OGC标准介绍 7
· GML示例
下面是一个包含点、线、面数据的GML的简单的例子，通过这个例子可以看到GML的组织情况：
<?xml version="1.0" encoding="UTF-8"?> <gml:FeatureCollection xmlns:gml="http://www.opengis.net/gml" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:fme="http://www.safe.com/gml/fme" xsi:schemaLocation="http://www.safe.com/gml/fme MyGML.xsd"> <gml:boundedBy> <gml:Envelope srsName="EPSG:4326" srsDimension="2"> <gml:lowerCorner>-0.012611143330844 -0.050087654172727</gml:lowerCorner> <gml:upperCorner>0.051158411625351 0.019154661096934</gml:upperCorner> </gml:Envelope> </gml:boundedBy> <gml:featureMember> <fme:point gml:id="id9e2fb700-bbdb-48a5-8d99-f68b89886f4a"> <fme:Id>0</fme:Id> <gml:pointProperty> <gml:Point srsName="EPSG:4326" srsDimension="2"> <gml:pos>0.027363801567048 -0.028672505120255</gml:pos> </gml:Point> </gml:pointProperty> </fme:point> </gml:featureMember> <gml:featureMember> <fme:line gml:id="id4e0efc7b-d652-4d3c-8466-1a2419bca6d2"> <fme:Id>0</fme:Id> <gml:curveProperty> <gml:LineString srsName="EPSG:4326" srsDimension="2"> <gml:posList>0.051158411625351 0.019154661096934 0.039736998797366 -0.050087654172727</gml:posList> </gml:LineString> </gml:curveProperty> </fme:line> </gml:featureMember> <gml:featureMember> <fme:polygon gml:id="idf9b78df6-c402-4bc9-afd4-22762401c565"> <fme:Name>0</fme:Name> <gml:surfaceProperty> <gml:Surface srsName="EPSG:4326" srsDimension="2"> <gml:patches> <gml:PolygonPatch> <gml:exterior> <gml:LinearRing> <gml:posList>0.016656227040926 -0.024151529209121 0.003569191508859 0.005829679464227 -0.012611143330844 -0.01177833197886 -0.004283029810438 -0.042473378954071 0.018321849744836 -0.049373815870979 0.016656227040926 -0.024151529209121</gml:posList> </gml:LinearRing> </gml:exterior> </gml:PolygonPatch> </gml:patches> </gml:Surface> </gml:surfaceProperty> </fme:polygon> </gml:featureMember> </gml:FeatureCollection>
在这个例子中描述了一个GML的FeatureCollection，由于这是由FME导出的数据，因此其中还包含“fme”的命名空间和“fme:point”、“fme:line”、“fme:polygon”类型，这些类型在这个XML包含的XSD中进行了描述，实际对应的类型为基于GML标准的点、线、面：
<?xml version="1.0" encoding="UTF-8"?> <schema xmlns="http://www.w3.org/2001/XMLSchema" xmlns:gml="http://www.opengis.net/gml" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:fme="http://www.safe.com/gml/fme" targetNamespace="http://www.safe.com/gml/fme" elementFormDefault="qualified"> <import namespace="http://www.opengis.net/gml" schemaLocation="http://schemas.opengis.net/gml/3.1.1/base/gml.xsd"/> <element name="point" type="fme:pointType" substitutionGroup="gml:_Feature"/> <complexType name="pointType"> <complexContent> <extension base="gml:AbstractFeatureType"> <sequence> <element name="Id" minOccurs="0" type="integer"/> <element ref="gml:pointProperty" minOccurs="0"/> <element ref="gml:multiPointProperty" minOccurs="0"/> </sequence> </extension> </complexContent> </complexType> <element name="line" type="fme:lineType" substitutionGroup="gml:_Feature"/> <complexType name="lineType"> <complexContent> <extension base="gml:AbstractFeatureType"> <sequence> <element name="Id" minOccurs="0" type="integer"/> <element ref="gml:curveProperty" minOccurs="0"/> <element ref="gml:multiCurveProperty" minOccurs="0"/> </sequence> </extension> </complexContent> </complexType> <element name="polygon" type="fme:polygonType" substitutionGroup="gml:_Feature"/> <complexType name="polygonType"> <complexContent> <extension base="gml:AbstractFeatureType"> <sequence> <element name="Name" minOccurs="0"> <simpleType> <restriction base="string"> <maxLength value="30"/> </restriction> </simpleType> </element> <element ref="gml:surfaceProperty" minOccurs="0"/> <element ref="gml:multiSurfaceProperty" minOccurs="0"/> </sequence> </extension> </complexContent> </complexType> </schema>
而描述数据范围的“gml:boundedBy”属性则直接由“gml:Envelope”类型的对象来描述；另外，数据的坐标节点、空间参考等属性则直接由GML的类型进行描述。
更简单一点，如果我们需要自己写一个GML来描述一个地理数据，写出来的文件内容可能是这样的：
<City><!--描述某城市的数据--> <gml:boundedBy> <gml:Envelope> <gml:pos>100 200</gml:pos> <gml:pos>230 250</gml:pos> </gml:Envelope> </gml:boundedBy> <gml:featureMember> <Bridge gml:id="bridge 1"><!--某桥对象--> <span>100</span><!--属性信息：该桥的跨度--> <height>200</height><!--属性信息：该桥的高度--> <gml:curveProperty><!--几何信息--> <gml:LineString gml:id="line 24"> <gml:pos>100 200</gml:pos> <gml:pos>200 200</gml:pos> </gml:LineString> </gml:curveProperty> </Bridge> </gml:featureMember> </City>
