[RPG] How to know what terrain is where in the Forgotten Realms

However the book says nothing about where these portals might appear.

I'm guessing that that is deliberate. It can really break your game when a player flips open his stack of books and points out a portal exactly where they're trying to go. These things show up at the speed of plot, and that's a good thing.

However, if you do want to make it arbitrary, here's how I'd do it. Any location major enough to have a background feat listed in the FRPG has a known portal in its capital. Access to these portals may vary from place to place, but they're always there.

Taking some measurements, the scale is clearly wrong. Looking at the castle-shaped building in the section labelled 12 with a true-distance measuring tool (I'm using GIMP's Measure Tool), I find that its central block is about 12 pixels from river-side front to back. Measuring the scale using the same tool, 15 pixels is 20 feet, making the castle sans towers a mere 16 feet deep. That castle's footprint is smaller than my livingroom! And I doubt the intention was to model that castle after this one:

^{“A view of Broadway Tower” by Newton2, licensed under CC BY 2.5}

All the other buildings have similar problems, with the smallest being 4′×4′. That's unbelievably tiny even by shack or shed standards, and I doubt they are supposed to be sheds anyway.

Clearly the scale is wrong, by an order of magnitude.

The trouble with many maps in big-name books is that there are many maps to produce, and typically these are handled by the art director as art rather than as true cartography. Drawing each bridge and building produces a particular map style that is labour-intensive and carries prestige, and is therefore sought after by art directors at big-name publishers. Rarely do the end readers actually try to orient on these maps in any but the most hand-wavey way anyway, and they do make the book extremely pretty. And, possibly more to the point, they make the book look how the buyer expects a campaign setting to look, with the text broken up by many maps.

Producing quality cartography is hard. So, quite probably there was no coherent intention of the kind you're trying to divine analytically from the map's properties, since any usability intentions were probably far behind business requirements like meeting production schedules and short-turnaround art orders, if usability was even a contender in that competition for business attention. Most likely, the maps are simply incoherent when looked at more than cursorily, and there is no intent — only the question of what you should do with them. And the easiest is to just use them abstractly, as a guide to layout and civic character.