The permanent adjustment of dumpy level is an important process in surveying used to ensure that the instrument maintains correct geometric relationships between its main components. Unlike temporary adjustments, which are performed every time the instrument is set up, permanent adjustments are carried out occasionally to correct instrumental errors. These adjustments ensure that the line of sight, bubble axis, and vertical axis of the dumpy level maintain proper alignment. Proper permanent adjustment is essential for obtaining accurate levelling results in civil engineering and surveying projects.
What Is Permanent Adjustment of Dumpy Level
Permanent adjustment refers to the correction of the internal settings of the dumpy level so that the instrument operates according to its designed principles. These adjustments are not required frequently and are usually performed when the instrument shows consistent measurement errors. The purpose of permanent adjustment is to ensure that the line of collimation is perfectly horizontal when the bubble is centered.
Objectives of Permanent Adjustment
Permanent adjustments are necessary to maintain the accuracy and reliability of the instrument.
Main Objectives
- To ensure the axis of the bubble tube is perpendicular to the vertical axis
- To ensure the line of collimation is parallel to the bubble axis
- To maintain correct alignment of the instrument components
- To eliminate systematic instrument errors
These objectives help maintain accurate leveling measurements.
Fundamental Conditions of Permanent Adjustment
For a dumpy level to function correctly, certain geometric conditions must be satisfied.
1. Axis of Bubble Tube Perpendicular to Vertical Axis
When the bubble is centered, the axis of the bubble tube must be perpendicular to the vertical axis of the instrument. This ensures that the telescope remains horizontal when the instrument is rotated.
2. Line of Collimation Parallel to Bubble Axis
The line of collimation (line of sight through the telescope) must be parallel to the bubble axis. When the bubble is centered, the line of sight becomes perfectly horizontal.
3. Crosshair Adjustment
The horizontal crosshair must be truly horizontal, and the vertical crosshair must be vertical. This ensures correct alignment when taking staff readings.
Types of Permanent Adjustments
Permanent adjustment of dumpy level mainly involves the following adjustments.
1. Adjustment of Bubble Tube
This adjustment ensures that the bubble tube axis is perpendicular to the vertical axis of the instrument. The instrument is leveled and then rotated by 180 degrees. If the bubble moves from the center, half of the error is corrected using the leveling screws and the remaining half using the capstan screws of the bubble tube.
2. Collimation Adjustment
Collimation adjustment ensures that the line of sight is parallel to the bubble axis. This is checked using the two-peg test, which is a standard procedure in surveying. If an error is detected, the crosshair ring is adjusted to correct the line of collimation.
3. Crosshair Adjustment
The crosshairs must be properly aligned so that the horizontal hair remains horizontal and the vertical hair remains vertical. If the crosshairs are tilted, they must be adjusted by rotating the diaphragm ring.
Two-Peg Test for Permanent Adjustment
The two-peg test is a common method used to check the accuracy of a dumpy level and adjust the line of collimation.
Procedure of Two-Peg Test
- Select two points A and B on level ground about 60 to 90 meters apart.
- Place the instrument midway between the two points.
- Take staff readings on both points.
- Move the instrument near one point and take readings again.
- Compare the readings to detect collimation error.
If an error exists, adjustments are made to correct the line of sight.
Importance of Permanent Adjustment
Permanent adjustment is necessary to maintain instrument accuracy over time.
Key Benefits
- Ensures accurate levelling measurements
- Maintains instrument reliability
- Reduces systematic errors
- Improves surveying accuracy
- Extends instrument life
These adjustments help surveyors obtain correct elevation data.
Common Signs That Permanent Adjustment Is Needed
Certain indications suggest that permanent adjustment may be required.
Signs of Instrument Error
- Inconsistent staff readings
- Bubble does not remain centered after rotation
- Incorrect leveling results
- Errors detected during two-peg test
When these issues appear, the instrument should be checked and adjusted.
Precautions During Permanent Adjustment
Permanent adjustments should be performed carefully to avoid damaging the instrument.
Important Precautions
- Perform adjustments on stable ground
- Use proper tools for adjustment screws
- Avoid excessive tightening of screws
- Verify results after adjustment
- Follow correct testing procedures
These precautions ensure safe and accurate instrument adjustment.
Difference Between Temporary and Permanent Adjustment
| Adjustment Type | Purpose | Frequency |
|---|---|---|
| Temporary Adjustment | Prepare instrument for use | Every setup |
| Permanent Adjustment | Correct instrument errors | Occasionally |
| Temporary adjustments prepare the instrument for daily surveying work, while permanent adjustments maintain long-term accuracy. |
Applications of Properly Adjusted Dumpy Level
A properly adjusted dumpy level is used in many engineering applications.
Major Applications
- Building foundation leveling
- Road construction surveys
- Railway alignment
- Canal construction
- Land surveying projects
These applications require precise elevation measurements.
FAQs About Permanent Adjustment of Dumpy Level
What is permanent adjustment of dumpy level?
Permanent adjustment is the process of correcting the internal alignment of the dumpy level to ensure accurate measurements.
What is the two-peg test?
The two-peg test is a method used to check and correct the line of collimation error in a dumpy level.
How often is permanent adjustment required?
Permanent adjustment is performed occasionally when instrument errors are detected.
Why is permanent adjustment important?
It ensures that the instrument provides accurate leveling results.
Conclusion
The permanent adjustment of dumpy level is essential for maintaining the accuracy and reliability of the instrument in surveying operations. These adjustments ensure that the bubble axis, line of collimation, and crosshairs are correctly aligned. Procedures such as the two-peg test help detect and correct instrument errors. By performing permanent adjustments when necessary, surveyors can ensure precise elevation measurements and reliable surveying results in civil engineering projects.
