Types of bearing in surveying

A bearing is the horizontal angle that a survey line makes with a reference direction (meridian). Bearings are classified in three ways.

1. According to meridian

• True bearing: measured from the true (geographic) meridian, the line through the geographic North and South poles. It is fixed for a place.
• Magnetic bearing: measured from the magnetic meridian, the direction of a freely suspended magnetic needle. It is read with a compass. True bearing = magnetic bearing ± declination.
• Arbitrary bearing: measured from an arbitrary meridian, usually the direction of the first line of a small survey.
• Grid bearing: measured from the central meridian of a map grid (used in large surveys).

2. According to designation

• Whole circle bearing (WCB): angle measured clockwise from the north end of the meridian, $0^\circ$ to $360^\circ$ (prismatic compass).
• Quadrantal bearing (QB): acute angle ($0^\circ$–$90^\circ$) from the north or south end towards east or west, e.g. N 35° E, S 50° W (surveyor's compass).

3. According to path followed by the line

• Fore bearing (FB): bearing of a line in the direction of progress of the survey.
• Back bearing (BB): bearing of the line in the opposite direction.

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

If the FB is less than $180^\circ$, add $180^\circ$; if more, subtract it. Example: FB of AB = $120^\circ$, so BB = $300^\circ$.