# GML overview

The RoadLink feature is attributed with a [*roadStructure*](https://docs.os.uk/os-downloads/products/transport-network-portfolio/os-open-roads/os-open-roads-technical-specification/code-lists-and-enumerations/roadstructurevalue) with a data type of [*RoadStructureValue*](https://docs.os.uk/os-downloads/products/transport-network-portfolio/os-mastermap-highways-network-fundamentals/os-mastermap-highways-network-code-lists/roadstructurevalue). The following table lists the codes which are used to populate this field and gives a description for each code.

This section describes the GML format for OS Open Roads. We recommended you read this in conjunction with the Open Geospatial Consortium’s (OGC's), [OpenGIS® Geography Markup Language](https://portal.ogc.org/files/?artifact_id=20509) [Encoding Standard v3.2.1.](https://portal.ogc.org/files/?artifact_id=20509)

The XML specifications that GML is based on are available from the World Wide Web Consortium (W3C) website: <https://www.w3.org/XML/>.

Information about Unicode and UTF-8, the character encoding that we use, is available on the Unicode Consortium website: <http://www.unicode.org/>.

## Schema overview and location <a href="#id-3.1-schema-overview-and-location" id="id-3.1-schema-overview-and-location"></a>

XML schemas are used to define and validate the format and content of the GML data. The GML v3.2.1 standard 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 schema, [*OSOpenRoads.xsd*](https://www.ordnancesurvey.co.uk/xml/open/roads/1.0/OSOpenRoads.xsd)*,* which is referenced by the data, is available on the [OS Open page](https://www.ordnancesurvey.co.uk/xml/open/index.html) of the XML file resources section of our website.

The OS schema imports the INSPIRE Transport Networks Data Specification, which in turn imports the GML 3.2.1 schemas. These in turn import schemas produced by the W3C, which are available from the W3C website at:&#x20;

<http://www.w3.org/XML/1998/namespace.html>

All of these schemas are defined in XML Schema Definition (XSD) language, as defined by the W3C.

The OS application schema uses the XML namespaces detailed in the table below. A link to each definition is provided in the last column.

<table data-full-width="true"><thead><tr><th>Prefix</th><th width="410">Namespace identifier</th><th>Definition available at</th></tr></thead><tbody><tr><td>gml</td><td><code>http://www.opengis.net/gml</code></td><td><a href="http://schemas.opengis.net/gml/3.2.1/gml.xsd">http://schemas.opengis.net/gml/3.2.1/gml.xsd</a></td></tr><tr><td>xsi</td><td><code>http://www.w3.org/2001/XMLSchema– instance</code></td><td>Built into XML <a href="http://www.w3.org/TR/xmlschema-1/">http://www.w3.org/TR/xmlschema-</a> <a href="http://www.w3.org/TR/xmlschema-1/">1/</a></td></tr><tr><td>xlink</td><td><code>http://www.w3.org/1999/xlink</code></td><td><a href="http://www.w3.org/1999/xlink.xsd">http://www.w3.org/1999/xlink.xsd</a></td></tr><tr><td>xml</td><td><code>http://www.w3.org/XML/1998/namespace</code></td><td><a href="http://www.w3.org/2001/xml.xsd">http://www.w3.org/2001/xml.xsd</a></td></tr><tr><td>net</td><td><code>urn:x-inspire:specification:gmlas:Network:3.2</code></td><td><a href="http://inspire.ec.europa.eu/schemas/net/4.0/Network.xsd">http://inspire.ec.europa.eu/schemas/net/4.0/Netwo</a> <a href="http://inspire.ec.europa.eu/schemas/net/4.0/Network.xsd">rk.xsd</a></td></tr><tr><td>tn</td><td><code>http://inspire.ec.europa.eu/schemas/net/3.0</code></td><td><a href="https://inspire.ec.europa.eu/schemas/net/3.2/Network.xsd">https://inspire.ec.europa.eu/schemas/net/3.2/Netw</a> <a href="https://inspire.ec.europa.eu/schemas/net/3.2/Network.xsd">ork.xsd</a></td></tr><tr><td>tn-ro</td><td><code>urn:x-inspire:specificationn:gmlas:RoadTransport Network:3.0</code></td><td><a href="http://inspire.ec.europa.eu/schemas/tn-ro/3.0/RoadTransportNetwork.xsd">http://inspire.ec.europa.eu/schemas/tn-</a> <a href="http://inspire.ec.europa.eu/schemas/tn-ro/3.0/RoadTransportNetwork.xsd">ro/3.0/RoadTransportNetwork.xsd</a></td></tr><tr><td>os</td><td><code>http://namespaces.os.uk/product/1.0</code></td><td><a href="https://www.ordnancesurvey.co.uk/xml/schema/product/1.0/OSProduct.xsd">https://www.ordnancesurvey.co.uk/xml/schema/pr</a> <a href="https://www.ordnancesurvey.co.uk/xml/schema/product/1.0/OSProduct.xsd">oduct/1.0/OSProduct.xsd</a></td></tr><tr><td>highway</td><td><code>http://namespaces.os.uk/mastermap/highwayNetwork/1.0</code></td><td><a href="https://www.ordnancesurvey.co.uk/xml/schema/highwaysnetwork/v1/LinearHighwayNetwork.xsd">https://www.ordnancesurvey.co.uk/xml/schema/hi</a> <a href="https://www.ordnancesurvey.co.uk/xml/schema/highwaysnetwork/v1/LinearHighwayNetwork.xsd">ghwaysnetwork/v1/LinearHighwayNetwork.xsd</a></td></tr><tr><td>road</td><td><code>http://namespaces.os.uk/Open/Roads/1.0</code></td><td><a href="https://www.ordnancesurvey.co.uk/xml/open/roads/1.0/OSOpenRoads.xsd">https://www.ordnancesurvey.co.uk/xml/open/road</a> <a href="https://www.ordnancesurvey.co.uk/xml/open/roads/1.0/OSOpenRoads.xsd">s/1.0/OSOpenRoads.xsd</a></td></tr></tbody></table>

### Simple Features Profile – Level 1 <a href="#id-3.2-simple-features-profile-level-1" id="id-3.2-simple-features-profile-level-1"></a>

OS Open Roads conforms to the GML 3.2.1 Simple Features Profile – Level 1.

GML is designed to support a wide variety of capabilities, ranging from simple contextual mapping to products that include complex geometric property types, and even spatial and temporal topology. The [GML](https://portal.opengeospatial.org/files/?artifact_id=42729) [Simple Features Profile](https://portal.opengeospatial.org/files/?artifact_id=42729) defines a restricted subset of GML, which allows for greater interoperability.


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