Sketching a stereogram of a line

This chapter is designed to give you practice in visualizing the way in which the stereographic projection works. At the end of it you will be plotting approximate stereograms of your own. Once you understand how to sketch these approximate stereograms you will find plotting the accurate stereograms (dealt with later) very easy. Do not skip this section even if it seems to you rather simple.

Let us sketch a stereogram showing a linear structure which plunges at 25° in a direction 120°.

Stage 1: Write down the orientation of the linear structure in standard form. In this example we simply write 25–120 (Fig. 7a).

Stage 2: Sketch a miniature map with a north mark pointing up the page. Draw on it the standard map symbol to record the orientation of the linear structure. This is an arrow pointing towards bearing 120 with none 25 written next to the arrow head (Fig. 7b).

Stage 3: Draw a circle (a freehand one of diameter 3 or 4 cm will do) which is to be the primitive circle of our sketch stereogram. Mark the north direction at the top of it and place a small cross to mark the centre (Fig. 7d).

Stage 4: This is where our powers of visualization come in. The circle in Figure 7d is our view of the lower half of a hollow sphere seen from the zenith (top) point, that is we are looking vertically down into the lower hemisphere (Fig. 7c).