Q: Where can I get the grid shift file needed for apltran for describing the Ordnance Survey National Grid projection?

A:

The NTv2 file describing the OSTN02 transformation can be downloaded from the Ordnance Survey at:

​https://www.ordnancesurvey.co.uk/business-and-government/help-and-support/navigation-technology/os-net/ostn02-ntv2-format.html

This is required if you wish to re-project data into Ordnance Survey National Grid.