> 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/areas-and-zones-portfolio/boundary-line/boundary-line-technical-specification/gml.md).

# GML

Boundary-Line layers are supplied in Geography Markup Language (GML) v.3.2.1.

GML is based on XML (eXtensible Mark-up Language) and an understanding of the [XML schema](https://www.w3schools.com/xml/schema_intro.asp) will be helpful in understanding the GML format.

{% hint style="info" %}
See the following Geography Markup Language v3.2.1 [OpenGIS® Geography Markup Language (GML) Encoding Standard](https://portal.ogc.org/files/?artifact_id=20509) (PDF download) document by the Open Geospatial Consortium (OGC) for full details about GML.
{% endhint %}

Boundary-Line GML files are only created for the following layers:

* County
* District\_borough\_unitary
* Parish

## Schema overview and location <a href="#id-6.2-schema-overview-and-location" id="id-6.2-schema-overview-and-location"></a>

XML schemas are used to define and validate the format and content of the GML. The GML v.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 `AdministrativeUnits.xsd` application schema which is referenced by the data, is available on the [INSPIRE website](https://inspire.ec.europa.eu/schemas/au/4.0/AdministrativeUnits.xsd). It imports the GML 3.2 schemas which rely on [XML as defined by W3C](https://www.w3.org/XML/1998/namespace.html).

Style sheets have been created and provided for Boundary-Line GML. You can find these in the Ordnance Survey GitHub repository at <https://github.com/OrdnanceSurvey/Boundary-Line-stylesheets>.


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