How do you map very large cities?

[Jürgen Hubert]

I'm currently getting a lot of work done for Urbis, and one of my higher priorities is fleshing out the cities in a particular region - which eventually means mapping them out.

So far, I've only down two very rough sketches of two of these cities which don't show much beyond the basic geography and the layout of the city districts relative to each other (see attachments). However, eventually - once I've come up with sufficient individual locations for these cities to make it worth the effort - I want to create better maps with more detail.

Yet compared to most fantasy cities, these cities are huge - on average, they have about a million inhabitants. So obviously, mapping every street is right out - for one thing, it would take me forever, and for another my computer's RAM would die screaming in agony (though maybe I will get a better one next year...).

So the trick is finding the right level of abstraction - and that's where I could use your help. How did you map cities this large and provide enough detail without getting bogged down in those same details too much?