Temporary & Permanent Adjustment (Levelling Instrument)
In levelling, proper adjustment of the instrument is essential to obtain accurate results. Adjustments are of two types: temporary and permanent. Temporary adjustments are performed at every setup, while permanent adjustments ensure the correct relationship between different parts of the instrument. For ITI Surveyor trainees, understanding both types is very important.
Temporary Adjustment
Temporary adjustment is the process of setting up the levelling instrument at a station before taking observations. It is done every time the instrument is shifted.
Steps of Temporary Adjustment
1. Setting up the Instrument
The instrument is mounted on a tripod and placed at a suitable height. The tripod legs are spread firmly on the ground.
2. Levelling
Levelling ensures that the instrument is perfectly horizontal.
- Adjust levelling screws
- Bring the bubble to the center
- Check in all directions
3. Focusing
Focusing is done to remove parallax error and obtain a clear image.
- Focus eyepiece for clear crosshairs
- Focus objective lens on the staff
Permanent Adjustment
Permanent adjustment ensures that the fundamental axes of the instrument are correctly aligned. These adjustments are not required frequently but should be checked periodically.
Main Conditions of Permanent Adjustment
1. Axis of Bubble Tube
The axis of the bubble tube should be parallel to the line of collimation.
2. Line of Collimation
The line of collimation should be horizontal when the bubble is centered.
3. Telescope Axis
The telescope axis should be correctly aligned to ensure accurate sighting.
Importance of Adjustment
- Ensures accuracy in levelling work
- Reduces errors in measurement
- Improves reliability of results
Difference Between Temporary and Permanent Adjustment
| Temporary Adjustment | Permanent Adjustment |
|---|---|
| Done at every setup | Done occasionally |
| Includes levelling and focusing | Includes alignment of instrument parts |
| Simple and quick | Requires skill and precision |
Precautions
- Ensure proper levelling before taking readings
- Handle instrument carefully
- Check adjustments regularly
- Avoid parallax error
- Keep instrument clean and safe
Importance in Surveyor Trade
Both temporary and permanent adjustments are essential for accurate levelling. Proper adjustment ensures correct measurement of elevations and improves the quality of survey work.
Conclusion
Temporary adjustment prepares the instrument for use, while permanent adjustment maintains its accuracy. Both are important aspects of levelling in surveying.
For ITI Surveyor trainees, mastering these adjustments is necessary for accurate and professional fieldwork.