About Lesson
Working Principle of open-cross staff
- Used to lay out right angles from a chain line.
- Sight object through the narrow slit in line with the horse hair on the wider slit.
- Rotate the instrument until the ranging rod is visible in the slit line.
- Then sight through the perpendicular pair of slits, and have a helper place a ranging rod in that line of sight.
- The line joining the base of the cross-staff and second ranging rod is perpendicular to the original chain line.
Key Points to Remember
- Open Cross-Staff is basic but effective for right angle setting.
- No angular measurement is done — only visual alignment.
- Essential for setting offsets and boundaries in chain surveying.