I have 1:50k and 1:25k maps of the same area, but they don't quite 'line-up' - i.e. if I draw a route along a, say, bridleway on the 1:25k and the scale out to the 1:50k, the route's no longer on the bridleway its slightly off to one side. Same if I do it the other way round.
Any ideas? I assume it's something to do with the datum in the maps.

