There is a software package called Bryce, it's been through a number of developers over its lifetime. Currently it is released by DAZ (you can find it here). At various stages in its life it has been a free download, as I write this post, that is the case.
Go, download it. It's a great 3D tool, and one of the best landscape modelling tools available.
Muck around with it, especially the terrain tools, have a look online for some of the other great tutorials for it. It's one of the easiest 3D modelling programs to learn, and while it may not be as powerful as professional tools like Maya....it's free.
I love creating maps using a graphic manipulation tool such as Photoshop and Bryce.
Here's a little something I've been working on, I'll use a few steps to show you how I did it.
The basic premise behind this map is simple. There is a concept called a Dyson sphere.
It is basically a solid spherical shell of matter surrounding a star. Most interpretations of the Dyson sphere have inhabitants living on the inside of the shell...able to utilize the light of the star within to stay warm, grow crops, etc. My problem with that theory is simple...gravity says anything inside the shell will be pulled to the centre of the mass, ie. into the star. I think it would make more sense for inhabitants to live on the outside of the Dyson sphere, thus pulled down onto it, much like people are pulled down onto the Earth.
But if people live on the outside of the sphere, how do they get their light and warmth?
Simple, there are holes punctured in the shell, with reflective mirrors that point the light back to the surface.
Using this assumption, areas closest to the holes would be hottest, and the temperature would gradually get colder as distance to the hole increased. After a certain point, there would be icy waste as far as the eye could see.
If there were a few holes punctured around the sphere, there could be many self contained civilisations scattered across the surface of the sphere. If the remainder of the internal star's energy was captured and stored, it could be used to puncture wormholes in reality, perhaps luring assorted alien races to it...maybe maintaining a force field because it is actually a galactic prison...I don't know, there is a lot of potential in the idea and I've kept coming back to it after years of thought.
For all intents and purposes, each circular temperate zone is a world of its own isolated from the others by vast icy wastes (possibly connected by the wormholes empowered by the sphere's store energy). If we assume a typical pseudo-medieval fantasy setting, then there would obviously be a fallen advanced civilization from the past (who might still have members alive, but might have become extinct for one reason or another). The civilization capable of crafting such a world (and manipulating matter/energy transformation) would probably have left behind some incredible artefacts, perhaps viewed as magical by the current inhabitants. Nanotech swarms might respond to the vocal commands of those who share the blood of the ancients, thus creating pseudo-magical effects.
Anyway, enough with the theory of how a "flatworld" with "magic" might be feasible given "realistic physics"...let's look at how the map was created.