Design And Implementation Of Map Symbol Library Extension And Graph Editor Based On SVG

With the rapid development of geographic information technology, Geographic Information System(GIS) applications in various industries are becoming increasingly widespread. And with the continuous expansion of the GIS application fields, the various walks of life demand for various aspects of GIS software also alter from day to day. The map is the basis of GIS, which is the carrier of geographic spatial information, and map symbol is the language of the map, what’s more, it is an important means of displaying geographic spatial information. The design of map symbol is directly related to the quality of map cartography, thus affecting the user to understand the map content intuitively.In order to meet the needs of the map symbols in the domain of new industry or specific business scope, it is not enough to establish a set of map symbol library, it also need to expand the function of custom symbols in the original system of map symbol. Therefore, it is an important link in the application development of GIS software to research on map symbol system with extensible function about custom map symbols, and to design GIS map symbol library with visual editing tool.In this paper, SVG, a vector graphics format file with open standard, is used as the basis point symbol of the map symbol library extending custom symbol function. The map symbol system is designed in accordance with the idea which regards point symbols as the smallest graph element so that it achieve the expansion of the entire map symbol library based on SVG graph element. And the visual editing tool for SVG point symbol has been designed and implemented to provide users with the function of customizing graph element based on SVG point symbol, so that completing the expansion of custom symbol function based on SVG about map symbol library. According to the principle of regarding point symbol as graph element, this paper has designed concrete point symbols covering 4 basic types:geometry graphics, TrueType character graphics, image element, SVG custom graphics.The linear symbols can be divided into the general line and the pixel line these two basic symbols. The general line symbol provides five common types and a custom line style, several line caps such as round, square, diamond and arrowhead, several line joins such as round, bevel and miter; According to the primitive line extension of the custom line and gives the corresponding method of drawing.The polygon symbols can be divided into contour symbols and fill patterns, among them, the contour lines can be achieved by the existing line symbol system, filling pattern can be used as a primitive symbol or symbols filling line and can be realized through a certain primitive organization.Based on the point, line, surface these three kinds of basic symbols, three kinds of complex symbols were derived as follows:combination of point symbol, line symbol combination, combination of symbol.According to the combination of line symbol, parallel broken line method is presented.Using of open source software to accomplish the map symbol base which is based on the SVG file format to extension, and support for cross platform map symbol rendering engine Linux (GDI+Flat API).Using Qt interface development framework, visualization editing tools which is based on SVG is designer and implemented, so as to improve the map symbol library custom symbol function expansion.The application examples have shown that the solution proposed in this paper is feasible and effective for the map symbol library extension and graph editor based on SVG All types of point symbols, line symbols and surface symbols have been displayed correctly in the two-dimensional map, especially custom symbols (point, line, polygon) based on SVG graph element. The results achieve satisfying performance in both displaying expression of map symbols and the function that the graph editor on SVG, reaching the expected requirements of GIS infrastructure software.