Question

The least count of a self-reading levelling staff is

• A. 0.5 mm
• B. 0.15 cm
• C. 5.0 cm
• D. 5.0 mm

Hint:

The least count of a levelling instrument refers to the smallest value that can be accurately read. For a self-reading staff, this is typically 5 mm, i.e., 0.005 m.

Answer 1

The Correct Option is D

Step 1: Understanding the levelling staff.
A self-reading levelling staff is a graduated staff which is read directly by the surveyor using an optical level. The smallest division on the staff determines its least count.
Step 2: Standard least count.
Most self-reading levelling staffs are graduated in 5 mm intervals.
Hence, the least count is \( \boxed{5.0 \text{ mm}} \).
Step 3: Eliminate incorrect options.
(1) 0.5 mm: too fine for a staff marking.
(2) 0.15 cm = 1.5 mm: again, finer than usual graduation.
(3) 5.0 cm = 50 mm: too coarse.