最近几年来，地理信息系统无论是在理论上还是应用上都处在一个飞速发展的阶段。 GIS被应用于多个领域的建模和决策支持，如城市管理、区划、环境整治等等，地理信息成为信息时代重要的组成部分之一； “数字地球”概念的提出，更进一步推动了作为其技术支撑的GIS的发展。 与此同时，一些学者致力于相关的理论研究，如空间感知、空间数据误差、空间关系的形式化等等。 这恰好说明了地理信息系统作为应用技术和学科的两个方面，并且这两个方面构成了相互促进的发展过程。
XML Schema 参考手册 完整 XML Schema 参考手册 The unique element specifies that an attribute or element value (or a combination of attribute or element values) must be unique within a specified range. The value must be unique or zero. The unique element must contain the following elements in order: The oselector element contains an XPath expression that specifies a set of elements in which the value specified by the field element must be unique. There must be one and only one selector element. Each field element contains an XPath expression that specifies a value (attribute or element value) that must be unique to the set of elements specified by the selector element. If there are multiple field elements, the combination of field elements must be unique. In this case, the value of a single field element may not be unique to the selected element, but the combination of all fields must be unique. There must be one or more field elements. 父元素： Element (? The symbol declares that the element can appear zero or once in the unique element. ) Attribute Description Id Optional. Specifies the unique ID of the element. Name Necessary. Specify a name for the element. Any attributes Optional. Specifies any other attributes with the non-schema namespace.Definition and usage ¶
Element information ¶
Grammar ¶