# Transformation between OS Net v2001 and v2009 realisations

As detailed in [ETRS89 realised through OS Net](/more-than-maps/a-guide-to-coordinate-systems-in-great-britain/ordnance-survey-coordinate-systems/etrs89-realised-through-os-net.md) the OS Net ETRS89 realisation was originally related to ITRF97 at epoch 2001.553. This original realisation is now designated “OS Net v2001”. In 2016 it was updated to a realisation related to ITRF97 at epoch 2009.756, designated “OS Net v2009”. The average shift between the two sets of coordinates is small (rms of all components \~18mm). A file of the shifts for individual OS Net stations is available on the OS website. The transformation parameters below can also be used (with equation 3 from the 'Helmert datum transformations' page) to move any coordinates related to OS Net v2001 to be compatible with OS Net v2009.

#### OS Net v2001 to OS Net v2009 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>+ 0.0054</td><td>-0.0117</td><td>-0.0010</td><td>+ 0.000</td><td>+ 0.0000</td><td>+ 0.0000</td><td>+ 0.0000</td></tr></tbody></table>

{% hint style="info" %}
The overall RMS of this transformation, when computed from the two OS Net data sets, is 12.2 mm. The RMS residual components in the East, North, Up system are 6.9 mm, 9.2 mm, 17.8 mm respectively.
{% endhint %}

There is a range of options for transforming coordinates computed under OS Net v2001 to be more compatible with OS Net v2009. In increasing order of accuracy they are:

1. Apply the 3 parameter transformation given in the table above.
2. Apply a simple mean shift in X, Y and Z based on the nearest *n* OS Net stations with *n* based on the most likely number of OS Net stations used in the original processing. The OS Net station coordinate shifts (in X, Y, Z) are in a file on the OS web site. Both v2001 and v2009 coordinates are given in the OS Net coordinate file on the OS web site so shifts can also be computed from there.
3. (Equal accuracy to 4 below). Apply weighted mean shifts based on the nearest *n* OS Net stations with *n* based on the most likely number of OS Net stations used in the original processing. Weighting can be based on the distance to the OS Net stations.
4. (Equal accuracy to 3 above). Apply a simple mean shift in X, Y and Z based on the OS Net stations actually used in the processing.
5. Apply weighted mean shifts based on the OS Nets actually used in the processing. Weighting can be based on the distance to the OS Net stations.
6. Re-process the raw GNSS data using the OS Net v2009 coordinates.


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