Where does the slogan 'You Are Here' come from?

Go to a map inside of a museum, airport, train station, shopping mall, or any large venue. There will be a red dot, an arrow, or something, with a note that says "You are here". That tells you where you are located on that map, so you can find your way to your destination. If the map did not tell you where you were standing, you would have a difficult time getting to your destination! Hence, "You are here". Some people don't relate to maps (I have discovered) and this answer will leave them as baffled as ever. If you are not a map person, ask somebody for directions...