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:

​http://www.ordnancesurvey.co.uk/oswebsite/gps/osnetfreeservices/furtherinfo/ostn02_ntv2.html

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