Geography Markup Language (GML) is a standardized XML grammar that provides a specialized vocabulary for working with geospatial data. GML enables the encoding of geographic features, and provides a vendor-neutral means of modeling, transporting and storing the raw data that features are composed of, such as points, lines, polygons, attribute descriptions, temporal objects and coverages.

Using GML, you can encode the description of the location or spatial extent of a river, roadway or bridge. A GML record includes a geometry representing a bridge or roadway, as well as supporting information for the geometry such as the number of lanes of a road.

With GML, you can transfer those descriptions through the Internet, and control how they are displayed. Like XML, GML clearly separates the content from the presentation; in other words, GML delivers the actual geographic data (information about the properties and geometries of objects in the physical world) as opposed to the visualization of that data (colours, line weights, symbols, etc.). Since GML is an XML application, it can be styled into a variety of presentation formats, including vector and raster graphics, text, sound and voice. While maps are one of the most common presentations of GML, GML is not dependent on any particular graphical specification.

The GML specification is developed and maintained by the Open Geospatial Consortium, Inc. (OGC), an international industry consortium of over 240 companies, government agencies and universities participating in a consensus process to develop publicly available geo-processing specifications.

Before you start OGC advises that parts of the GML specification use the XML schema language from the World Wide Web Consortium (W3). The GML implementation specification is not intended as an introduction to XML schema. You should first have a reasonable knowledge of XML schema to understand the GML specification.