Web Mapping Illustrated

Name

Geography Markup Language Files (GML)

Synopsis

• Also known as Geographic Markup Language and GML/XML.

• GML is a text-based, XML format that can represent vector and attribute data.

• This is an Open Geospatial Consortium specification for data interchange.

File listing

GML files are usually a single text file with a GML filename extension (coal_dep.gml). Some may use XML as the filename extension.

XML schema documents often accompany GML files that have been translated from some other format (e.g., using the ogr2ogr utility).

Example of text in a GML file

GML uses sets of nested tags to define attributes and geometry coordinates:

      <gml:featureMember>
        <Coal_Deposits fid="1">
          <UNKNOWN>0.000</UNKNOWN>
          <NA>0.000</NA>
          <ID>2</ID>
          <ID2>2</ID2>
          <MARK>7</MARK>
          <COALKEY>110</COALKEY>
          <COALKEY2>110</COALKEY2>
          <ogr:geometryProperty>
            <gml:Point>
              <gml:coordinates>78.531,50.694</gml:coordinates>
            </gml:Point>
          </ogr:geometryProperty>
        </Coal_Deposits>
      </gml:featureMember>

Data access/connection method

• GML access is available in MapServer through OGR.

• The CONNECTIONTYPE OGR parameter must be used.

• The path to the GML file is required, including a file extension.

• There can be multiple layers in a GML file, including multiple feature types.

ogrinfo examples

Here's an example that uses ogrinfo on a single GML file:

                  > ogrinfo /data/gml/coal_dep.gml
    Had to open data source read-only.
    INFO: Open of 'coal_dep.gml'
    using driver 'GML' successful.
    1: Coal_Deposits

Here's an example that uses ogrinfo to examine the structure ...