Geography Markup Language (GML)
In simple terms,
The Geography Markup Language (GML) is a way to exchange geographical information between computer system which use different software from different software providers
More specifically, GML is an XML encoding for the transport and storage of geographic information, including both the geometry and the properties of geographic features, between distributed systems.
Penn State Dept. of Geography has an excellent online introductory GML course here.
GML 3.3 Adopted January 2012
GML 3.2.1 (Approved July 2007)
GML 2.1.2 (deprecated)
Profiles and Application schema
GML is a large, rich, expressive language designed to have the ability to express any geographic concept in common usage. Therefore, unless you are developing a universal, comprehensive GML parsing application, the proper way to use GML is probably not to learn the entire encoding specification right away, but to start with an application schema, or profile of GML. As explained here, "Profiles live in the GML namespaces (http://www.opengis.org/gml) and define restricted subsets of GML. Application schemas are XML vocabularies defined using GML and which live in an application-defined target namespace. Application schemas can be built on specific GML profiles or use the full GML schema set." Basically, profiles and application schemas are smaller subsets of the GML schema designed by a specific information community and tailored to a small number of uses.
Status of GML 3.2.1 and ISO 19136
ISO 19136 (GML 3.2.1) has been published as an International Standard. GML 3.2.1 was approved as an OGC standard by the members on July 27, 2007. There is an associated document that contains the revision notes: changes from 3.1.1 to 3.2.1.
Current GML Work
There is a GML 3.3/4.0 Standards Working Group (SWG). During 2008, this group had an open call for formal Change Requests. The public call for CRs closed in September 2008. The SWG has collected all of the CRs and has begun discussions on the next revision to GML. Scheduled completion date is the end of the first quarter 2010.
GML Application Schemas and Profiles
The pages listed below describe existing and emerging GML profile efforts from various user communities. Look on the references page for guidance on developing your own GML profile, and feel free to use this site as a workspace for your development. Web pages and discussion forums are available for your use.