well. Google Maps and Openstreetmap are integrated. I haven't looked
at the way they are integrated (and you might need a different kind of
integration), but if your use case matches that of the built-in map
implementations, I'd look up how they're done. If there are two
already, most likely a third one can work relatively easy.
Very often, I'd expect a real GIS system to have a different focus
than the built-in maps though, so the way to integrate
that functionality would differ completely, and 100%
depend on the use case implemented.
