Dear Geoff, Given that you have checked the Ordnance Survey's "A guide to coordinate systems in Great Britain" you will know everything that I know on this and, no doubt, much more besides.
What I didn't quite deduce from the document is how individual features get into the grid squares! You address this directly and eloquently... the location of the geographical features is indeed achieved by satellite systems using the ETRS89 datum but this is then used to calculate positions in the OSGB36 TRF which may then be transferred to maps using a TM projection. I have just dug out some survey data I paid good money for, for a sundial survey by an OS-licensed surveyor. The many pages of spreadsheet that I was sent strongly suggest that this is pretty much exactly what goes on. Very best wishes Frank --------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial
