If the initial fractal isn't quite right, you can run it through one or more post-processing steps. Typical post-processing steps are non-linear scaling and/or smoothing with a digital filter. There are a vast number of ways in which you can blur or smooth the terrain by filtering. I'm not going to try to discuss them here; basically, look up some sources on signal processing, try some things, see what looks good.
You can even generate terrain using nothing but a digital filter. Fill your map with random values, then run your filter over it several (3 to 10) times. [thanks to Karl Meissner for that one.]
Digital filtering is also closely related to wavelets. So far, I haven't seen anything earthshattering published about synthesizing terrain with wavelets, but it seems to show a lot of promise. If I find anything, I'm put it up here.
Up to the terrain synthesis page
© 1996 cburke@mitre.org