Calculation of Included Angle, Local Attraction, Magnetic Declination, True Bearing and Closing Error
In compass surveying, accurate measurement and correction of bearings are essential for reliable results. Surveyors must understand how to calculate included angles, detect local attraction, apply magnetic declination, determine true bearings, and identify closing errors. These concepts are fundamental for Surveyor trade trainees.
Included Angle
The included angle is the angle between two consecutive survey lines in a traverse. It is calculated from the bearings of those lines.
Calculation of Included Angle
- Included Angle = Back Bearing of previous line − Fore Bearing of next line
- If the result is negative, add 360°
Example:
F.B. of AB = 60°
F.B. of BC = 120°
B.B. of AB = 60° + 180° = 240°
Included Angle at B = 240° − 120° = 120°
Local Attraction
Local attraction is the deviation of the magnetic needle due to nearby magnetic materials such as iron, electric lines, or machinery. It affects the accuracy of compass readings.
Detection of Local Attraction
- If F.B. − B.B. ≠ 180°, local attraction exists
- Stations where difference is exactly 180° are considered free from local attraction
Correction of Local Attraction
Corrections are applied to the bearings at affected stations based on error values. One station is assumed correct, and others are adjusted accordingly.
Magnetic Declination
Magnetic declination is the angle between the true meridian and the magnetic meridian.
- East Declination: Magnetic north is east of true north
- West Declination: Magnetic north is west of true north
Relation
- True Bearing = Magnetic Bearing + Declination (East)
- True Bearing = Magnetic Bearing − Declination (West)
True Bearing
True bearing is the angle measured with respect to the true meridian. It is free from magnetic errors and is used for accurate mapping and calculations.
Closing Error
Closing error occurs in a closed traverse when the survey does not return exactly to the starting point. It indicates errors in measurement or calculation.
Types of Closing Error
- Linear Closing Error
- Angular Closing Error
Angular Closing Error
For a closed traverse:
Sum of interior angles = (2n − 4) × 90°
Where n = number of sides
Difference between measured and theoretical sum gives angular error.
Linear Closing Error
It is the distance between starting and ending points of the traverse. It can be corrected using graphical or analytical methods.
Importance of These Concepts
- Ensures accuracy in compass surveying
- Helps in error detection and correction
- Improves reliability of survey data
- Essential for traverse calculations
Precautions
- Check for local attraction before calculations
- Apply correct declination values
- Ensure proper measurement of bearings
- Verify calculations carefully
Application in Surveyor Trade
These calculations are widely used in compass traversing, map preparation, and field surveys. Surveyors use these concepts to ensure accuracy and reliability in their work.
Conclusion
Calculation of included angles, correction of local attraction, application of magnetic declination, and identification of closing errors are essential aspects of compass surveying. For ITI Surveyor trainees, mastering these concepts is crucial for accurate fieldwork and professional success.