We recently published an article on the basics of Coordinate and Projections for UK users in ArcGIS. In response to this article we received a really interesting follow-up question:
“Please can you provide some more information on why the Petroleum transformation should be used? Are there any cases where another transformation in the list should be used?”
When transforming data between WGS-84 and BNG it can be confusing which options to choose. At ArcGIS 10 you are presented with 7 different options.
So, what are the differences and which one should you choose?
Essentially each option has a different accuracy depending on the output accuracy you require on your data but also the geographic area in which it resides. This is best summarised using the following table (reproduced by kind permission from Jim Sibbald).
* with configuration
As you can see the OSGB_1936_To_WGS_1984_Petroleum has the best accuracy around the UK out of the box within ArcGIS.
However, what if you are collecting data from a GPS device in WGS-84 and when transforming you require greater accuracy than ±2 meters? The most accurate transformation between British National Grid and WGS 1984 would be using the OSTN02 transformation.
At present, out of the box, ArcGIS doesn’t support this type of transformation. If you require this level of accuracy there are a number of options depending on the version of ArcGIS that you’re using. However, after working in collaboration with DGC and Ordnance Survey, Esri UK have released OSTN02 support within ArcGIS desktop.
Configuring OSTN02 Transformation in ArcGIS 10.1
Users of ArcGIS 10.1 will be able to take advantage of OSGB_1936_To_WGS_1984_7 supported as shown in Figure 1. This requires the user to download the .grb file from Ordnance Survey and to place it in the appropriate folder.
Figure 1 – OSTN02 in NTv2 in ArcGIS 10.1
To utilise this transformation method simply download and paste the OSTN02_NTv2.gsb into a folder called ‘C:\Program Files\ArcGIS\Desktop10.1\pedata\ntv2\uk’. You will need to create the folder called ‘uk’ which must be in lower case. Next time you restart ArcGIS 10.1 you will be able to use this transformation from the Geographic Coordinate Systems Transformations dialog box.
Configuring OSTN02 Transformation in ArcGIS 10
To add this support in ArcGIS 10 you will need to download and install an add-in. More information about this add-in can be found in the following blog article: OSTN02 supported in ArcGIS desktop.
In both cases, if you are using ArcGIS Server then remember that you need to install it in both areas, especially if you are publishing maps from an MXD with it applied.
However, you also need to be aware that the accuracy of the Petroleum and NTv2 (OSTN02) transformations also fluctuates around different parts of the UK.
Summary
As such, the OSTN02 transformation is the most accurate but not included by default. With most UK data requiring an accuracy of approximately ± 2 meters the Petroleum transformation is fine for common applications and uses.
Jim Sibbald