For newcomers to nautical charts, there is a tendency to think that a chart is simply a topographical map of the water. Not quite. Topographical maps have contour lines, showing increasing elevation. The more lines, the higher the elevation. More lines in a condensed area denotes cliffs or steep hillsides. But the lines are all drawn in the same way, regardless of height.
Nautical charts have lines for the depth contours, but they are much more sophisticated. Lines with different dot patterns mean different depths. For example:
Depths from 0 to 3 fathoms (6 feet per fathom) or 18 feet are bounded by an unbroken series of dots: ………………At 24 feet or 4 fathoms, that pattern changes to a series of four dots, space four dots: …. …. ….At 5 fathoms it is five dots, space five dots: ….. ….. …..
There are various patterns of dots continuing through different depth levels. And to make it more colorful, the shallow waters from 0 to 10 fathoms (60 feet) are colored light blue. Deeper water is white, and fathoms are noted by simple numbers, for example, 60 = 360 feet.
And that’s just the beginning of how different patterns of lines show depth. I’d tell you more today, but it’s a deep subject! :)