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 key element specifies that an attribute or element value (or set of values) must be a key within the specified range. The scope of the key is the containing element in the instance document. The key means that the data should be unique, non-zero, and always exist within the specified range. The key element must contain the following elements in order: The selector element contains an XPath expression that specifies a set of elements in which the value specified by field must be unique. 父元素： Element (? The symbol declares that in the key element, the element can appear zero or once, and the + symbol declares that the element must appear one or more times. ) Attribute Description Id Optional. Specifies the unique ID of the element. Name Necessary. Specifies the name of the key element. Any attributes Optional. Specifies any other attributes with the non-schema namespace.Definition and usage ¶
Element information ¶
Grammar ¶