GML对象的层次结构
<p class="MsoNormal"><span> 下面的</span><span lang="EN-US">UML</span><span>图表示了</span><span lang="EN-US">GML</span><span>对象的层次结构，各个对象的模式文档基本上都是围绕着图中的对象分类来进行组织的。</span>
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<p class="MsoNormal"><span> 图中以下划线开头的项（如</span><span lang="EN-US">Feature</span><span>）可看作该类的典型对象实例。例如，元素</span><span lang="EN-US">gml:_Feature</span><span>应该解释为“任何</span><span lang="EN-US">GML</span><span>要素”，它可以用来定义</span><span lang="EN-US">GML</span><span>特性的值是“任何要素”的变量或模板。</span>
<span> 地理要素包含一系列的空间和非空间属性，要素模式（</span><span lang="EN-US">Feature Schema</span><span>）为创建</span><span lang="EN-US">GML</span><span>的要素和要素集合提供了一个框架，它定义了抽象和具体的要素元素及类型，并通过</span><span lang="EN-US"><include></span><span>元素引入了几何模式和时态模式中的定义和声明。几何模式（</span><span lang="EN-US">Geometry Schema</span><span>）详细地描述了地理要素的几何模型。时态模式（</span><span lang="EN-US">Temporal Schema</span><span>）扩展了</span><span lang="EN-US">GML</span><span>的核心元素，使其包括描述地理数据时态特征的元素，它提供一种框架用于描述动态要素的时间变化。覆盖模式（</span><span lang="EN-US">Coverage Schema</span><span>）支持从时空域到属性值的映射，属性类型对于时空域里的所有地理位置是公共的，时空域直接由坐标空间中的许多直接位置组成。拓扑模式（</span><span lang="EN-US">Topology Schema</span><span>）定义了反映地理要素之间关系的拓扑对象。</span>
<span><br></span>