It's possible to join all these maps and end up with 3 separate mapsets that are all well below 4GB.

I took a close look at the GB Discoverer maps (National Parks, National Trails and UK 50k) and they all contain a set of 67 maps that are exactly equal. This is the autorouting layer of GB that every single map contains. It becomes visible when zooming in in Basecamp outside the topo area of the map. There is also an OVERVIEW_mdr.img in every map that's equal as well.
The 25k maps also contain a 50k portion (visible when zooming out) that can be deleted because we already have the full UK 50k map available.
When deleting all duplicates and the 50k layer your 25k maps can be joined and the data will be reduced by 2/3.

You can achieve this final result:


  • GBDiscoverer50K All.img: 3.2GB
  • National Parks.img: 2.2GB
  • National Trails.img: 1.2GB