Offsetting by Optical Square:

If location D is fixed, denoting some important ground fixture, e.g. an electrical pole and offset is required from it on survey line. A surveyor aligns the instrument along AB and then moves sideways along line AB, till, once again, the ranging rod at B and image of D coincide locating the position of C on ground such in which CD is perpendicular to AB. The position of C is transferred on ground along with the help of the plumb bob that was hanging from the centre of instrument.

Figure(b): Offsetting by Optical Square

The continuous use of optical square over a period of time and the rough handling of instruments may cause the included angle between the mirrors M₁ and M₂ to change from original 45^o. This will induce errors within setting the offsets. If the angle becomes, say, 40^o, the offset will be at 80^o in place of 90^o along with the survey line, although if involved angle becomes 50^o, the offset angle becomes 100^o. The instrument could be tested for accuracy and adjusted extremely simply by procedure described in following paragraph.