Introduction to Geographic Markup Language (GML)

This week we are looking at the Geographic Markup Language. This is an XML based language for expressing geographic features in a way that is interoperable. As with most XML languages, there are two parts to the language - the schema that describes the document and the instance document that contains the actual data.
A general GML document is described using a GML Schema. This allows you to describe a generic geographic data set which contains points, lines and polygons. However, the developers of GML have a vision where communities get together and define application schemas. These are specialized extensions of GML. Instead of lines you refer to roads, highways, and bridges. Provided everyone in your community agrees to use the same schemas you can exchange data easily and be sure that a road is still a road when they view it.
This is a new and expanding area. A list of public application schemas can be found here (http://en.wikipedia.org/wiki/GML_Application_Schemas).