Description

Optical Square:

It is more accurate than a cross–staff. Also, it is convenient to use for setting out right angles. It has a circular box with three slits.

The instrument is set on the line whose perpendicular is to be set out. Two slits point towards the ranging rod at the end of the survey line.

Then another ranging rod is to be set at such a point that the two images coincide with each other. This point is on the perpendicular of the initial line.