Introduction

The emphasis in this book is the geometrical description and analysis of geological structures, especially by graphical means. The basis of much of this is descriptive geometry: the art of accurately drawing three-dimensional objects and of graphically solving associated space problems. It is based on the idea of depicting such objects by means of projections. Everyday examples of projections are shadows and photographs. Both are the result of projecting various parts of an object to a plane by rays of light. These rays are projectors which connect points on the object with the corresponding points on the image plane.

By projecting an object to an image plane, a view of that object is obtained. Alternatively, a view may be thought of as an actual picture of the object obtained along a line of sight perpendicular to the corresponding image plane.

Orthographic projection

The simplest type of projection, and the one used most in engineering as well as for many purposes in geology, is the orthographic projection. Orthographic means “drawn at right angles” and refers to parallel projectors that are perpendicular to an image plane (Fig. A.1a). The most important property of this projection is that the images of objects appear in their true shape.