> For the complete documentation index, see [llms.txt](https://docs.os.uk/osngd/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/osngd/getting-started/os-ngd-fundamentals/coordinate-reference-systems.md).

# Coordinate reference systems

## Coordinate reference systems available for OS Select+Build

OS Select+Build is the download service that gives you access to OS NGD data; it's available via the [OS Data Hub](https://osdatahub.os.uk/). 

The GeoPackage and CSV formats accessible via OS Select+Build are available in four different CRSs dependent on your data selection:

* British National Grid (BNG: EPSG: 27700)
* British National Grid + ODN Height (EPSG: 7405)
* European Terrestrial Reference System (ETRS89: EPSG: 4258)
* World Geodetic System (WGS84: EPSG: 4326)

### Default coordinate reference systems for OS Select+Build

The following table outlines the default CRS used by each OS NGD collection in OS Select+Build:

<table><thead><tr><th width="293">Collection</th><th width="403">Default Coordinate Reference System for OS Select+Build</th></tr></thead><tbody><tr><td>OS Boundaries</td><td><p>British National Grid (EPSG: 27700)</p><p> </p></td></tr><tr><td>OS Building Features</td><td><p>British National Grid (EPSG: 27700)</p><p> </p></td></tr><tr><td><span data-gb-custom-inline data-tag="emoji" data-code="1f195">🆕</span> OS Functional Areas</td><td>British National Grid (EPSG: 27700)</td></tr><tr><td>OS GB Address</td><td><p>British National Grid (EPSG: 27700)</p><p> </p></td></tr><tr><td><span data-gb-custom-inline data-tag="emoji" data-code="1f195">🆕</span> OS GB Postcodes</td><td>British National Grid (EPSG: 27700)</td></tr><tr><td>OS Islands Address</td><td>European Terrestrial Reference System (EPSG: 4258)</td></tr><tr><td>OS Named Features</td><td><p>British National Grid (EPSG: 27700)</p><p> </p></td></tr><tr><td>OS Land Features</td><td><p>British National Grid (EPSG: 27700)</p><p> </p></td></tr><tr><td>OS Land Use Features</td><td><p>British National Grid (EPSG: 27700)</p><p> </p></td></tr><tr><td><span data-gb-custom-inline data-tag="emoji" data-code="1f195">🆕</span> OS NI Postcodes</td><td>European Terrestrial Reference System (ETRS89: EPSG: 4258)</td></tr><tr><td>OS Structure Features</td><td><p>British National Grid (EPSG: 27700)</p><p> </p></td></tr><tr><td>OS Routing and Asset Management Information (RAMI)</td><td><p>British National Grid (EPSG: 27700) for all feature types, except the Average And Indicative Speed Feature Type which is British National Grid + ODN Height (EPSG: 7405)</p><p> </p></td></tr><tr><td>OS Transport Features</td><td><p>British National Grid (EPSG: 27700)</p><p> </p></td></tr><tr><td>OS Transport Network</td><td>British National Grid + ODN Height (EPSG: 7405) for most feature types in the collection; British National Grid (EPSG: 27700) for the Ferry Terminal, Path, Road, Road Junction, Street and Pavement Link Feature Types</td></tr><tr><td>OS Water Features</td><td><p>British National Grid (EPSG: 27700)</p><p> </p></td></tr><tr><td>OS  Water Network</td><td>British National Grid + ODN Height (EPSG: 7405) for the Water Link and Water Node Feature Types; British National Grid (EPSG: 27700) for the Water Link Set Feature Type</td></tr></tbody></table>

### Selecting a coordinate reference system

When [creating a new data package in OS Select+Build](/osngd/getting-started/downloading-with-os-select+build.md#data-packages), you can choose what CRS you'd like to receive the data in.

If you don't choose a particular CRS for your data package, OS Select+Build will automatically select the default CRSs for the feature types in your data package for you.

