So, I was fiddling with a satnav I have for my boat, and became interested in how the map data was accessed.
On very old satnavs we used to have floppy disks that had the map info on them, and for a new area we had to put in a new disk.
I had made backups of those to have at home, incase well, someone messed around with them intoxicated and it flew over board (yes, that actually happened one time we used the boat as transport to an island festival, there 'might' have been beer involved..)
[According to our laws this is perfectly legal, and I believe it also recently became legal in the UK.]