As far as proof goes... that's actually become less complicated, with digital works.
The map you found on google, for example, was posted on a certain day/time to these forums. The person who posted them can prove the account is his, the time stamp in the forums proves he had completed the map on a certain date and time, and it would be up to anyone else to prove they'd completed it *prior* to that. (Which they couldn't, since they didn't...)
I've also seen (very rarely, in very expensive/high profile cases) dating done on inks, papers, and canvases to prove if one artwork/manuscript/whatever was created before another.
There was even a case when a man proved his ownership of an invention by showing a preliminary sketch in one of his old diaries/journals.
Posting online just makes it easier, because *everything* is stamped with a time/date, even if you can't see it
And the easiest (imho) way to prove something was created, for example, by me, and not by you, is that I could probably recreate it (or something very similar) while you could not
Just like I could not recreate one of Max or GP's masterpieces (and yes, darn you all, I've tried D: )