Precise methods for plotting lines 2. Where the line is known from its pitch

Both pitch and plunge are angles between a given line and the horizontal. The difference is that the plunge is measured in an imaginary vertical plane whereas the pitch is measured in the plane which contains the line (Fig. 12a). Consequently, on the stereogram (Fig. 12b) both angles are measured from the plotted line L to the primitive circle; the plunge is the angle in a great circle which is a diameter (vertical plane) whilst the pitch is measured in the great circle representing the dipping plane which contains the line. Again the procedure is explained with the aid of an actual example. A lineation defined by aligned amphibole crystals pitches 35S on a foliation plane which dips 015/30SE (Fig. 12c). 1 Plot the foliation plane 015/30SE as a great circle on the tracing paper (Fig. 12d, for method see pp. 20–1). 2 Rotate the net under the overlay until one of its great circles coincides with that for the plotted plane on the tracing paper (Fig. 12e). 3 Starting from the primitive circle, count out the angle of pitch (here, 35°) inwards along the great circle. This gives the plotted position of the line (Fig. 12e). Note that the pitch is 35S, the ‘S’ indicating that the pitch is measured downwards from the southern end of the strike line of the plane. This is why we start our counting from the southern end of the great circle (Fig. 12e).