# Datum, ellipsoid and projection information

#### Shape and size of biaxial ellipsoids used in the UK

<table data-full-width="true"><thead><tr><th>Name</th><th>Semi-major axis a (m)</th><th>Semi-minor axis b (m)</th><th>Associated datums and projections</th></tr></thead><tbody><tr><td>Airy 1830</td><td>6 377 563.396</td><td>6 356 256.909[10]</td><td>OSGB36, National Grid</td></tr><tr><td>Airy 1830 modified</td><td>6 377 340.189</td><td>6 356 034.447</td><td>Ireland 65, Irish National Grid</td></tr><tr><td>International 1924 aka Hayford 1909</td><td>6 378 388.000</td><td>6 356 911.946</td><td>ED50, UTM</td></tr><tr><td>GRS80 aka WGS84 ellipsoid</td><td>6 378 137.000</td><td>6 356 752.3141</td><td>WGS84, ITRS, ETRS89.</td></tr></tbody></table>

The ellipsoid squared eccentricity constant e² is computed from *a* and *b* by equation (B1).

#### Transverse Mercator projections used in the UK

<table data-full-width="true"><thead><tr><th width="189">Projection</th><th width="249">Scale factor on central meridian (F₀)</th><th>True origin, Φ₀ and λ₀</th><th width="369">Map coordinates of true origin (metres), E₀ and N₀</th><th>Ellipsoid</th></tr></thead><tbody><tr><td>National Grid</td><td>0.9996012717</td><td><p>lat 49° N</p><p>long 2° W</p></td><td><p>E 400 000</p><p>N -100 000</p></td><td>Airy 1830</td></tr><tr><td>Irish National Grid</td><td>1.000035</td><td><p>lat 53°30′ N</p><p>long 8° W</p></td><td><p>E 200 000</p><p>N 250 000</p></td><td>Airy 1830 modified</td></tr><tr><td>UTM zone 29</td><td>0.9996</td><td><p>lat 0°</p><p>long 9° W</p></td><td><p>E 500 000</p><p>N 0</p></td><td>International 1924 or WGS84</td></tr><tr><td>UTM zone 30</td><td>0.9996</td><td><p>lat 0°</p><p>long 3° W</p></td><td><p>E 500 000</p><p>N 0</p></td><td>International 1924 or WGS84</td></tr><tr><td>UTM zone 31</td><td>0.9996</td><td><p>lat 0°</p><p>long 3° E</p></td><td><p>E 500 000</p><p>N 0</p></td><td>International 1924 or WGS84</td></tr></tbody></table>

### EPSG codes for common datums and coordinate operations in GB

Many geographic information systems (GIS) and other methods of handling digital geographic data use unique identifiers - Spatial Reference System Identifier (SRID) - for datums, coordinate systems etc.. See [*https://en.wikipedia.org/wiki/Spatial\_reference\_system#Identifier*](https://en.wikipedia.org/wiki/Spatial_reference_system#Identifier). A commonly used SRID Authority is the EPSG (European Petroleum Survey Group) who maintain an online registry of SRIDs at [*https://epsg.org/home.html*](https://epsg.io/).

#### EPSG codes of ellipsoids, datums and coordinate operations in common use in the Great Britain.

| Datum (and associated coord ref system) or operation name                        | EPSG SRID code |
| -------------------------------------------------------------------------------- | -------------- |
| Airy 1830 ellipsoid                                                              | 7001           |
| GRS80 ellipsoid                                                                  | 7019           |
| WGS84 ellipsoid                                                                  | 7030           |
| OSGB36 Datum, 2D lat/long                                                        | 4277           |
| OSGB36 Datum, 2D National Grid                                                   | 27700          |
| OSGB36 Datum, 3D National Grid + ODN height                                      | 7405           |
| ODN Vertical Datum                                                               | 5701           |
| ETRS89 Datum, 2D lat/long                                                        | 4258           |
| ETRS89 Datum, 3D lat/long/Height                                                 | 4937           |
| ETRS89 Datum, 3D XYZ                                                             | 4936           |
| WGS84 Datum, 2D lat/long                                                         | 4326           |
| WGS84 Datum, 3D lat/long/Height                                                  | 4979           |
| WGS84 Datum, 3D XYZ                                                              | 4978           |
| OSTN15 transformation, native 1km grid                                           | 7953           |
| OSTN15 transformation, NTv2 version                                              | 7709           |
| OSGM15 transformation, ODN                                                       | 7711           |
| OSGM15 transformation, Orkney Datum                                              | 7712           |
| OSGM15 transformation, “Newlyn offshore”                                         | 7713           |
| OSGM15 transformation, Lerwick Datum                                             | 7714           |
| OSGM15 transformation, Stornoway Datum                                           | 7715           |
| OSGM15 transformation, St. Marys Datum (Scilly Isles)                            | 7716           |
| OSGM15 transformation, Douglas Datum (IoM)                                       | 7717           |
| WGS84 < - > OSGB36 Helmert transformation (section 6.6)                          | 1314           |
| ETRS89 lat/long + ODN height compound CRS                                        | 9424           |
| ETRS89 lat/long + ODN (Offshore) height compound CRS                             | 9425           |
| ETRS89 lat/long + Orkney height compound CRS                                     | 9426           |
| ETRS89 lat/long + Lerwick height compound CRS                                    | 9427           |
| ETRS89 lat/long + Stornoway height compound CRS                                  | 9428           |
| ETRS89 lat/long + Douglas height compound CRS                                    | 9429           |
| ETRS89 lat/long + St. Marys height compound CRS                                  | 9430           |
| British Isles height ensemble (approx. MSL height across whole of British Isles) | 1288           |
| ETRS89 lat/long + British Isles height ensemble compound CRS                     | 9542           |


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