being reflected from H, the ray passes through the pin hole and becomes visible to the eye. The observer looking through the hole a can directly see the ranging rod at B through the un-silvered portion of the mirror H and he image of the ranging rod placed at O. Thus when both the ranging rods coincide, the line OD becomes perpendicular to the chain line. If they do not coincide, the optical square has to move back and forth to get the correct position of D.