The Cartographer's Lie

What is The Cartographer's Lie?

Somewhere on today's map, one hex is drawn as something it is not. You get four survey notes, and every one of them is true. Only one hex can satisfy all four at once; that hex is the liar. "The lie touches exactly 0 reefs." "The lie is drawn on reef or heath." Put those together and every reef is dead. You are not guessing, you are proving. One map a day, the same one for everyone, about three minutes. Free in the browser, no account.