map creation

[Brian McQueen]

I've through the tutorials and such and I've got a good idea of what's
going on now, but there is one big hole in my understanding.
Everything starts with a data set that is rendered as a map, but how
are those created?  They always seem to come from some download site,
but how are those data files made?  Can folks make them or is that too
difficult?  For tutorial purposes how would I make one?

Thanks

Brian