Another avoidance is to not "be" the cartographer and actually join the police as a deputy. Agree that the bard that drew this land obviously didn't know the place well, because he put in these implausible rivers. He probably put in other imaginary stuff too - look at that: he put in horses and eagles when anybody knows the dragons and griffins ate the horses and eagles ages ago. That still leaves you "needing" to be realistic in whatever part of the world the mapper honestly knows, but that could be a small subset of the map.