Theodolite checks – interpreting bubble behavior during horizontal rotation If, while rotating a theodolite in the horizontal plane, the bubble in the bubble tube occupies the same position throughout the rotation, what does this indicate about the instrument geometry?

Introduction / Context:
The accuracy of angular measurements with a theodolite depends on the relationship among its principal axes: the vertical (rotation) axis, the trunnion (horizontal) axis, and the line of collimation. Bubble tube observations provide quick diagnostics of these relationships. This question asks you to interpret a stable bubble during a full horizontal rotation of the instrument.

Given Data / Assumptions:

• The instrument is rotated 360 degrees about the vertical axis.
• The bubble in the level attached to the instrument maintains the same position throughout.
• We assume the bubble tube is aligned to indicate horizontality of the instrument base plate.

Concept / Approach:

If the vertical axis is perfectly vertical, the instrument’s base plane remains horizontal during rotation, and the bubble does not wander. Any tilt in the vertical axis would cause the bubble to move cyclically as the instrument turns. Therefore, a stationary bubble indicates that the rotation axis has been leveled to true vertical. Though other axis conditions are also important, this specific observation directly diagnoses the verticality of the rotation axis.

Step-by-Step Solution:

Verification / Alternative check:

Standard field checks use this observation to verify vertical axis adjustment before precise angle work; any drift indicates the need for footscrew correction or instrument servicing.

Why Other Options Are Wrong:

Trunnion axis horizontal: tested by level swings in vertical plane, not by this horizontal rotation bubble stability alone.

Line of collimation perpendicular to vertical axis: assessed via collimation tests (face left/right), not solely by bubble stability during rotation.

Common Pitfalls:

Assuming bubble constancy proves all adjustments; in reality it confirms only verticality of the rotation axis.

Final Answer:

The rotation axis is truly vertical