Short Answer:

In compass surveying, fore bearing is the angle measured in the direction from the starting point to the next point of a survey line. Back bearing is the angle measured in the reverse direction—from the end point back to the starting point.

Both fore bearing and back bearing help in checking accuracy and detecting any magnetic errors during surveying. Ideally, they should differ by 180°. These bearings are important for completing traverses, drawing accurate maps, and ensuring reliable field data in compass-based surveys.

Detailed Explanation:

Fore bearings and back bearings in compass surveying

In compass surveying, measuring the direction of a survey line is done using bearings. Bearings are the horizontal angles measured from a reference direction, usually magnetic north. To ensure complete and accurate surveying of a line, both directions of measurement are considered: the forward direction and the reverse direction. These are known as fore bearing (FB) and back bearing (BB) respectively.

Understanding fore and back bearings is essential for accurate plotting of survey lines, for closing traverses properly, and for identifying any possible errors due to local attraction or magnetic disturbances.

Fore bearing

Fore bearing is the angle measured from the starting point of a line to its forward end, with reference to magnetic north. It is the main reading taken when a surveyor sights forward from one station to the next.

Measured in the direction of progression in the survey.
Taken using a compass by facing the next point in the line.
If whole-circle bearing system is used, FB ranges from 0° to 360°.
If quadrantal system is used, it is noted with directions like N 40° E.

Fore bearings are recorded during fieldwork and used in the preparation of maps and traverse closure.

Back bearing

Back bearing is the angle measured in the opposite direction of a survey line, i.e., from the endpoint of a line back to its starting point.

It represents the reverse direction of the same line.
It is helpful in checking the correctness of the fore bearing.
In whole-circle bearing system:
BB = FB ± 180°
(Add 180° if FB < 180°, subtract 180° if FB ≥ 180°)
In quadrantal system: BB is the same angle but with the cardinal directions reversed (e.g., N 40° E becomes S 40° W).

Back bearings help verify measurements and ensure that the readings are consistent and free from local attraction errors.

Importance in compass surveying

Error checking: Comparing fore bearing and back bearing helps identify magnetic disturbances or local attraction, where the needle gets deflected due to nearby metal objects or electric fields.
Traverse closure: For closed traverses, bearings are essential to compute interior angles and close the polygon correctly.
Plotting accuracy: Bearings (both FB and BB) are used to draw the exact direction of lines on a map or plan.
Adjustment: If local attraction is present, adjustments are made based on the difference between fore and back bearings of the same line.

Example:

If the fore bearing of a line is 70°, then the back bearing should be:

BB = 70° + 180° = 250° (if FB < 180°)

If the FB is 210°, then:

BB = 210° – 180° = 30°

If the difference is not 180°, it indicates a local attraction or instrument error.

Conclusion:

Fore bearings and back bearings are the forward and reverse directions of a survey line measured using a compass. They are essential for checking accuracy, detecting errors, and ensuring correct survey plotting. Ideally, they differ by 180° in whole-circle readings. Understanding and using both types of bearings properly helps in achieving precise and reliable results in compass surveying work.