Waterlining is a general and safe method for determining the centerline C of a shape S. Below and on the left you can see how C (dashed red line) is automatically threaded through characteristic points of the waterlines (parallel lines in black). Each member or point P of C, as resulting from our process, is loaded with attributes: pointers to the sections of S to the left and right of C, as well as the distances R from each P to S. For more on these attributes and the manner in which they propagate, see PAGE. The right picture demonstrates how that distance attribute can be verified: circles on every P with radius R, while tangent to the edges of S, they never intersect them. The shape, in this case street casings, can be reconstructed by tracing the envelope of all those circles or by points at distances R from C. In the NEXT pages you will find further details of this centerlining.