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I have a TIFF file in the TM65 (Transverse Mercator) projection, and want to convert it to the OSGB (British National Grid) projection. I'm trying to do this in ArcGIS using the Project Raster tool.

I can select the input and output co-ordinate system fine, but it then asks me to choose a Geographic Transformation. It seems that I have to choose two - one to go from TM65 to WGS84, and then another to go from WGS84 to OSGB. My question is - which one should I choose? What are the differences between them? I have tried choosing some almost at random (and also tried matching them up - so I do TM65->WGS84_2 and then WGS84_2 to OSGB) and each time once the points are reprojected they are displaced from their true locations - the amount varies between fairly small (but large enough to be an issue) and quite large.

Is there any way I can get an accurate conversion between TM65 and OSGB? Am I using the wrong tool?

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up vote 4 down vote accepted

Currently, none of the supported transformations for OSGB36 cover the Republic nor Northern Ireland. TM65_To_WGS_1984_2 is probably the best choice for conversion to WGS84 as the older transformations have no known accuracy values.

You can find out about supported transformations in ArcGIS in technical article 21327.

If the data's offshore, there's a transformation from OSGB36 to ED50 and you could then piece together other transformations. Accuracies are several meters.

You might check the online EPSG Geodetic Parameter Dataset to see if they have any other options. Note: I'm on the subcommittee that maintains the dataset.

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Melita, you're tha bomb when it comes to these kinds of questions! – Chad Cooper Nov 8 '11 at 18:17

A good description on the problem can be found here:

The downloadable zip file contains a ntv2 grid for the datum shift.

Official information can be obtained from and and

Both Irish and OSGB datum shifts can be made with ntv2 datum shift grids, but the OSGB only covers England, Wales and Scotland with limited offshore coverage, while the Irish grid has values for the whole rectangle:

enter image description here

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