Compass surveying – definition of back bearing In routine field work, the back bearing (BB) of a line is related to its fore bearing (FB) by which relationship?

Introduction / Context:
Bearings denote the direction of a line with respect to a reference meridian. In traverse work, it is common to observe both fore bearing (from the rear station to the forward station) and back bearing (from the forward station back to the rear). This item checks your recall of the fundamental angular relation between these two quantities in whole-circle or quadrantal systems when properly corrected for station errors and local attraction.

Given Data / Assumptions:

• The same reference meridian is used for both observations.
• No local attraction or index error remains after correction.
• Bearings are expressed in degrees.

Concept / Approach:

Direction reversal rotates the line by a straight angle. On a plane, reversing the direction of a vector changes its bearing by exactly 180 degrees. Therefore, the back bearing equals the fore bearing plus or minus 180 degrees, adjusted into the 0°–360° range when reported as whole-circle bearings. The same principle applies for quadrantal bearings after converting appropriately.

Step-by-Step Solution:

Verification / Alternative check:

Check with a simple example: if FB = 60°, then BB should be 240°. Adding or subtracting 180° yields the correct reverse direction every time, confirming the rule.

Why Other Options Are Wrong:

±90° or ±270°: correspond to perpendicular or three-quarter turn, not a direction reversal.

±360°: brings you back to the same direction, not the reverse.

Common Pitfalls:

Forgetting to bring the result back into the 0°–360° range; mixing up magnetic and true bearings without applying declination corrections.

Final Answer:

Fore bearing ± 180°