Concept:An optical square is a precision instrument used to establish perpendicular directions in chain surveying.
Explanation:In chain surveying, surveyors often need to locate details that lie away from the main chain line.
The line joining such a detail to the chain line is usually a perpendicular or right angle.
To set out this right angle accurately, the optical square is used.
It works by reflecting light through mirrors or a prism, allowing the surveyor to see the chain line and the offset direction simultaneously.
The optical square is therefore the correct tool for laying lines at
90∘ to the main chain line.
Station pegs only mark survey stations, the offset staff measures short offset distances, and the steel band measures lengths along the line.
None of these instruments can set out a right angle.
Answer:A. Optical square