Optical Square:

It might be remembered that at least two surveyor's are required to set normal offsets while using cross staff. This drawback could be overcome if optical square is used for these reasons as only single surveyor could perform this task.

The optical square, as display in Figure (a), is constructed on the basis of simple law of optics that the angle among the first incident ray and the final reflected ray is twice that of the angle among a pair of plane mirrors. Therefore, in optical square two mirrors M¹ and M² are mounted on a frame such that the included angle between them is 45^o. The incident ray from object D is reflected from mirror M1 in line M₁M₂ to mirror M₂ and finally along AM₂ by mirror M₂. Since included angle between mirrors M₁ and M₂ is 45^o, the angle between rays DM₁ and AM₂ will be 2 × 45 = 90^o.

Figure: Optical Square

In order to set the offset at point C on survey line AB, the optical square is held horizontally over the station C with the help of a plumb bob, and line of sight is aligned along line AB by observing ranging rod at B through opening at G. Further the ranging rod at D is observed by mirrors. The image of rod D is made to coincide along with the image of rod B by moving ranging rods sideways (Figure). While the two images coincide, then the line CD becomes perpendicular to line AB.