It is one of important issues to acquire the reasonable sub-contour lines from a contour terrain map in order to reconstruct a 3--D terrain model as smooth as possible. Although in the fields of computer graphics and geographic information CAD systems many approaches have been developed to solve this problem, it still remains open. TO acquire reasonably interpolated sub--contour lines and reconstruct the smooth surface throughout the whole region of contour lines, we propose the following method: (l) employing the gradient controlled partial differential equations (PDE) in which the local parameters in each region held by every neighboring contour lines are controllable; (2) applying the two step iterative convergence algorithm which satisfies both the given elevation and smoothness with C continuity on every contour line. Usefulness of the proposed method is testified by some examples, the smooth saddle shapes with plural branches in one region and the reasonable sub--contour lines of a terrain map.
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