Theodolite components and functions: Select the correct consolidated statement regarding the tangent screw, the levelling head (tribrach), and the levelling screws used to set the vertical rotation axis.
Correct Answer: all the above
Introduction / Context:
Accurate angle measurement requires correct instrument set-up: centering, levelling, and precise pointing. Understanding the function of the tangent screw, tribrach (levelling head), and levelling screws ensures repeatable, high-quality observations with theodolites and total stations.
Given Data / Assumptions:
- Standard theodolite/total station mounted on a tripod.
- Normal field procedures: coarse clamping, fine pointing, and levelling.
- Vertical axis must be truly vertical to avoid angular errors.
Concept / Approach:
The tangent screw provides fine, controlled motion after clamping to precisely bisect targets. The instrument sits on the levelling head (tribrach), which carries the foot- (levelling) screws. By manipulating the levelling screws, the plate is tilted so that the instrument’s rotation axis becomes truly vertical, verified by the circular or tubular bubble.
Step-by-Step Solution:
1) Clamp the motion; use tangent screw for fine sighting.2) Mount on tribrach; center over station via optical plummet.3) Level with the three/four foot screws to align the vertical axis.Verification / Alternative check:
Two-peg test and face-left/face-right observations will expose mislevel or collimation issues, emphasizing correct use of these components.
Why Other Options Are Wrong:
- ‘‘None of these’’ contradicts standard instrument anatomy and procedure.
Common Pitfalls:
- Attempting fine pointing without clamping first, causing backlash effects.
- Levelling with only two screws rather than using the proper opposing-screw technique.
Final Answer:
all the above