For taking an oblique offset which makes an angle of 45° with the chain line, the instrument used is the
- AAdjustable cross-staff
- BOpen cross-staff
- CFrench cross-staffCorrect
- DAll of the above
Solution
Concept
Instrument for setting out oblique offsets
Formula
NONE
Step-by-step solution
1. An open cross-staff can only establish right angles (90 degrees) with a chain line. 2. A French cross-staff features an octagonal box with vertical sighting slits on eight sides. 3. This construction enables setting out angles of both 45 degrees and 90 degrees with the chain line. Answer: C
Explanation
A French cross-staff consists of an octagonal brass tube with vertical viewing slits on its sides. Unlike an open cross-staff which only allows sighting lines at right angles (90°), the French cross-staff allows sight lines at both 45° and 90°. This feature makes it suitable for setting out 45° oblique offsets along a survey chain line.
Why the other options are wrong
A) An adjustable cross-staff can set out any angle, but the French cross-staff is specifically fixed for 45° and 90°. B) An open cross-staff can only set out right angles (90°). D) An open cross-staff cannot measure a 45° angle.
Shortcut
French cross-staff measures 45° and 90° angles (Open cross-staff measures only 90°).
Common mistake
Confusing open cross-staff, which only sets 90-degree angles, with French cross-staff, which can set 45-degree angles.
Correct answer: C) French cross-staff