# Standards and CRS

## Standards

OS MasterMap Water Network Layer is based on the [INSPIRE Data Specification on Hydrography](https://inspire.ec.europa.eu/documents/Data_Specifications/INSPIRE_DataSpecification_HY_v3.1.pdf), which itself is based on the ISO TC211 family of open standards.

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

OS MasterMap Water Network Layer uses the British National Grid (BNG) spatial reference system. BNG 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 specify a vertical (height) reference system.

This water network product, unlike other OS MasterMap layers, contains height values for the geometry vertices. Several orthometric height datums are used in OS MasterMap data to define vertical spatial reference systems; the most common of these is Ordnance Datum Newlyn (ODN), which is used throughout mainland Britain. Height values on the features do not specify which vertical reference system is used, although information on the extents of additional datums can be provided.

OS MasterMap Water Network Layer is projected in the ESPG 7405 OSGB36 / British National Grid + ODN. This projection is used as it specifies the Z value as ODN.

In the Geography Markup Language (GML) data, this is represented by reference to its entry in the [EPSG registry](http://www.opengis.net/def/crs/EPSG/0/7405).

The GeoPackage product format is in the BNG projection (BNG: 27700).

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

Please see[ A Guide to Coordinate Systems in Great Britain](https://docs.os.uk/more-than-maps/deep-dive/a-guide-to-coordinate-systems-in-great-britain) on our More than Maps platform for more information on CRSs.


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