I've been trying to emit particles from a layer using Trapcode Particular. So far, so good. But they start emitting at the top of the layer and I'd like the particles to start emitting in the center and then randomly outwards. So that I can start with a few buildings, then zoom out and show the entire map.
1. PreComp your map layer
2. In your new PreComp, make the map solid white (color overlay) & your background black.
3. Make a new comp sized solid layer (color doesn't matter) and add a circle mask
4. Place the new masked solid layer on top of your map layer
5. Set the track matte for the map layer to Alpha Matte
6. Animate the circle mask to grow from the center of your map out to the edges.
Now bring this entire PreComp back into your main comp and use it as your emitter layer. Your houses should start emitting in the middle and finish once the mask reaches the edges of your map.
Hopefully that all makes sense. Let me know how it works out.