# GML overview

This section describes the GML format for OS Open Rivers. We recommend that you read this section in conjunction with the Open Geospatial Consortium (OGC) document, [OpenGIS® Geography Markup](https://portal.ogc.org/files/?artifact_id=20509) [Language (GML) Encoding.](https://portal.ogc.org/files/?artifact_id=20509)

The XML specifications on which the GML is based are available from the World Wide Web Consortium (W3C) website: [https://www.w3.org/XML/.](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/.](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. The GML v3.2.1 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 schema, *OSOpenRivers.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 Data Specification on Hydrography application schema, 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:

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

All these schemas are defined in XML Schema Definition language (XSD), 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>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 the XML schema: <a href="http://www.w3.org/TR/xmlschema-1/">http://www.w3.org/TR/xmlschema-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></tbody></table>

### Simple Features Profile – Level 0 <a href="#id-3.2-simple-features-profile-level-0" id="id-3.2-simple-features-profile-level-0"></a>

OS Open Rivers conforms to GML 3.2.1 Simple Features Profile – Level 0.

GML is designed to support a wide variety of capabilities, ranging from simple contextual mapping, such as OS OpenMap, to products that include complex geometric property types and even spatial and temporal topology. The Simple Features Profile of GML 3.2.1 defines a restricted subset of GML, which allows for greater interoperability.


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