A cylindrical core of intact rock taken from a drill hole may exhibit on its curved surface the traces of some planar structure; for sake of example it will be referred to here as bedding (Fig. 36a). The form of this curved intersection line allows the angle δ between the normal to bedding and the axis of the core to be measured.

Our aim is to establish the in situ attitude of bedding. Although the plunge and plunge direction of the borehole axis may be known, our estimation of the strike and dip of the bedding is frustrated by the fact that the core has undergone an unknown rotation about an axis parallel to the axis of the cylindrical specimen during its withdrawal from the borehole. In fact, the pole to bedding could correspond to any line on the surface of a circular cone of apical angle = 2δ and with an axis parallel to the core's axis (Fig. 36a).

The available data can be displayed stereographically (Fig. 36b). The axis of the borehole is plotted as a point (a1, Fig. 36b) which forms the centre of a small circle of apical angle 2δ. The pole to bedding lies somewhere on this small circle, but further information is needed to locate it. The additional information could perhaps be provided by a second borehole with axis a2 and cone angle 2δ2 (Fig. 36b).