Stereographic projection

3D Model

This model is a 3D depiction of how planar and linear geologic structures are plotted onto a circular grid using stereographic projection. A dipping planar surface (coloured red) such as a bedding plane is intersected with the lower hemisphere, and the curve of contact is then projected toward the zenith (yellow coloured cone). The intersection of this cone with the equatorial plane produces a great circle, representing the planar surface as a simple curve in 2D space.

The same plane can also be represented as a simple point by intersecting a normal to the plane (a line perpendicular to it - coloured blue in this example) with the lower hemisphere, and projecting that point to the zenith in the same fashion. A point is plotted where this projection intersects the equatorial plane, so our planar surface is now represented by a single point. Note that the point is 90° away from the great circle.

Linear geological features such as stretching lineations can be plotted in the same way the pole to a plane is.