I've been part of an implementation of a global map done in SVG. Only path data that was sent down was national borders. We had to do many multiple levels becau

I've been part of an implementation of a global map done in SVG. Only path data that was sent down was national borders. We had to do many multiple levels because the volume of data to get the paths drawn accurately was both too big and too slow until you zoomed in a bit and could start loading individual countries, then individual regions. Rendering rasterized tiles from a tile server onto a canvas means that, on the client, performance is identical regardless of zoom level. Basically, we went with SVG because only drawing the borders looked cool, and we could fake some clever zoom animations, and we couldn't afford to purchase custom tile data and run a tile server. As for interactive bits, those are frequently done with DOM nodes positioned over the map. Maps make for a pretty crap user experience for screen readers, so if you really wanted to make the information accessible, you're better off putting it in a separate interface. That, and I can honestly say I have yet to see a map that would make sense to have a crawler attempt to index.