Which of the following tests is used to make the line of sight perpendicular to the horizontal axis of a surveying instrument?
Correct Answer: A. Azimuth test
📚 Detailed Explanation: Azimuth Test Ensures Line of Sight Is Perpendicular to Horizontal Axis
Why A (Azimuth test) is correct: The azimuth test is performed to verify that the line of collimation (line of sight) is exactly perpendicular to the horizontal (transit) axis of the instrument. If not perpendicular, a collimation error exists and the instrument traces a cone rather than a plane when rotated horizontally.
Field Adjustment Tests for Theodolite and Level
| Test Name | Purpose | What It Corrects |
|---|---|---|
| Azimuth test (Plate level test) | Line of sight perpendicular to horizontal axis | Collimation error; ensures the line of sight sweeps a vertical plane |
| Spire test (Horizontal axis test) | Horizontal axis perpendicular to vertical axis | Trunnion axis error; eliminates tipping point error |
| Cross hair ring test (Reticle test) | Vertical cross hair perpendicular to horizontal axis | Ensures the vertical cross hair lies in a vertical plane; allows precise vertical pointing |
| Vertical arc test | Vertical circle index reads 0° when telescope is horizontal | Index error of the vertical circle |
How the Azimuth Test is Performed:
1. Set up the theodolite; centre and level it.
2. Bisect a well-defined target (point P) with the telescope in direct (face left) position. Note the horizontal circle reading.
3. Transit (plunge) the telescope and rotate 180° in azimuth; bisect the same target in reverse (face right).
4. If both readings give exactly 180° difference → line of sight is perpendicular to H-axis.
5. If not, an error exists equal to half the discrepancy → adjust the reticle laterally.
1. Set up the theodolite; centre and level it.
2. Bisect a well-defined target (point P) with the telescope in direct (face left) position. Note the horizontal circle reading.
3. Transit (plunge) the telescope and rotate 180° in azimuth; bisect the same target in reverse (face right).
4. If both readings give exactly 180° difference → line of sight is perpendicular to H-axis.
5. If not, an error exists equal to half the discrepancy → adjust the reticle laterally.
- Azimuth test: verifies that the line of sight is perpendicular to the horizontal axis.
- Spire test: horizontal axis perpendicular to vertical axis.
- Cross hair ring test: vertical cross hair in the correct plane.