> 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/water-portfolio/os-open-rivers/os-open-rivers-overview/os-open-rivers-data.md).

# OS Open Rivers Data

## Simplification <a href="#id-2.1-simplification" id="id-2.1-simplification"></a>

The detail within OS Open Rivers is automatically generalised from Ordnance Survey large-scale data. Map simplification is the process of reducing the scale and complexity of data whilst maintaining the important elements and characteristics.

OS Open Rivers simplification comprises the following processes:

* **Selection/omission of rivers** – Features that appear at higher resolutions are removed at lower resolutions, for example, small river channels.
* **Simplification of geometry** – The density of points along a line is reduced using an algorithm that creates a simplified curve. This curve includes sufficient points to retain the shape and connectivity of the original line at the intended usage resolution.

## Feature types <a href="#id-2.2-feature-types" id="id-2.2-feature-types"></a>

OS Open Rivers features are classified into two feature types.

* **WatercourseLink** – This feature represents the alignment of a watercourse.
* **HydroNode** – This feature explicitly represents the start, end, and junctions of watercourses, and places where related real-world attribution changes have been recorded, for example, the point where a watercourse becomes tidal. The HydroNode is coincident with the ends of related links.

Each feature type has associated attribution, and further detail on this can be found in the [OS Open Rivers technical specification](/os-downloads/products/water-portfolio/os-open-rivers/os-open-rivers-technical-specification.md).

## Coordinate reference system <a href="#id-2.3-coordinate-reference-system" id="id-2.3-coordinate-reference-system"></a>

The Geography Markup Language (GML), GeoPackage, and shapefile product formats enable the use of the British National Grid (BNG) coordinate reference system. In the GML data, this is represented by reference to its entry in the EPSG registry, as [http://www.opengis.net/def/crs/EPSG/0/27700.](http://www.opengis.net/def/crs/EPSG/0/27700)

The BNG spatial reference system uses the OSGB36 geodetic datum and a single Transverse Mercator projection for the whole of Great Britain. Positions on this projection are described using easting and northing coordinates in units of metres. The BNG is a horizontal spatial reference system only; it does not include a vertical (height) reference system.

The vector tiles format is in the Web Mercator (EPSG:3857) projection. This is a global coordinate reference system.

## Currency <a href="#id-2.4-currency" id="id-2.4-currency"></a>

OS Open Rivers is derived from Ordnance Survey large-scale data and is refreshed every six months.

## Completeness <a href="#id-2.5-completeness" id="id-2.5-completeness"></a>

Quality control procedures are undertaken at all stages of production to ensure that data is accurate, complete, and conforms to the specification. Quality control checks include automated data testing against the product specification and visual checks by operators.


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