The July update of ArcGIS Online, combined with the 10.3.1 release of the ArcGIS Platform, has brought a number of new and improved features. One, of particular relevance to many users in the UK and Ireland, is support for OSTN02 NTv2, TM75_to_WGS84 (4) and TM75_to_ETRS89 (3) transformations in ArcGIS online.
Why is the improved transformation support important?
Prior to the update, if you had data in OSGB 1936 (e.g. British National Grid) and applied the OSTN02 NTv2 transformation, ArcGIS Online would replace this with a default transformation, OSGB_1936_To_WGS_1984_Petroleum.
Now, following the update, if you have one of these transformations applied to your data before publishing from ArcGIS for Desktop or ArcGIS Pro, it will be retained.
Transformations window in ArcGIS Desktop
What is OSTN02 NTv2?
OSTN02 NTv2 (named OSGB_1936_to WGS_1984_7 in the ArcGIS platform) can be applied when transforming data between OSGB 1936 and WGS 1984 with better than 1 metre accuracy. This is most beneficial when you have data in both British National Grid and WGS 1984 Web Mercator and you need them to display together, for instance when using Esri Basemaps.
It’s important to be aware that this transformation has been available since 10.1 through configuration. However, it’s now included out of the box and can be accessed directly in the Transformations drop down menu.
When would you use OSTN02 NTv2 over the Petroleum projection?
The OSTN02 NTv2 transformation is more accurate than OSGB_1936_To_WGS_1984_Petroleum when used onshore.
However, be aware that if your data extends beyond 10km offshore, the transformation will not be applied. Therefore, in these circumstances, it may be better practice to use OSGB_1936_To_WGS_1984_Petroleum to ensure all your data is transformed correctly.
What are the NTv2 Transformations between TM75 and WGS 1984 or TM75 and ETRS 1989?
The equivalent transformation which can be applied between TM75/Irish Grid and WGS 1984 is called TM75_To_WGS_1984_4_NTv2, and between TM75/Irish Grid and ETRS89 it is TM75_To_ETRS_1989_3_NTv2, as shown below.
Figure 2 - Transformations using TM75 NTv2
For more information on coordinate systems and projections check the following article.