The two-peg test of a level is performed to check:
Correct Answer: A. The line of collimation is parallel to the axis of the bubble tube
📚 Detailed Explanation: Two-Peg Test Checks That Collimation Axis Is Parallel to Bubble Tube Axis
Why A (The line of collimation is parallel to the axis of the bubble tube) is correct: The fundamental requirement for a correctly adjusted level is that when the bubble is centred, the line of collimation (line of sight) must be horizontal. This requires the line of collimation to be parallel to the axis of the bubble tube. The two-peg test detects and allows correction of any deviation from this condition.
Two-Peg Test Procedure and Logic
| Step | Action | What Is Found |
|---|---|---|
| 1. Set pegs A and B | Place two pegs about 50–80 m apart on level ground | — |
| 2. Level at midpoint C | Set instrument exactly halfway between A and B; read staff at A and B | True height difference (collimation and curvature cancel); difference = true diff |
| 3. Level near peg A | Set instrument very close to peg A (within 2–3 m) | Read staff at A (nearly true reading, tiny error). Calculate what staff at B should read if instrument is correct. |
| 4. Read staff at B | Read actual staff at B from the near-A position | Compare actual vs. expected reading at B. Difference = total collimation error over full distance. |
| 5. Adjust | Use the diaphragm screws (cross-hair screws) to make the line of collimation read the expected (correct) value on the B staff | After adjustment: line of collimation is parallel to bubble tube axis ✓ |
What the test checks: The two-peg test verifies that the line of collimation is parallel to the bubble tube axis — i.e., when the bubble is centred, the line of sight is truly horizontal. It does NOT check the vertical axis or the plate bubble.
- Two-peg test checks: line of collimation parallel to axis of bubble tube.
- Detects collimation error (line of sight not horizontal even when bubble is centred).
- Correction: adjust the diaphragm screws (not the foot screws, not the bubble).