“In its original form OSTN02 consists of a plane 700 km by 1250 km grid of 1 km resolution with an eastings shift and northing shift at each grid node.
The plane grid form of OSTN02 is not easily compatible with some GIS systems. Some of these systems utilise a transformation data format known as NTv2. NTv2 format is a gridded format but the grid is realised on a latitude/longitude graticule and the shifts are expressed as differences in latitude and longitude.” OS
Ordnance Survey working with Defence Geographic Centre have produced an NTV2 version of their OSTN02 transformation.
To implement the transformation and make it available in ArcGIS Desktop:
1. Download the data file from OS website: http://www.ordnancesurvey.co.uk/business-and-government/help-and-support/navigation-technology/os-net/ostn02-ntv2-format.html
2. Using Windows Explorer, create a new folder called greatbritain (if it doesn’t already exist) using lowercase letters in (On a 64 bit machine this will be in Program Files (x86) ):
- 9.x – C:\Program Files\ArcGIS\pedata\ntv2\
- 10.0 – C:\Program Files\ArcGIS\Desktop10.0\pedata\ntv2\
3. Copy the NTv2 grid file(s) to the greatbritain folder. Note: There is no limit to how many .gsb files you can store in this folder.
4. Download GEOGTRAN.txt file. Paste that file in some folder on the C drive like C:\PEOBJ (please do not put the GEOGTRAN file to the Program Files Folder).Add an Environment Variable of PEOBJEDITHOME to the System Variables with the value of the folder where the GEOGTRAN file will reside (e.g. C:\PEOBJ ).
When you download and copy the file into the e.g. C:\PEOBJ – please remove the file extension (txt) leaving the file without extension.
The content of GEOGTRAN file is as follow:
GEOGTRAN, 208339, "OSGB_1936_To_WGS_1984_OSTN02_NTV2", PE_GCS_OSGB_1936, PE_GCS_WGS_1984, PE_MTH_NTV2, PE_PAR_NAME_DATASET, "Dataset_greatbritain/OSTN02_NTv2" GEOGTRAN, 208338, "OSGB_1936_To_ETRS_1989_OSTN02_NTV2", PE_GCS_OSGB_1936, PE_GCS_ETRS_1989, PE_MTH_NTV2, PE_PAR_NAME_DATASET, "Dataset_greatbritain/OSTN02_NTv2"
5. Open Arc Map and set the Layers projection to Geographic projection OSGB36
6. Add any data projected in OSGB36 (geographic) or cartesian British National Grid.
7. Add ESRI_STREETMAP_World_2D or any other data projected in WGS1984 – do not apply any trasformation:
As you can see there will be the data shift of approximately 100m.
8. Right click on Layers then go to Properties and click Transformation button and select one of the new transformation methods from the drop-down list – in our case it will be OSGB_1936_To_WGS_1984_OSTN02_NTV2:
9. Apply the selected transformation – as you can see the WGS84 data has been properly re-projected to OSGB36 using OSTN02 in NTv2 Transformation method:
The above method is working for ArcGIS Desktop 9.2 to 10.0 – it has been tested and is working fine. If you have any questions please do not hesitate to ask.
Please remember – if you have data in British National Grid (Easting, Northing), you need to project it first to OSGB36 (world projection) to use the method described above.