Scaling and fitting of two adjoining elevation maps

[Akubra]

I’m thinking of creating some elevation maps of Rautah, the planet I’m building. I want to have small scale maps (one of the planet and then one or two per continent), but I also would like large scale maps that could possibly be the start of a topographic atlas.

There are a few very good tutorials explaining how to create realistic elevation/topographic maps, but once I have one type of map, how do I make the other type?

As far as I can see there are two approaches:

1. A top-down approach: starting with a small scale map, and use it as a base for the large scale maps. But two problems arise here:
   
   • Scaling - I can’t just cut the small scale map in pieces and enlarge them. The terrain resolution would be horrible. Adapting the maps manually and letting Wilbur manipulate them to add some realism could be a conceivable solution to adjust this problem, although the manual interventions would be very tedious, and it would make the next problem worse.
   • Fitting - How do I make sure that two adjoining large scale maps fit at their edges? After passing them through Wilbur they won’t fit seamlessly.
   
   
2. A bottom-up approach: starting with the large scale maps and creating the small scale map by reducing their size and combining them. This would probably solve the scaling problem, but not the fitting problem. And of course, if following this approach, it would take much longer to produce a small scale map.

So the main question is how do I make two adjoining maps fit nicely together. Has anybody tried this before with good results? I’ve been thinking about this for some time and I can’t figure it out. I'd be very grateful if someone can help me out on this.

Cheers - Akubra

FYI: I have GIMP, Inkscape, Wilbur and I have recently downloaded QGIS, but I have never used it. If there is a solution with any of these that would be an extra bonus! If not, it will be an extra drive to learn something new.