Which of the following instruments is used for setting out right angles?
📝 Detailed Explanation: Surveying Instruments and Their Uses
This question tests your knowledge of the specific functions of various basic surveying instruments. While all are used in fieldwork, only one is designed for setting out right angles.
Prism Square
A prism square is a handheld optical instrument used to accurately establish a right angle (90°) from a point on a survey line. It is more precise than a simple cross-staff. It works on the principle of light reflection; the user looks through the instrument and sees two images (of ranging rods placed on the line and at the point to be offset). When the images of the two rods are perfectly aligned, the angle is exactly 90 degrees.
Analysis of Other Instruments
The other options are incorrect as they serve different purposes:
- Clinometer: This instrument is used to measure angles of elevation or depression, which is essential for measuring or setting out slopes and gradients.
- Offset Rod: An offset rod is essentially a ranging rod with notches or a hook at the top, used to help pull a chain or tape through obstacles like hedges or ditches. It is not an angle-measuring device.
- Plumb Bob: This is a simple tool consisting of a pointed weight on a string. It is used to establish a true vertical line, primarily for centering an instrument (like a theodolite) precisely over a station point on the ground.
Key Takeaway
For setting out right angles, surveyors rely on specific tools. While a simple cross-staff can do the job, a prism square or an optical square provides a much higher degree of precision, making them the correct choice for accurate work.