Q33. In which of the following plane, the telescope of the theodolite is turned in order to swing?
📚 Detailed Explanation: Swinging the Telescope — Horizontal Plane Motion
“Swinging” is a specific, named theodolite operation distinct from “transiting” (Q28). Knowing which plane each motion occurs in prevents the two from being confused.
Swinging the telescope means rotating it about the vertical axis, which sweeps the line of sight through the horizontal plane — left and right, as when turning to sight a new target at a different bearing or azimuth. This is the motion used whenever measuring a horizontal angle between two targets, or when re-orienting the instrument to face a new direction.
Why the Other Options Are Wrong
A — Horizontal axis: The horizontal (trunnion) axis is what the telescope rotates about for vertical tilting (transiting), not for swinging — swinging instead occurs about the vertical axis.
C — Inclined plane: There is no standard theodolite motion described as occurring in an “inclined plane” — the telescope’s two fundamental rotational motions are strictly in the horizontal plane (swinging) and the vertical plane (transiting/tilting).
D — Vertical plane: Vertical plane motion describes the telescope tilting up and down (about the horizontal/trunnion axis) — this is the motion used for vertical angle measurement, the opposite of swinging.
Key Concepts for Students
- Swing = horizontal plane (about vertical axis); Tilt/transit = vertical plane (about horizontal axis): These two motions are perpendicular to each other and serve entirely different measurement purposes — horizontal angle measurement uses swinging, vertical angle measurement uses tilting.
- Left swing vs. right swing: Swinging can occur clockwise (right swing, the standard convention) or counter-clockwise (left swing, less favoured — see Q47). Both are still horizontal-plane motions, differing only in direction.
- Practical use of swinging: Every time a surveyor rotates the theodolite to sight a different target for a horizontal angle measurement (whether using upper motion or lower motion, see Q12 and Q25), that rotation is technically a “swing” through the horizontal plane.