Something else that just occured to me. If the constellations actually mark the positions of the colonies then the observatory map gives them all the information they need to locate earth. Looking at the map would give them a bearing to each colony from earth. Since obviously they know the positions of the twelve colonies, even without distances you could triangulate the exact position of earth.
Even if you assume they weren't able to record the images they saw, Adama, Starbuck and Appolo are all trained as pilots. You would have to assume that even with a brief glance at the starmap they could recall enough to sketch it again fairly reliably.
That pretty conclusivly proves the constellations in the starfield on earth are merely metaphors for the colonies. It is an accurate representation of the view of the stars from earth, so it is useful to verify earth once they find it. But it's not helpful in finding earth unless you can relate the stars in that perspective to the perspective seen from Kobol or the colonies.
If you have accurate charts of all the stars in the galaxy, and a precise copy of the observatory map you could probably run a computer simulation that could compare the starfields from every spot in the galaxy. But that might take longer then any of them would be alive, even for a computer.
And you'd have to assume they recorded the observatory view and aren't just going by memory.