On a nautical chart, not all of the sea is known equally well. Some of it was surveyed last year by multibeam sonar; some was last sounded by a weighted line over the side of a boat in 1880. So charts carry a Zone of Confidence: a ladder of stars on each patch of water, six for "fully surveyed, trust it" down to a bare U for "nobody has checked."
Quietly brilliant: the confidence is drawn straight onto the data. No methodology section, no footnote about survey vintage. You glance and you can see which parts to trust, compressed into an icon a tired navigator can read at three in the morning.