> For the complete documentation index, see [llms.txt](https://docs.os.uk/more-than-maps/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/more-than-maps/a-guide-to-coordinate-systems-in-great-britain/from-one-coordinate-system-to-another-geodetic-transformations/approximate-wgs84-to-osgb36-odn-transformation.md).

# Approximate WGS84 to OSGB36/ODN transformation

The following Helmert parameters transform WGS84 (or ETRS89 or ITRS, the differences are negligible here) coordinates to ‘something like’ OSGB36 and ‘something like’ ODN heights. The error is up to 3.5 metres (95%) both horizontally and vertically. This transformation is for use with equation (3). Note the remarks made in the [Helmert datum transformations page](/more-than-maps/a-guide-to-coordinate-systems-in-great-britain/from-one-coordinate-system-to-another-geodetic-transformations/helmert-datum-transformations.md).

#### ETRS89 (WGS84) to OSGB36/ODN Helmert transformation parameters.

<table data-full-width="true"><thead><tr><th>tX(m)</th><th>tY(m)</th><th>tZ(m)</th><th width="127">s (ppm)</th><th>rX(sec)</th><th>rY(sec)</th><th>rZ(sec)</th></tr></thead><tbody><tr><td>- 446.448</td><td>+ 125.157</td><td>- 542.060</td><td>+ 20.4894</td><td>- 0.1502</td><td>- 0.2470</td><td>- 0.8421</td></tr></tbody></table>

{% hint style="info" %}
OSGB36 is an inhomogeneous TRF by modern standards. Do not use this transformation for applications requiring better than 3.5 metre (95%) accuracy in the transformation step, either vertically or horizontally. Do not use it for points outside Britain.
{% endhint %}

{% hint style="info" %}
OSGB36 does not exist offshore.
{% endhint %}


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