The new OSTN15 NTv2 transformation from the Ordnance Survey provides a greater degree of accuracy for some UK data.
This article explains how to add this transformation to ArcMap, ArcGIS Pro or ArcGIS Server.
If you have the latest software version (starting at ArcGIS Pro 1.4.1, ArcMap 10.5, or ArcGIS Enterprise 10.5), you can simply download and install the ArcGIS Coordinate Systems Data from the appropriate product's download section in MyEsri.
If you are using an earlier version of the ArcGIS products listed above then it will be necessary to download the NTv2 files from the OS website, and carry out some additional steps, in order to make the transformation available. To achieve this, please follow the instructions below (note this is only necessary if you are using earlier versions of the software):
1. Create a new Windows folder called ‘PEOBJ’
2. Add an Environment Variable called ‘PEOBJEDITHOME’ to the System Variables (Control Panel > System > Advanced System Settings > Advanced tab) with the variable value set as the folder path created in step 1. where the GEOGTRAN file will reside (e.g. C:\PEOBJ)
3. In your PEOBJ folder, create a text file without any extension called GEOGTRAN. The contents of the GEOGTRAN file should be:
4. Download zipped NTv2 format files from the Ordnance Survey here and extract to a temporary location.
5. Navigate to the following location as appropriate:
for ArcMap, C:\Program Files (x86)\ArcGIS\Desktop10.5\pedata\ntv2\uk
for ArcGIS Pro, C:\Program Files\ArcGIS\Pro\Resources\pedata\ntv2\uk
for ArcGIS Server, C:\Program Files\ArcGIS\Server\pedata\ntv2\uk
6. Copy the following files from the temporary location to the folder in step 5.
7. You can now use the OSTN15 NTv2 transformations:
"OSGB_1936_To_ETRS_1989_2 + ETRS_1989_To_WGS_1984" or
"ETRS_1989_To_WGS_1984 + OSGB_1936_To_ETRS_1989_2"