Common Errors in Levelling and Their Elimination
In levelling, accurate determination of elevations depends on correct observation and proper handling of instruments. However, various types of errors may occur during fieldwork. These errors can affect the accuracy of results if not properly identified and corrected. For ITI Surveyor trainees, understanding common errors and their elimination is essential.
Types of Errors in Levelling
Errors in levelling can be classified into three main categories:
- Instrumental Errors
- Personal Errors
- Natural Errors
1. Instrumental Errors
These errors occur due to defects or improper adjustment of the levelling instrument.
Common Instrumental Errors
- Incorrect adjustment of bubble tube
- Collimation error (line of sight not horizontal)
- Faulty or damaged levelling staff
Elimination
- Check and adjust instrument regularly
- Perform permanent adjustments
- Use properly calibrated staff
2. Personal Errors
These errors are caused by the surveyor due to carelessness or lack of skill.
Common Personal Errors
- Incorrect staff reading
- Parallax error
- Improper levelling of instrument
- Incorrect booking in level book
Elimination
- Take readings carefully
- Remove parallax by proper focusing
- Ensure correct leveling of instrument
- Record data immediately and clearly
3. Natural Errors
These errors occur due to environmental factors beyond the control of the surveyor.
Common Natural Errors
- Curvature of the earth
- Atmospheric refraction
- Wind and temperature changes
- Settlement of tripod
Elimination
- Apply curvature and refraction corrections
- Avoid observations in extreme weather
- Ensure firm setup of tripod
- Use shorter sight distances
Other Common Errors
1. Collimation Error
Occurs when the line of sight is not horizontal even after leveling.
Correction
- Check by two-peg test
- Adjust instrument properly
2. Staff Not Held Vertical
Leads to incorrect readings.
Correction
- Use staff bubble
- Ensure vertical position of staff
3. Unequal Back Sight and Fore Sight Distance
Leads to systematic errors.
Correction
- Keep BS and FS distances approximately equal
Importance of Error Elimination
- Improves accuracy of levelling
- Ensures reliable results
- Reduces need for rework
- Essential for construction and planning
Precautions
- Handle instruments carefully
- Check adjustments before use
- Take repeated readings if necessary
- Maintain proper field records
Conclusion
Errors in levelling are unavoidable, but they can be minimized by proper techniques and precautions. Understanding the types of errors and their elimination methods helps in achieving accurate survey results.
For ITI Surveyor trainees, mastering error detection and correction is essential for professional and precise surveying work.