RESEARCH AND EXPERIMENT OF PROGRESSIVE CARTOGRAPHIC GENERALIZATION
Q. Guo, Z. Yang, X. Lu, C. Zheng
School of Resource and Environment Science, Wuhan University, Wuhan, China
The basic idea of progressive cartographic generalization is decomposing the cartographic generalization of large scale change into progressive selection and simplification of map objects. The process can be seen as continuous changes and finally obtains abrupt change generalized results by means of progressive changes. The progressive generalization includes the progressive change of the spatial objects and abstraction of their attributes from roughness to precise according to their hierarchy.
Generalization process can be totally controlled because accepting or rejecting is performed on minimum detailed data in every step. It also makes the data transformation and decision-making exercisable. For the control of cartographic generalization process, the progressive cartographic generalized results in every step must be evaluated. It makes all the results of every step are available for further generalization. This method is more suitable in the cartographic generalization process of web mapping, which can results in maps of discretional scale. The realization of progressive spatial abstraction is the theoretical foundation and technical guarantee of progressive transfers of spatial vector data in the net, as well as the algorithm basis of spatial data structure in real-time and continuous cartographic generalization.
In this paper, the experiments of contour and building outline simplification are accomplished successfully with this idea, and the analyses of results are given out.
In the process of contour generalization, firstly the topographic characteristic lines are extracted with help of constrained Delaunay triangular networks, and then a progressive generalization algorithm of contour constrained by topographic characteristic lines is proposed. The details is designing error measurement of vertex, boundary, triangular and polyhedron in Delaunay triangular networks, progressively accepting or rejecting elements according to a given threshold.
In the process of building outline generalization, firstly the building outline is preprocessed, and an angle between two adjacent edges on a building outline is a right angle when it suits to a certain corresponding condition. Then the buildings outline is generalized progressively with help of a minimal graphical unit. In process of the whole simplification, the absolute value of changed area of a building is taken as a constrained condition, and a graphic unit is an edge of a building, which may be deleted for building simplification according to a given threshold and an operation. The elementary spatial feature of a building outline after simplification is maintained, as well as the spatial relationship between buildings and other map objects.
The experiment indicates that the algorithm of progressive graphic simplification is accord with the rule of human cognition and the custom of understanding space. It can be widely used in map compilation, multi-scale spatial information visualization and geographical space abstraction.