Before there were satellites, there were three things: the lead line, the compass, and a set of eyes that could read the color of the water. We've kept two of them alive.

The color is a depth

Deep water runs dark blue, and it lightens as the bottom comes up. A patch of green is a shoal; a brown smear is a bar. The old pilots read the whole chart off the water's color without ever looking down.

Why we still teach it

Electronics tell you a number. The water tells you the truth. When they disagree — and eventually they always do — trust the water.