If there is single point A (figure 19.a) the gradient of the line through point A is lost. Therefore it is necessary to compute the transformation for a number of lines with a common point A, but with different gradients. By transforming these lines, a figure as presented in figure 19.b is generated. Points A and B have one common line, this is the point of intersection in figure 19.b. All collinear points have a common line, with the same gradientφand radius r in the r, φ-coordinate system.Using an accumulator (see figure 20) the number of common lines can be detected . If a point in the accumulator exceeds a threshold, this position characterizes a line. The line coordinates are transformed to the x, y-coordinate system. Along this line the flat points are searched and marked as belonging to a flat cluster.