The true dip of a planar structure is its angle of slope measured in a vertical section perpendicular to the strike. In Figure 15b the true dip is labelled T.

On any vertical section with another trend the plane will appear to dip at a different angle, the angle of apparent dip (labelled A in Fig. 15b). As an extreme example, if bedding planes with a true dip of 60° are viewed in cross-section on a vertical plane parallel to their strike they will show an apparent dip of 0°.

Figure 15a shows dipping beds of Jurassic limestone at Bridgend, South Wales. The stone wall constructed on a bedding plane (left of photograph) is aligned at an angle to the strike of the bedding plane and therefore its base is tilted at an angle equal to the apparent dip for the direction of the wall. The angle of apparent dip is less than the true dip.

The angle of apparent dip depends on two factors:

1. 1 the angle of true dip;

2. 2 the angle between the plane of section and the strike direction.

This is illustrated in Figure 15c where a true dip of 40° yields an apparent dip of 33°, 19° or 0° depending on the trend of the plane of cross-section.

The stereographic projection is used to solve two problems relating to apparent dip.