The imaginary line passing through the intersection of cross hairs and the optical centre of the objective lens is called:
Correct Answer: B. Line of collimation
📚 Detailed Explanation: Line of Collimation = Line Through Intersection of Cross Hairs and Optical Centre of Objective
Why B (Line of collimation) is correct: The line of collimation (also called the line of sight) is the imaginary line that passes through the intersection of the cross hairs (at the diaphragm) and the optical centre of the objective lens. When the instrument is correctly adjusted and levelled, this line is horizontal and defines the reference for all staff readings.
Important Lines in a Level Instrument
| Line / Axis | Definition |
|---|---|
| Line of collimation (Line of sight) | Line through: intersection of cross hairs + optical centre of objective lens. Should be horizontal when instrument is levelled. |
| Axis of the bubble tube | Tangent to the upper curve of the bubble tube at its mid-point; should be horizontal when bubble is centred |
| Vertical axis (Axis of rotation) | The axis about which the instrument rotates in plan; should be truly vertical |
| Axis of the telescope | The geometric axis of the telescope tube; should coincide with the line of collimation |
Collimation Error: If the line of collimation is not parallel to the axis of the bubble tube, a collimation error exists. Even when the bubble is centred, the line of sight is not horizontal. This is eliminated by the two-peg test and by reciprocal levelling.
- Line of collimation = through intersection of cross hairs + optical centre of objective.
- Also called the line of sight; must be horizontal when the instrument is properly levelled.
- Collimation error = line of collimation NOT parallel to bubble tube axis.