Relationship Between Fore Bearing and Back Bearing
Understanding the relationship between Fore Bearing (F.B.) and Back Bearing (B.B.) in compass surveying for whole circle and quadrantal bearing systems.
Introduction
In compass surveying, bearings are used to represent the direction of survey lines with respect to a reference meridian. The direction measured in the forward direction of a survey line is called the Fore Bearing (F.B.), while the bearing measured in the reverse direction is called the Back Bearing (B.B.).
The relationship between Fore Bearing and Back Bearing is essential for checking the accuracy of field observations and identifying local attraction errors.
Whole Circle Bearing (W.C.B.)
In the Whole Circle Bearing system, bearings are measured clockwise from the north direction and vary from 0° to 360°.
Relationship for Whole Circle Bearing
B.B. = F.B. + 180° (When F.B. < 180°)
B.B. = F.B. − 180° (When F.B. > 180°)
Explanation
Since the back bearing represents the reverse direction of the same line, it differs from the fore bearing by exactly 180°.
Examples
| Fore Bearing (F.B.) | Calculation | Back Bearing (B.B.) |
|---|---|---|
| 60° | 60° + 180° | 240° |
| 220° | 220° − 180° | 40° |
Quadrantal Bearing (Reduced Bearing)
In the Quadrantal Bearing system, bearings are measured from either north or south towards east or west, and the value always lies between 0° and 90°.
Relationship for Quadrantal Bearing
B.B. = F.B.
Change North ↔ South
Change East ↔ West
Explanation
In quadrantal bearings, the angle remains the same while the cardinal directions are reversed.
Examples
| Fore Bearing (F.B.) | Back Bearing (B.B.) |
|---|---|
| N45°E | S45°W |
| S30°E | N30°W |
| N60°W | S60°E |
Importance of Fore Bearing and Back Bearing
- Used for checking the accuracy of compass observations
- Helps detect local attraction in surveying
- Essential in traverse computations
- Improves precision in field measurements
- Important for plotting survey lines correctly
Local Attraction Check
In an ideal condition:
Difference Between F.B. and B.B. = 180°
If the difference between Fore Bearing and Back Bearing is not exactly 180°, it indicates the presence of local attraction due to magnetic disturbances.
Conclusion
The relationship between Fore Bearing and Back Bearing is a fundamental concept in compass surveying. In Whole Circle Bearing systems, the back bearing differs by 180°, while in Quadrantal Bearing systems, the angle remains unchanged but the directions are reversed.
Understanding these relationships is essential for accurate traverse surveying, field calculations, and detection of local attraction errors.
Author
Mohan Dangi ( Gold Medalist )
Civil Engineer | Geotechnical Engineer | Content Creator
References
- B.C. Punmia – Surveying Volume I
- S.K. Duggal – Surveying
- R. Agor – Surveying and Levelling
- IS Codes Related to Surveying Practices
- Standard Civil Engineering Surveying Manuals
Disclaimer
This article is intended for educational and informational purposes only. Surveying methods and bearing calculations may vary depending on instrument type, field conditions, and applicable standards. Readers are advised to refer to standard surveying textbooks and professional engineering guidelines before applying these concepts in practical surveying work.
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