Spatial Relation Rules and Progressive Graphic Simplification of Contours

Contour lines are a discrete representation of relief, but topographic stir face is a continuous surface. Contours must be used to represent relief according to some rules. In this paper, two kinds of rules are shown. One is mathematical rule, the other is cartographic rule. With respect to these rules, the relationship between contour lines can be found. The direction of contour can be adjusted automatically. After that, the elevation of left part of contour line is higher than that of right part. The contour tree structure is saved in certain table. The topographic terrain represented by contour lines is not complete. Based on the spatial relationships and the characteristic points of contour lines, the ridge lines and ravine lines can be linked partly. The rivers must be the ravine lines. The intersection between contours and rivers are the characteristic points. The important degrees of characteristic points are given according to complex degree of bends in contour lines. The important degrees of the ridge lines and ravine lines are given according to the important degrees of characteristic points associated with that. This information is stored in two structure tables. The basic idea of progressive graphic simplification of contour lines is that the result of sudden change can be got from the progressive change. The self-closed contour can be handled by using the same method of self-closed linear feature progressive generalization. In generalization of other contours, firstly, the characteristic points, which are not associated with ridge and ravine, are handled progressively; the minimal bend is deleted with respect to the area of bend and important degree of the characteristic point. Then bends associated with ridges and ravines are handled according to important degree of the characteristic points and lines from small important degree to large important degree. The special cases in this method are shown and the algorithms of resolving these problems are given.