Optical Square
- A precision surveying instrument used to set out right angles to a chain line using the principle of reflection.
- Compact round brass box (approx. 5 cm diameter, 1.25 cm deep).
- Contains:
a) Horizontal mirror (H) – half silvered (top: plain, bottom: silvered).
b) Index mirror (I) – fully silvered.
c) Mirrors placed at a 45° angle to each other.
- Three openings: a – viewing hole, b – object or ranging rod slot and c – for incoming light ray from object
- Comes with a metal cover to protect from dust and moisture.
Working Principle
- Based on reflection of light at 45°.
- Light from object O enters through slot c, reflects off index mirror (I) → then reflects from mirror H → reaches the observer’s eye through hole a.
- Observer simultaneously sees: Direct image of ranging rod at point B through plain part of mirror H. and Reflected image of ranging rod at point O.
- When both images coincide, the line OD is perpendicular to the chain line AB.
Usage
- More accurate than cross staff.
- Effective for longer distances.
- Used during chaining to locate offset points perpendicular to a baseline.
Key Points to Remember
- Operates using double reflection.
- Mirrors are set at 45° to each other.
- Ensures accurate perpendicular alignment.
- Used in chain surveying for locating features or boundaries at right angles.