Objectives In the generalization method of contours, the indirect generalization of contours based on three dimension (3D) Douglas-Peucker (3DDP) algorithm has a good application prospect. This method achieves the purpose of deleting the sub-points of the surface and reserving the main points by setting thresholds, but this threshold updating method affects the efficiency of indirect generalization of contours, and there is no consideration of the semantic characteristics of surface points in selection.

Methods Aiming at above problems, we presented an indirect generalization method for contours using the importance sequence of all points, and topographic feature lines considering terrain slopes extracted by an optimal position convergence method were taken as constraints of generalization. First, data was converted into 3D discrete points and sorted. Second, no points were deleted, and the importance of geometric weight and semantic weight of all discrete points was calculated to form an importance sequence which ordered all points from the largest to the smallest important values. Finally, according to the generalization target scale, the reserved points in the importance sequence were determined to achieve the indirection of contours.

Results Experimental results show that before the point selection method is improved, it takes about 18 minutes to select points by setting thresholds every time, which affects the generalization efficiency. However, the proposed method only takes about 25 minutes to queue all points by their importance values. Besides, by adding semantic information, the topological variation of generalized contours is reduced.

Conclusions The proposed method can not only improve the efficiency for the indirect generalization of contours, but also effectively prevent the topological variation of contours.