## Coordinate reference systems available for the OS NGD APIs

### OS NGD API – Features

The GeoJSON format accessible via OS NGD API – Features is available in four different CRSs:

* British National Grid (BNG: EPSG: 27700).
* World Geodetic System (WGS84: EPSG: 4326)
* British National Grid + ODN Height (EPSG: 7405)
* Web Mercator (EPSG: 3857)

### OS NGD API – Tiles

The vector tiles format accessible via OS NGD API – Tiles is available in two different CRSs:

* British National Grid (BNG: EPSG: 27700)
* Web Mercator (EPSG: 3857)

## Definitions for the coordinate reference systems available in the OS NGD

<table><thead><tr><th width="274">Coordinate Reference System</th><th>Definition</th></tr></thead><tbody><tr><td>British National Grid <br>(BNG: EPSG: 27700)</td><td>The British National Grid (BNG) spatial reference system uses the OSGB36 geodetic datum and a single Modified 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.</td></tr><tr><td>British National Grid + ODN Height (EPSG: 7405)</td><td>The BNG with ODN height reference uses the OSGB36 geodetic datum and a modified Transverse Mercator projection for the whole of Great Britain. Positions in this system are described by easting and northing coordinates plus a height value, all in units of metres. Please note, this will only be available for some OS NGD feature types.</td></tr><tr><td><p>European Terrestrial Reference System </p><p>(ETRS89: EPSG: 4258)</p></td><td><p>The European Terrestrial Reference System uses the ETRS89 geodetic reference system which is the EU-recommended frame of reference for European data. Positions on this projection are described using latitude and longitude, and coordinates are provided in degrees. ETRS89 is a horizontal spatial reference system only; it does not specify a vertical (height) reference system. </p><p><br>Please note, this coordinate system is used for the OS Islands Address Collection due to the coverage of this data only being Northern Ireland, Isle of Man, and the Channel Islands. Similarly, this coordinate system is used for the OS NI Postcodes Collection due to the coverage of this data being Northern Ireland.</p></td></tr><tr><td>World Geodetic System (WGS84: EPSG: 4326)</td><td>The WGS84 spatial reference system uses the WGS84 geodetic datum. Positions on this projection are described as latitude and longitude, and coordinates are provided in decimal degrees. There will also be an option to receive this reference system via CRS 84, meaning the coordinates will be provided as longitude / latitude rather than latitude / longitude.</td></tr><tr><td>Web Mercator (EPSG: 3857)</td><td>Web Mercator is not a recognised geodetic system, but it is used for many visualisation and web mapping use cases. It uses the WGS84 geodetic datum, and positions on this projection are described as easting and northing, all in units of metres.</td></tr></tbody></table>

## :new: Use of geographic coordinate reference systems in the OS NGD

Where OS provides the option to receive data with geometries expressed in latitude and longitude format, they will be transformed into the ETRS89 (European Terrestrial Reference System 1989, EPSG: 4258). Where the option is stated to provide WGS84 (World Geodetic System 1984, EPSG: 4326), the embedded coordinate reference system (CRS) value of 4326 will be provided; however, the data will actually be in ETRS89 (EPSG: 4258).&#x20;

In practice, the difference will be negligible – that is, significantly smaller than the stated accuracy of the data being transformed. Both OS NGD APIs offer data as Web Mercator (EPSG: 3857), but this will be a projection using the same ETRS89 data, labelled as WGS84, as the base CRS.

The reason OS does this is due to the shifting nature of WGS84, which is adjusted yearly (to account for global tectonic motion) and would therefore require additional date metadata to be a faithful representation of this CRS. ETRS89 is a 'plate fixed' static system that allows for a consistent transformation to be performed. Many software applications / libraries require data to be provided in WGS84 however, so by embedding the WGS84 CRS, we are facilitating these use cases without compromising the positional accuracy of our data.

{% hint style="info" %}

#### Guide to coordinate reference systems in Great Britain

An in-depth [guide to coordinate systems in Great Britain](https://docs.os.uk/more-than-maps/a-guide-to-coordinate-systems-in-great-britain) is available from our More Than Maps site.
{% endhint %}


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