brynn-
To make a "random world" the easiest way is to use some form of fractal noise - like Perlin noise.
I like to use gimp and the
felimage noise plugin mainly because it supports generating noise with a spherical mapping. This is necessary to represent a globe on a rectangular map. The easiest way of doing this is by using an equirectangular projection. This is a rectangle map that has a width equal to double its height, so the width and height map directly from degrees longitude and latitude.
Another option is to use dedicated world building software that will generate terrain heightfields.
Wilbur is a great free one.
Fractal terrains is a brilliant commercial product. These are overkill for most of the maps I make as I don't tend to require accurate height-field models unless I am doing 3d work.
Working with a large raster image and detailed spherical noise, I just randomly regenerate a bunch to get a distribution that works with what I want (many small islands, large continents, etc.) To get a shoreline, I just threshold the noise. In Gimp I can just use the slide and watch the map be turned into B&W and decide what value I like.
Depending on the desired map style, I need to go from the equirectangular to something else. For the Torchio map I wanted a two hemisphere projection (like looking t the two opposite sides of the sphere from a large distance). There a a bunch of map specific tools to do these re-projections. I will use
Flex Projector,
Hugin or just gimp with custom coded reprojections using mathmap.
There is much discussion at the
Cartographers' Guild in the forums on this, with such topics as "How to get your rivers in the right place" (basically, they flow as downhill as possible and merge with more and more rivers, until they reach a basin) or "Where do deserts appear?" (depends on mountains and latitudes). There are a large number of armchair geophysicists there helping to answer such questions, and a couple of real ones, too...
Hope that answered a few questions!
-Rob A>