Q.58 Which of the following options represents an INCORRECT relation between the fundamental axes of a theodolite?
- AThe axis of the plate level must be perpendicular to the vertical axis.
- BThe axis of the altitude level must be parallel to the line of collimation.
- CThe axis of the striding level must be perpendicular to the transit axis.Correct
- DThe trunnion axis must be perpendicular to the line of sight.
Solution
Concept
In a theodolite, the axis of the striding level must be parallel to the horizontal (transit/trunnion) axis, not perpendicular to it.
Step-by-step solution
1. Plate level axis must be perpendicular to the vertical axis. Correct. 2. Altitude (telescope) level axis must be parallel to the line of collimation. Correct. 3. Striding level axis must be parallel to the transit (horizontal) axis. Option (C) says perpendicular, so it is wrong. 4. Trunnion (horizontal) axis must be perpendicular to the line of sight. Correct. 5. The incorrect relation is (C).
Explanation
The striding level sits on the trunnion axis and is used to check that this axis is horizontal. So its axis must be parallel to the transit axis. Saying 'perpendicular' is wrong, so option (C) is the incorrect relation.
Why the other options are wrong
(A) Correct relation: plate level axis ⊥ vertical axis. (B) Correct relation: altitude level axis ∥ line of collimation. (D) Correct relation: trunnion axis ⊥ line of sight.
Shortcut
A striding level 'rides' on the trunnion axis, so it must be parallel to it.
Common mistake
Mixing up the relations. Only the striding level axis is parallel to the horizontal axis. The plate level axis is perpendicular to the vertical axis.
Correct answer: C) The axis of the striding level must be perpendicular to the transit axis.