基于XSLT的图示表达规则的设计与实现

地理信息的符号化在ISL/dIS19117中称为地理信息的图示表达,包括表达目录、表达规范和表达服务3个主要部分。其中表达目录组织一系列的表达规则,用于测试地理要素的空间要素与地图显示环境．如显示时间、显示比例尺、地图显示的方向等,并调用表达服务。表达规范描述地图符号的图形参数,为表达服务提供参数属性值。表达服务提供符号生成操作,其函数参数值来源于表达规范。表达规范与表达目录相对独立,共同构成了表达信息。同一表达目录可以调用不同的表达规范,从而生成不同的地图。同样表达规范也可以被不同的表达目录调用。表达信息与地理要素独立,并分开存储在不同的地方。图示表达规则是根据制图知识,推理地理信息的几何与属性,并从图式规范中匹配出相应的地图符号。本文结合表达规则与可扩展样式单语言转换(XSLT．eXtensible Style Sheet Language Transformation)的特点,提出了基于XSLT的图示表达规则模型,采用XSLT描述表达规则,能便于表达规则的网络共享与实时交互。并以地理标记语言GML为对象．通过XSLT将GML转换为基于XML的自定义地图制图标记语言MapML,在此基础上,对MapML进行了符号化,生成SVG地图。实验表明通过XSLT能有效测试与分析地理数据的空间特性与专题属性,并顾及了地图显示比例尺。

Portrayal Rules of Geographic Information Based on XSLT: Design and Implementation

Symbolization of geographic information in GIS is named with geographic information portrayal in ISO/DIS 19117. This International Standard defines a feature-centered rule based portrayal mechanism. Instances of features are portrayed based on rules, which make use of geometry and attribute information. Portrayal information needs to be presented to portray a dataset containing geographic data. The portrayal information is handled as portrayal specifications applied according to specific portrayal rules. The portrayal mechanism makes it possible to portray the same dataset in different ways without altering the dataset itself. The portrayal specifications and portrayal rules are not part of the dataset. The portrayal rules can be stored in a portrayal catalogue. The portrayal specifications can be stored separately and referenced from the portrayal rules. The portrayal rule mechanism can be used to handle portrayal issues that have to be solved as they happen, such as how to automatically place text on maps, and how to specially represent the feature instances according to time of a day, direction or scale. The portrayal rules in the portrayal catalogue can be tested on the attributes of the feature instances in the dataset. A portrayal rule should be applied as a query statement that returns TRUE, or FALSE. New portrayal rules should be applied until one returns TORE. The portrayal specification associated with that particular portrayal rule shall then be applied. If no portrayal rule returns TRUE, then the default portrayal specification shall be used. A portrayal service is used to portray a feature instance or instances. The portrayal rules, in the light of mapping knowledge, match the cartographic specifications with geographic information, and find the symbol in the portrayal specification in order to represent the geographic object. Integrating the portrayal rules and extensible style sheet language (XSLT), we can describe the new model, the portrayal rules based on XSLT, where the portrayal rules are presented by the XSLT markup language. In the experiment, the Geographic Markup Language (GML) is transformed into the MapML by XSLT, where the MapML is the user-defined markup language representing the map-making data. Finally, the MapML is symbolized in the symbols based on SVG. The experimentation indicates that the portrayal rules based on XSLT can satisfy the needs of geographic information portryal.