Research On Automated Coastline Simplification Methods

Linear feature simplification is not only one of the most classic issues but also an important content in cartographic generalization.Realizing coastline automatic simplification is more difficult than some other ordinary linear features simplification,because coastline is a special kind of geographic linear feature with multiple geometric configurations,geographic characteristics,and application categories.However,it is important and necessary to realize automatic coastline simplification because this is a key step in automatic chart generalization.Besides,theories and methods of geographic-feature-oriented generalization can be enriched by researches of coastline simplification.Additionally,automatic coastline simplification is the key technical support for integrative production of nautical chart,geospatial data integration of land and sea and stepless zooming of GIS and ECDIS.This thesis mainly makes a systematic research on automatic coastline simplification for nautical chart(1)Recent researches of domestic and overseas on linear feature simplification and coastline simplification are summarized and analyzed firstly.Furthermore,aiming at weaknesses and difficulties of current coastline simplification researches,main contents of this thesis are drawn,and general technical route of this research is struck out too.(2)Comprehensive theories of coastline simplification,including the shape,constitution and types of coastline and basic units,constrains and the progressive process of coastline simplification,is conducted.Considering effects of scale on functions,geometric and geographic characteristics of coastline,a scheme of coastline simplification based on scale division is presented,which contains key points of coastline simplification at small,medium and large scale.Indeed,the following researches are conducted according to the scheme.(3)A progressive simplification method based on bend units is proposed for coastline at small scale.The double-oblique-dividing-curve method,an improvement of the oblique-dividing-curve method,is put forward to recognize bend units of coastline at small scale whose shape is simpler than at large scale.Indeed,bend units divided by this method are more accurate and complete for coastline at small scale.Land and sea coordinated representation is taken into consideration,order of priority on bends deletion is adjusted and specific bends deletion strategies are devised based on relationships of bend units in order to avoid simplifying excessively.In brief,this proposed method can realize the progressive simplification automatically,which is also verified by contrast experiments and compatibility experiments.(4)A progressive simplification method for estuary coastline at medium scale is proposed.As the scale increases the shape of coastline becomes more and more complex.Therefore,it is difficult but necessary to research complex coastline with broken patterns.Estuary coastline,a type of representative complex coastline,is the main object.Firstly,a binary tree of estuary skeletons is structured by Delaunay triangulation network to represent the pattern of the estuary coastline.Secondly,leaf streams of the structured model are removed or partial removed gradually to simplify small bends and parts of the coastline enough.Thirdly,narrow details of the bends-simplified coastline are exaggerated to avoid visual conflicts of the estuary.In a word,with this proposed method,complex coastline can be simplified progressively to reach a result which mainly conforms to the rule of expanding land area and decreasing sea area and conforms to the representation balance between geometry and geography.(5)A progressive simplification method based on parallels is proposed for artificial coastline at large scale.Firstly,representation characteristics of man-made structures which can be distinguished at large scale on the artificial coastline are analyzed.And then a method based on parallels is proposed to recognize protruding man-made structures and extract centre line automatically.Secondly,dimension-reducing and option methods are used by order to simplify protruding man-made structures with constraints and the progressive procedure of simplification.So it keeps man-made elements of artificial coastline sufficient after simplification.Besides,the rest of artificial coastline can be simplified by deleting bend units contained in the sea to ensure simplified results can be used securely in navigation.Thirdly,displacement and deletion are used to avoid special conflicts in simplification and ensure topology consistency.In conclusion,with this proposed method,the artificial coastline and its man-made structures can been simplified progressively.