Understanding which Transformation to choose in ArcGIS

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).

table.PNG

* 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. 

transformation_v101.png

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.

Details of the copyright and liability of the files are here.

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

OSTN02 supported in ArcGIS desktop

Are you using GPS captured data within ArcGIS desktop?

Ever wanted a more accurate transformation for your GPS captured data when displayed on a BNG basemap?

Making this possible has been a personal challenge for myself and a number of colleagues for sometime as over the past 12 months we have seen an increasing number of customers requesting this capability.

I am happy to announce, after working in collaboration with DGC and Ordnance Survey, Esri UK have released OSTN02 support within ArcGIS desktop.   For those of you not familiar with the intricacies of transformations in the UK, OSTN02 is a very accurate grid based transformation between British National Grid and WGS84 (Lat/Long). It can be used when collecting data from a GPS that is to be stored in a British National Grid feature class or for transforming British National Grid data to WGS84.

Aware that the Defence Geographic Centre (DGC) had previously carried out some work in this area I approached them to see if they were willing to share the transformation with the rest of the GIS user community.  DGC were happy for this to happen and Ordnance Survey have provided quality assurance, ensuring the transformation works as expected.

For those that are interested, the OSTN02 NTv2 transformation file can be downloaded from myEsriUK. You will also find a full set of instructions on how to configure the files at this location.  We have long supported the transformation in ArcPAD and you can download the relevant files for ArcPAD from this location too.

Thanks to DGC and Ordnance Survey for helping to make this happen.  I hope you are able to make use of the new transformation in your work. 

NB: In ArcGIS desktop the OSTN02 NTv2 transformation is called OSGB_1936_To_WGS_1984_7