Also known as GML. Geography Markup Language is a set of XML components designed to encode geospatial data. It is another way to store and retrieve GIS data in a flat file format. For example, GML files can be used for describing the boundaries of your school, the path you recorded with your GPS on a road trip along the California Coastline, or the topography of a mountain.

XML is a great way to provide an extensible format for holding meta-data, which is probably why the XML based RSS blogs have been so successful.

Other forms of GIS data formats are Autodesk’s Data Interchange File (DXF), Digital Line Graphs (DLG), Topologically Integrated Geographic Encoding and Referencing Files (TIGER), and GeoTIFF. There are many other GIS formats (see , many of which are proprietary. One of the nice things about GML is that it's an open format that anyone can read, and therefore, likely to be widely adopted by GIS applications. Because it's XML, it can be easily parsed by XML Libraries, which are available in almost every programming language.

Is GML widely adopted yet? Maybe not, but documentation is available and in this programmer's opinion, it will be.

A great example of a GML document is The site includes a Java Map Viewer with the example loaded.