Stage 1- - the structural line or plane is projected onto a sphere Let us start by projecting a structural line. Figure 6a shows such a line as it is observed, e.g. in the field. Alongside it we imagine a hollow sphere. The line is now shifted from the outcrop and, without rotating it, placed at the centre of the sphere (Fig. 6c). Now the length of the line is increased downwards until the line meets the sphere's surface. The point where the line and sphere meet is called the spherical projection of the line. This point is always located in the lower half of the sphere and its exact position depends on the orientation of the line.

A plane (Fig. 6b) is projected in the same way. It is translated to the sphere's centre and extended down until it touches the surface of the lower hemisphere (Fig. 6d). The line of contact is now a circle; a circle on the sphere with the same radius as the sphere itself. Such circles on a sphere which are produced by the intersection of a plane which passes through its centre are called great circles.

Thinking of the Earth as an approximate sphere, lines of longitude are great circles and so is the equator (Fig. 6e). However, there are other circles on the globe such as the polar circles and the tropics which do not qualify as great circles because they have a smaller radius than the Earth.