Research On Automated Cartographic Generalization Of Submarine Relief

Nautical chart is an effective tool to describe and transmit ocean spatial information,and it plays an important role in ocean project construction,science research,resource development and ocean shipping.Nautical chart is also regarded as the “eye” of a vessel,so it effect the navigation safety and navigation efficiency easily.As a difficult problem of chart cartography and Marine Geographical Information System,chart cartographic generalization is also needed in marine big data analysis,marine spatial data mining,thematic information extractiing and so on.However,the automation degree of chart cartographic generalization is still low,which limits the efficiency of chart producing and dffects the quality of chart product seriously.Therefore,studying chart cartographic generalization is essential and urgent.Focusing on the generalization of submarine relief features,which has the biggest workload and most factors need to be considered,cartographic generalization algorithms for depth contours and soundings are studied in this paper to increase the automation degree of chart cartographic generalization and enrich its theory.The main contents and innovations of this thesis are as the followings:(1)Research on sounding selection method based on chart production data.Methods to recognize the local shoalest soundings,local deepest soundings and control soundings are improved.The main characteristics of complex submarine features is analysed,and an automated recognition method of complex seabed region for sounding selection is proposed.Considering the characteristic of chart soundings,a sounding selection method based on TIN simplification is proposed,which takes full advantage of TIN in terrain quantitative analysis and proximity analysis.Firstly,sounding selection method based on “selecting line” is proposed under the constraint of standard space,from which,initial selecting result is obtained.Next,an optimization model to improve the initial selecting result is built under the constraints of terrain error and defined space.The proposed method respects the constraint of navigation safety and improves the terrain express precision effectively.In addition,the selected soundings are organized as a rhomboidal pattern,and the distribution density of selecting soundings is consistent with cartographic request.(2)Research on sounding selection method based on multi-beams surveying data.The characteristics of sounding selection based on multi-beams surveying data are analyzed.Terrain feature points,including ridge point,valley point and terrain changing point,are recognized based on terrain section lines which are constructed considering the arranging characteristic of muilt-beams surveying points;Moreover,a rhombus seeking model is built considering slope value,slope direction and corresponding depth value.Finally,sounding selecting result is obtained by thinning the initial selection result which consists of feature soundings and soundings acquired by rhombus seeking.The proposed method respects the constraint of navigation safety and preserves the terrain feature points effectively.In addition,the direction and length of diagonal line of rhombus are adapted with the terrain slope and terrain aspect.(3)Research on the automated aggregation method of depth contour.Firstly,the conditions for aggregating depth contours are analyzed.To detect the related contours need to be aggregated,a simple detecting method based on region graph is proposed,and the method can avoid the complex process of recognizing submarine features.Then,the current aggregation method based on delaunay triangulation is improved by expanding bridge areas and filling small “holes”,which can avoid the arising of little closed depth contour whose interior doesn’t contain any sounding and indistinct “bottle neck” effectively.Next,the integration of depth contour simplification and aggregation is relized by merging bends need to be generalized and brige areas.Finally,smoothness of aggregation result is improved by smoothing operation.(4)Research on the simplification method of depth contours.As to the problems that detecting bend need to be generalized is imprecise and bend simplification is imcomplete,a simplification method for depth contour based on Delaunay triangulation is prposed.Firstly,method of detecting bends need to be generalized is improved.Bends need to be generalize are divided into two categories: small bends and visual conflicting regions,which are recognized using the bend binary tree and the constrained Delaunay triangulation.Secondly,the generalization methods are enriched by designing a variety of generalization operators,including bend deleting,exaggeration,constructing convex hull for bends and splitting depth contours in “bottle neck”.Finally,a new simplification model worked on depth contour is built based on the request of depth contour simplification.As to the problems that scale-driven is week and smoothness is poor,a simplification method for depth contour under the constraint of largest curvature is designed.The relation between curvature and legibility of bend expressing is analysed,and the objective relation between simplification parameter and scale is built.Based on the constraint of largest curvature,a method to detect the curve segments need to be generalized is designed,which increases the accuracy of detecting results.Simplification methods for convex bends and concave bends are realized by densifying vertices and moving vertex progressively,which makes the simplification results smoother.The proposed method is scale-driven,and it fits the special cartographic environment of nautical chart whose scale are arbitrary.Besides,the proposed method is free from the trouble of setting threshold.(5)Research on smoothing method of depth contours considering navigation safety.In view of the special request of navigation safety for smoothing depth contour,a smoothing method based on the fitting of multi-segment Bezier curves is proposed.To maintain the bend characteristic of origin depth contours,the smoothing of depth contour is conducted based on the bend segment.Then the methods to smooth convex bend,concave bend and the curve segment linking neighbor bends are studied and designed considering navigation safety and geometric continuity.Besides the constraint of navigation safety is observed,the proposed method can also preserve the bend characteristic and key shape points of the origin depth contour,and it is also free from topological errors.In addition,the smoothing results can be adjusted flexibly.