Adjustment of Closing Error and Precautions in Using Prismatic Compass
In compass surveying, errors may occur due to inaccuracies in measurement, local attraction, or instrument handling. These errors lead to closing error in a traverse. Proper adjustment of closing error and correct handling of the prismatic compass are essential for achieving accurate results. For Surveyor trade trainees, understanding these concepts is very important.
Closing Error
Closing error occurs when a closed traverse does not return exactly to its starting point. It indicates the presence of errors in measurement of bearings or distances.
Adjustment of Closing Error
Adjustment of closing error is the process of distributing the error among the survey lines to obtain accurate results.
1. Graphical Method
In this method, the traverse is plotted on a drawing sheet. The closing error is represented by the gap between the starting and ending points. This gap is then adjusted proportionally along the survey lines.
- Simple and easy method
- Suitable for small surveys
2. Bowditch’s Method (Compass Rule)
This is the most commonly used method for adjusting closing error. The correction is distributed proportionally based on the length of each survey line.
Correction ∝ Length of the line
- Suitable when both linear and angular measurements are of equal accuracy
- Provides balanced adjustment
3. Transit Method
In this method, corrections are distributed based on the latitude and departure of each line.
- Used when angular measurements are more accurate than linear measurements
Steps in Adjustment
- Calculate the closing error
- Determine total length of traverse
- Apply corrections proportionally
- Adjust coordinates of each station
- Recheck the closure
Importance of Adjustment
- Improves accuracy of survey data
- Ensures proper closure of traverse
- Helps in correct plotting and mapping
Prismatic Compass
The prismatic compass is an important instrument used in compass surveying for measuring bearings. Proper handling and precautions are necessary to obtain accurate readings.
Precautions in Using Prismatic Compass
- Keep the compass away from magnetic materials such as iron, electric wires, and vehicles
- Ensure proper centering over the station
- Level the instrument correctly
- Allow the magnetic needle to come to rest before taking readings
- Take readings carefully to avoid parallax error
- Protect the instrument from dust, moisture, and damage
- Check for local attraction at each station
- Avoid working in strong wind conditions
Common Mistakes to Avoid
- Taking readings before the needle stabilizes
- Improper alignment of sight vane
- Ignoring local attraction
- Incorrect recording of bearings
Importance in Surveyor Trade
Adjustment of closing error ensures accuracy in traverse surveys, while proper use of prismatic compass ensures reliable bearing measurements. Both are essential skills for surveyors in fieldwork.
Conclusion
Closing error is a common issue in surveying, but it can be minimized and adjusted using proper methods like Bowditch’s rule. At the same time, careful handling of the prismatic compass is necessary to avoid errors.
For ITI Surveyor trainees, mastering these techniques is essential for accurate and professional surveying work.