Theodolite Adjustment: Key Principles

Proper adjustment of a theodolite is crucial for obtaining accurate measurements in surveying. Several adjustments ensure the instrument's axes are in their correct relationship with each other.

Correct Adjustment Explained

The fundamental requirement for accurate horizontal angle measurement is that the line of collimation must be perpendicular to the horizontal axis (also known as the trunnion axis).

• Line of Collimation: This is the imaginary line passing through the optical center of the objective and the center of the eyepiece.
• Horizontal Axis (Trunnion Axis): This is the axis about which the telescope rotates in a vertical plane.
• Condition: When the line of collimation is perpendicular to the horizontal axis, the plane of revolution of the line of collimation is vertical. This ensures that when the instrument is correctly leveled, horizontal angles are measured in a truly horizontal plane.

Analysis of Options

• Option 1: Axis of plate level is perpendicular to the horizontal axis. This is incorrect. The axis of the plate level should be parallel to the horizontal axis.
• Option 2: Line of collimation is perpendicular to the horizontal axis. This is the correct and essential adjustment for accurate measurements.
• Option 3: Line of collimation is parallel to the vertical axis. This is incorrect. The plane containing the line of collimation and the horizontal axis should be vertical, but the line of collimation itself is not parallel to the vertical axis.
• Option 4: Line of collimation is perpendicular to the vertical axis. This is incorrect. The perpendicularity requirement is with the horizontal axis, not the vertical axis.

Therefore, the correct statement regarding proper theodolite adjustment is that the line of collimation must be perpendicular to the horizontal axis.