> For the complete documentation index, see [llms.txt](https://docs.os.uk/os-downloads/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.os.uk/os-downloads/products/land-and-terrain-portfolio/os-mastermap-sites-layer/os-mastermap-sites-layer-technical-specification/gml.md). # GML GML is an XML (Extensible Mark-up Language) grammar for expressing geographic features. GML serves as a modelling language for geographic systems as well as an open interchange format for geographic transactions on the Internet. * More information can be found on the Open Geospatial Consortium (OGC): . * The XML specifications that GML is based on are available from the World Wide Web Consortium (W3C): [http://www.w3.org](http://www.w3.org/). * Information about Unicode and UTF-8, the character encoding we have chosen, is available on the Unicode Consortium website: [http://www.unicode.org](http://www.unicode.org/). ## XML schema XML schemas are used to validate the format and content of the GML. The GML 3.2.1 specification provides a set of schemas that define the GML feature constructs and geometric types. These are designed to be used as a basis for building application-specific schemas, which define the data content. The Ordnance Survey application schemas that are referenced by the data are available from our website at: [http://www.ordnancesurvey.co.uk/xml/schema.](http://www.ordnancesurvey.co.uk/xml/schema) These schemas make use of XML Schema Definitions (XSDs) and Document Type Definitions (DTDs) produced by the W3C that are available from the W3C website at: [http://www.w3.org/XML/1998/namespace.html.](http://www.w3.org/XML/1998/namespace.html) ## Namespaces xlink – `http://www.w3.org/1999/xlink` gml – `http://www.opengis.net/gml` osgb – `http://www.ordnancesurvey.co.uk/xml/namespaces/osgb` xml – `http://www.w3.org/XML/1998/namespace` --- # Agent Instructions This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com. ## Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation by asking a question. Perform an HTTP GET request on the following URL with the `ask` and `goal` query parameters: ``` GET https://docs.os.uk/os-downloads/products/land-and-terrain-portfolio/os-mastermap-sites-layer/os-mastermap-sites-layer-technical-specification/gml.md?ask=&goal= ``` `ask` is the immediate question: it should be specific, self-contained, and written in natural language. `goal` is what the user is ultimately trying to achieve, the reason they need the answer. Sharing it helps GitBook give you a better, more relevant answer. A goal is most helpful when it describes the outcome the user wants rather than restating the question. For example, with `ask=how do I create an API token`, a goal like `automate deployments from our CI pipeline` lets GitBook tailor the answer to that use case. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.