The method used to plot lines depends on the way in which the line's orientation was measured and recorded. As was explained on p. 8, the orientation of a line can be recorded by means of either its plunge or its pitch. The latter measurement is restricted to the situation where the line lies upon a plane that can also be measured, e.g. striae on a fault plane.

The concept of plunge is very straightforward. It is simply a line's angle of tilt away from the horizontal measured in a vertical plane. The linear structure in Figure 11a is not horizontal; it plunges at an angle of 30°, i.e. it makes an angle of 30° with the horizontal in a vertical plane (Fig. 11b). The plunge direction of the fold axis in Figure 11a is 200°, which is the direction of down-tilting parallel to the trend or strike of the same vertical plane.

The plotting procedure

Let us take an actual example of such a plunge measurement. The linear structure in Figure 11a plunges at 30° towards direction 200° (written 30–200). This means that the line in question is tilted at 30° from the horizontal if measured in a vertical plane which trends (strikes) in direction 200°. Therefore to plot this line we:

1. 1 Start, as always, by sketching the stereogram (Fig. 11c).

2. 2 For precise plotting, mark the north and 200° directions on the edge of the tracing sheet (Fig. 11d).

3. […]