# 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 %}


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET 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?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.