Permanent Adjustment of Theodolite
Permanent adjustment of a theodolite refers to the process of setting the fundamental relationships between the different parts of the instrument so that accurate measurements can be obtained. Unlike temporary adjustments, these adjustments are not required at every setup but must be checked periodically.
For ITI Surveyor trainees, understanding permanent adjustments is essential to ensure the accuracy and reliability of theodolite observations.
Objective of Permanent Adjustment
- To maintain correct alignment of instrument parts
- To eliminate systematic errors
- To ensure accurate angle measurement
Fundamental Lines of Theodolite
Permanent adjustments are based on maintaining proper relationships between the following lines:
- Vertical axis
- Horizontal axis (trunnion axis)
- Line of collimation
- Axis of plate level
- Axis of telescope level
Main Conditions of Permanent Adjustment
1. Plate Level Adjustment
The axis of the plate level should be perpendicular to the vertical axis.
- If the bubble is centered, the instrument is level
- If not, adjust using levelling screws
2. Collimation Adjustment
The line of collimation should be perpendicular to the horizontal axis.
- Check by sighting a distant object
- Adjust crosshairs if necessary
3. Horizontal Axis Adjustment
The horizontal axis should be perpendicular to the vertical axis.
- Ensures proper rotation of telescope
- Checked by observing vertical alignment
4. Telescope Level Adjustment
The axis of the telescope level should be parallel to the line of collimation.
- Ensures accurate vertical angle measurement
5. Vertical Circle Index Adjustment
The vertical circle should read zero when the line of sight is horizontal.
- Adjust index frame if necessary
Importance of Permanent Adjustment
- Ensures accuracy in angle measurement
- Reduces systematic errors
- Improves reliability of survey work
- Essential for precise engineering projects
Precautions
- Handle the instrument carefully
- Check adjustments regularly
- Avoid rough handling or shocks
- Keep the instrument clean and dry
- Use proper tools for adjustment
Difference Between Temporary and Permanent Adjustment
| Temporary Adjustment | Permanent Adjustment |
|---|---|
| Done at every setup | Done occasionally |
| Includes centering, levelling, focusing | Includes alignment of instrument parts |
| Simple and quick | Requires skill and precision |
Importance in Surveyor Trade
Permanent adjustment is essential for maintaining the accuracy of the theodolite. Without proper adjustment, all measurements may contain errors, affecting the quality of survey work.
Conclusion
Permanent adjustment of theodolite ensures correct alignment of its components and accurate measurement of angles. It is a crucial aspect of surveying that helps maintain precision in fieldwork.
For ITI Surveyor trainees, understanding and applying these adjustments is important for professional and accurate surveying practices.