Q.67 To measure vertical angles, the theodolite must be levelled with reference to
- Athe altitude bubble on the telescope
- Bthe altitude bubble on the index frameCorrect
- Cthe altitude bubble on both the telescope and the index frame
- Dthe bubble tube on the plate level
Solution
Concept
For vertical angles, the theodolite is levelled using the altitude bubble fixed on the index (vernier) frame.
Step-by-step solution
1. Plate level bubbles are used to level the instrument for horizontal angles. 2. For vertical angles, the vertical circle index must be truly horizontal. 3. This is checked with the altitude bubble on the index (vernier) frame. 4. So the answer is B.
Explanation
Vertical angles are read on the vertical circle using the index frame (vernier frame). The altitude bubble on the index frame makes the index line exactly horizontal. So levelling with this bubble gives correct vertical angles.
Why the other options are wrong
A: In the usual transit theodolite, the altitude bubble is on the index frame, not the reference used here. C: Two altitude bubbles are not needed. D: The plate level is used for horizontal angles.
Shortcut
Horizontal angle = plate level; vertical angle = altitude bubble on index frame.
Common mistake
Choosing the plate level, which only levels the horizontal plate.
Correct answer: B) the altitude bubble on the index frame