![]() Planes project as curves that are actually perfectly circular arcs called cyclographic traces or just great circles. A family of planes dipping at various increments is shown in Fig. 2b), which shows the method for stereographic projection of a dipping plane. To image features on a sheet of paper, these traces and points are projected from a point at the summit or zenithof the sphere onto the equatorial plane. A plane intersects the sphere in a trace that is a great circle that bisects the sphere precisely. Principle of stereographic projectionįor stereographic projection, a line or a plane is imagined to be surrounded by a projection sphere (Fig. Principle of the stereographic projection. In later labs we will use a Schmidt net, which constructs an equal-area projection. We will start with a Wulff net, which is used for the construction of the true, or equal-angle stereographic projection. ![]() There are several varieties of stereonet available. ![]() Protect yourself and others from the thumb tack by keeping it embedded in an eraser while not in use. It is convenient to place an old-fashioned thumb tack through the centre of the net. The stereonet may be reinforced with card to extend its life. It is sometimes helpful to reinforce the centre with adhesive tape on the back of the tracing paper. ![]() Mark E, S and W (or 090, 180 and 270) points at 90° intervals around the primitive. Mark the centre with a cross, and mark a north arrow on the primitive at the top of the page. To construct a stereogram, take a sheet of tracing paper and draw a circle, with the same radius as an available stereonet. For practical reasons we usually turn the tracing paper and keep the net fixed, but it is important to remember that in reality, the projection has a fixed orientation and the net should be rotated to make measurements. To measure angles, we need to rotate the net relative to the tracing paper. It is used to measure angles on the projection. The stereographic net or stereonet is the 3-D equivalent of a protractor. The projection itself, or stereogram, is usually drawn on tracing paper, and represents a bowl-shaped surface embedded in the Earth. There are two parts to any stereographic projection. Wullf net for plotting and measuring features on a stereographic projection Stereogram basics The on-line visualization tool at may also help. Try to imagine that you are looking down into a bowl-shaped depression in the Earth’s surface. However, if you have already met the stereographic projection in mineralogy, the lower hemisphere may take a little getting used to. There is a good reason for this: the lower hemisphere represents the region beneath the Earth’s surface where the rocks have not yet been eroded away. Whereas crystallographers use an upper hemisphere projection, structural geologists always use the lower hemisphere. However, there is one important difference. and is a popular method used by crystallographers as a tool for representing crystal form. Stereographic projection has been in use since the second century B.C. However, it is extremely useful, as orientation problems are very common in structural geology. Unlike structure contouring and other map-based techniques, it preserves only the orientation of lines and planes with no ability to preserve position relationships. Stereographic projection is a powerful method for solving geometric problems in structural geology.
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