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 keyref element specifies that the value of an attribute or element (or set of values) corresponds to the value of a specified key or unique element. The keyref element must contain the following elements in order: There must be one and only one selector element. 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. Refer Necessary. Specifies the name of the key or unique element defined in the schema (or other schema indicated by the specified namespace). The refer value must be a qualified name (QName). Types can include namespace prefixes. Any attributes Optional. Specifies any other attributes with the non-schema namespace. XML Schema 参考手册 完整 XML Schema 参考手册 Definition and usage ¶
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