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。
Compound elements that contain only text can contain text and attributes. This type contains only simple content (text and attributes), so we will add simpleContent elements to this content. When using simple content, we have to define an extension or qualification within the simpleContent element, like this: 提示： Use extension or restriction elements to extend or restrict the basic simple types of elements. Here is an example of a XML element, “shoesize”, which contains only text: The following example declares a compound type whose content is defined as an integer value, and the “shoesize” element contains an attribute named “country”: We can also set a name for the complexType element and have the type attribute of the “shoesize” element refer to this name (by using this method, several elements can refer to the same compound type): A compound element that contains only text ¶
<shoesize country="france">35shoesize>
<xs:element name="shoesize"> <xs:complexType> <xs:simpleContent> <xs:extension base="xs:integer"> <xs:attribute name="country" type="xs:string" /> xs:extension> xs:simpleContent> xs:complexType> xs:element>
<xs:element name="shoesize" type="shoetype"/> <xs:complexType name="shoetype"> <xs:simpleContent> <xs:extension base="xs:integer"> <xs:attribute name="country" type="xs:string" /> xs:extension> xs:simpleContent> xs:complexType>