Q.69 Identify the correct statements regarding theodolite. i. The axis of the plate level must lie in a plane parallel to the vertical axis. ii. The horizontal axis must be perpendicular to the vertical axis. iii. The vertical circle vernier must read zero when the line of collimation is horizontal. iv. The axis of the altitude level must be perpendicular to the line of collimation.
- AOnly i and iii
- BOnly i and iv
- Ci, ii, iii and iv
- DOnly ii and iiiCorrect
Solution
Concept
A theodolite in good adjustment must satisfy fixed relations between its fundamental lines (axes).
Step-by-step solution
1. Plate level axis must be perpendicular to the vertical axis, not parallel. So statement i is wrong. 2. Horizontal (trunnion) axis must be perpendicular to the vertical axis. So statement ii is correct. 3. Vertical circle vernier must read zero when the line of collimation is horizontal. So statement iii is correct. 4. Axis of altitude (telescope) level must be parallel to the line of collimation, not perpendicular. So statement iv is wrong. 5. Only ii and iii are correct, so the answer is D.
Explanation
In a correct theodolite, the plate level axis is perpendicular to the vertical axis and the horizontal axis is perpendicular to the vertical axis. The altitude level axis must be parallel to the line of collimation. The vertical circle must read zero when the telescope is horizontal.
Why the other options are wrong
A: Statement i is wrong (the plate level axis is perpendicular to the vertical axis). B: Statements i and iv are both wrong. C: Statements i and iv are wrong, so all four cannot be correct.
Shortcut
Plate level ⟂ vertical axis; altitude level ∥ line of collimation. Any statement saying the opposite is false.
Common mistake
Mixing up 'parallel' and 'perpendicular' for the plate level and the altitude level.
Correct answer: D) Only ii and iii