Geology

The direction of a line on the ground is given by the bearing of the line, which is the horizontal angle between the line and a reference, commonly north in the quadrant scale, or 000o (marked as 0o on the card) in the azimuth scale. The reference, however, can also be the south (S) in the quadrant scale, when reading the bearing (i.e., trend) of south-trending linear objects. The position of 'E' and 'W' are reversed on the circular card; 'E' lies left of the 0o mark (i.e., at 9 o'clock), and 'W' is to the right of the 0o (i.e., 3 o'clock) mark on the card. The reversal is designed to make the correct reading of the bearing possible. To appreciate this fact, notice that the north-seeking end of the needle always stays pointing north even when the compass dial is rotated. For example, to read a bearing of 045o, we level the dial and then turn right of north, but the north-seeking end of the needle turns to the left of 0o, which is actually east on the dial; so we read a correct bearing.

The vertical angle between the magnetic vectors relative to the level (horizontal) ground is the magnetic inclination, which varies with latitude; it is 90° at the north magnetic pole, and 0° at the magnetic equator

Measuring the bearing of a line between two points

Commonly we want to measure the trend and plunge of a line connecting two points, e.g., a line connecting a person and another person, or another landmark (e.g., house, tower, smoke stack). To do this we can either use the eye-level or waist-level sighting. The eye-level sighting was described above. For the waist-level sighting, we put the lid against our body, and tilt the lid while holding the box horizontally by centering the bull's eye level. Position the target on the black line on the mirror, and after centering the round level, read the trend.

Measure the plunge of this line as follows. Flip the compass (box is vertical) while the lid and sighting arm are folded. Look through the hole in the sighting tip and through the sighting window, and then center the clinometer level while shooting to a specific point on the target. If the two persons have the same height, intersect the other person's eyes with the black line on the sighting window. If we are sighting (shooting) to another person who is shorter than we are, say by 5 cm, then we shoot 5 cm above that person's eye level (at forehead or head level). If the other person is taller by 5 cm, then we shoot to the mouth level of that person.