We present a new method for approximation of a given surface by a cyli
nder surface. It is a constructive geometric method, leading to a mono
rail representation of the cylinder surface. By use of the Gaussian im
age of the given surface, we determine a projection plane. In the orth
ogonal projection of the surface onto this plane, a reference curve is
determined by use of methods for thinning of binary images. Finally,
the cylinder surface is constructed as follows: the directrix of the c
ylinder surface is determined by a least squares method minimizing the
distance to the points in the projection within a tolerance given by
the reference curve, and the rulings are lines perpendicular to the pr
ojection plane. An application of the method in ship design is given.
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