1. A computer implemented method for digitally producing displayable contour lines from a plurality of height points above a polygon net, wherein four selected adjacent height points of the plurality of height points (A, B, C, D) each having a height value (z.sub.A, z.sub.B, z.sub.C, z.sub.D)make up a saddle surface (A-B-C-D, r, s) having a saddle point above a quadrangle (A.sub.0, B.sub.0, C.sub.0, D.sub.0) of the polygon net (x, y values), and wherein a first and second contour lines (u.sub.1, v.sub.1 and u.sub.2, v.sub.2, respectively) are those lines which are obtained by a plane (A.sub.1 -B.sub.1 -C.sub.1 -D.sub.1 or A.sub.2 -B.sub.2 -C.sub.2 -D.sub.2, respectively) of a given plane height (z.sub.1 or z.sub.2, respectively) intersecting the saddle surface (A-B-C-D, r, s), comprising the following steps of a) calculating an intersection (17, 5, 18, 1) for each line connecting adjacent pairs of said height points (a, b, c, d), with the plane (A.sub.1 -B.sub.1 -C.sub.1 -D.sub.1 or A.sub.2 -B.sub.2 -C.sub.2 -D.sub.2, respectively) of the given height (z.sub.1 and z.sub.z, respectively), b) calculating a saddle point height (z.sub.M) of a saddle point (M) positioned above the center of the quadrangle (A.sub.0, B.sub.0, C.sub.0, D.sub.0) with said saddle point height being calculated as the mean value of the height values (z.sub.A, z.sub.B, z.sub.C, z.sub.D) of the height points (A, B, C, D) of the saddle surface (A-B-C-D, r, s), c) comparing the saddle point height (z.sub.M) of the saddle point (M) with the height value (z.sub.A) of a selected one of the height points (A, B, C, D) of the saddle surface (A-B-C-D, r, s) and with the given plane height (z.sub.1) of the plane (A.sub.1 -B.sub.1 -C.sub.1 -D.sub.1) intersecting the saddle surface (A-B-C-D, r, s), d1) and, if the given plane height (z.sub.1) lies between the height (z.sub.M) of the saddle point (M) and the height value (z.sub.A) of the height point (A), determining the first contour line (u.sub.1) as a connection between two neighbouring intersections (17, 1) such that the connection intersects the straight line connecting the saddle point (M) and the height point (A); d2) determining the second contour line (u.sub.2 or v.sub.2) as a connection between two neighbouring intersections (17, 5 or 18, 1) such that the second contour line does not intersect the connecting line of the saddle point (M) and the height point (A) if the given height (z.sub.2) does not lie between the height (z.sub.M) of the saddle point (M) and the height (z.sub.A) of the height point (A); e) calculating a first and a second intersection (F.sub.1 or G.sub.1 or F.sub.2 or G.sub.2, respectively) of the connecting line of the saddle point (M) and the height points (A, B, C, D) with the plane (A.sub.1 -B.sub.1 -C.sub.1 -D.sub.1 and A.sub.2 -B.sub.2 -C.sub.2 -D.sub.2, respectively) of a given height (z.sub.1 and z.sub.2, respectively), f) and passing the first and second contour lines (u.sub.1 or v.sub.1 and u.sub.2 or v.sub.2, respectively) through the first or second intersection (F.sub.1 or G.sub.1 and F.sub.2 or G.sub.2, respectively); g) repeating steps a-d1 for each group of four adjacent height points from said plurality of height points, determining first and second intersections and first and second contour lines for each of said groups, (h) determining based on the first and second contour lines from all of said groups, a first and second neighbor intersection for each of the intersections interconnected by the groups of first and second contour lines; (i) displaying on a computer display device the contour lines for said height points in accordance with the relationships determined by the neighbouring intersections to form open and closed contour-line polygons.