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.

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:

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