[naturalman1975]

One possible reason - if you are in an area which you know is not fully charted, you’re probably expected to follow different procedures than if you’re in an area that you know to be accurately charted. Don’t follow the right procedures, and hit something... you’re in trouble.

[Tallguy]

So would that be a “known unknown” or a “regular unknown”?

[Gay State Conservative]

Yes,that makes sense. Having just watched (for the 10th time) The Hunt for Red October the other day you'd think I would have figured that out on my own!