I threw together a quick script to generate the coordinates for the center of each map overlay and it doesn't match what they have in their addon, so I'm not entirely sure what they did.
I don't think the achievements require you to reveal every existing overlay, so it would need to be cross referenced with that database. I'll poke around some more and see if I can figure it out.
http://semlar.com/addons/Pinlight.zip if you want to play around with it.
I honestly have no idea how accurate it is in terms of "exploring" an area.
Edit: I made some adjustments and I think I got it closer to what they have.
