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<html><head><title>GML - Geography Markup Language</title></head><body bgcolor="#ffffff"><h1>GML - Geography Markup Language</h1>OGR has limited support for GML reading and writing. Update of existingfiles is not currently supported. OGR is based on GML 2.0. The reading onlyworks if OGR is built with Xerces linked in. XML validation is disabled bydefault. GML writing is always supported, even without Xerces.<p>The GML driver has no coordinate system support at this time.<p>In contrast to most GML readers, the OGR GML reader makes no effort to readthe format XML Schema definition of the feature classes in a GML file.Instead it attempts to automatically discover them and their associatedproperties by scanning the file and looking for "known" gml objects in thegml namespace to determine the organization. While this approach is errorprone, it has the advantage of working for GML files even if the associatedschema (.xsd) file has been lost.<p>The first time a GML file is opened it is completely scanned in order todetermine the set of featuretypes, the attributes associated with each andother dataset level information. This information is stored in a .gfs filewith the same basename as the target gml file. Subsequent accesses to thesame GML file will use the .gfs file to predefine dataset level informationaccelerating access. To a limited extent the .gfs file can be manually editedto alter how the GML file will be parsed. Be warned that the .gfs file willbe automatically regenerated if the associated .gml file has a newer timestamp.<p>When prescanning the GML file to determine the list of feature types, andfields, the contents of fields are scanned to try and determine the typeof the field. In some applications it is easier if all fields are just treatedas string fields. This can be accomplished by setting the configurationoption <b>GML_FIELDTYPES</b> to the value <b>ALWAYS_STRING</b>. Configurationoptions can be set via the CPLSetConfigOption() function or as environmentvariables.<p><h2>Creation Issues</h2>On export all layers are written to a single GML file all in a singlefeature collection. Each layer's name is used as the element name forobjects from that layer. Geometries are always written as the ogr:geometryProperty property on the feature.<p>The GML writer supports the following creation options:<ul><li> <B>XSISCHEMAURI</B>: If provided, this URI will be inserted as theschema location. Note that the schema file isn't actually accessed by OGR, soit is up to the user to ensure it will match the schema of the OGR produced GML data file.<p><li> <B>XSISCHEMA</B>: This can be EXTERNAL, INTERNAL or OFF and defaults toEXTERNAL. This (attempts) to write a BXML compatible GML schema file toa corresponding .xsd file (with the same basename). The generated schema isstill experimental. If INTERNAL is used the schema is written within the GMLfile, but this is even more experimental and almost certainly not valid XML.OFF disables schema generation (and is implicit if XSISCHEMAURI is used). <p></ul><h2>Example</h2>The ogr2ogr utility can be used to dump the results of a Oracle query toGML:<pre>ogr2ogr -f GML output.gml OCI:usr/pwd@db my_feature -where "id = 0"</pre><P>The ogr2ogr utility can be used to dump the results of a PostGIS query toGML:<pre>ogr2ogr -f GML output.gml PG:'host=myserver dbname=warmerda' -sql "SELECT pop_1994 from canada where province_name = 'Alberta'"</pre><h2>See Also</h2><ul><li> <a href="http://www.opengis.net/gml/">GML Specifications</a><p><li> <a href="http://xml.apache.org/xerces2-j/index.html">Xerces</a><p></ul></body></html>⌨️ 快捷键说明
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