Bearings and local attraction explained

Whole circle bearing (WCB)

The bearing of a line measured clockwise from the north end of the meridian, $0^\circ$ to $360^\circ$. Example: a line pointing south-west may have WCB $225^\circ$. This is the system of the prismatic compass.

Quadrantal bearing (QB), or reduced bearing

The acute angle ($0^\circ$ to $90^\circ$) between the line and the nearer end (north or south) of the meridian, measured towards east or west. It is written with the quadrant letters, e.g. N 40° E, S 30° W. This is the system of the surveyor's compass.

Fore bearing (FB) and back bearing (BB)

The bearing of a line in the direction of progress of the survey is its fore bearing; the bearing of the same line in the opposite direction is its back bearing. They differ by exactly $180^\circ$:

$$\text{BB} = \text{FB} \pm 180^\circ$$

Use $+$ if FB $< 180^\circ$ and $-$ if FB $> 180^\circ$. Example: FB of AB = $65^\circ 30'$, BB = $245^\circ 30'$.

Local attraction

The deviation of the magnetic needle from the magnetic meridian caused by magnetic substances near the station, such as iron objects, steel structures, power lines, or rocks containing iron ore. Bearings observed at an affected station are in error by the same amount.

• Detection: the FB and BB of a line differ by $180^\circ$ if neither end is affected; otherwise one or both stations are affected.
• Correction: (i) the line with FB − BB = $180^\circ$ is taken as free; bearings of the other lines are corrected station by station from it, or (ii) the included-angle method is used, because the angle at an affected station is not affected.