M&M,My 6th edition says"areas believed to have a depth of less than 6 ft".I know this was not what you asked but I did do a little math.As far a I can tell,the most narrow line was 1/32 of an inch.Based on the scale I calculated that as about 40 ft.I know of areas that are over 6 ft deep at 20 ft offshore so the blue area is probably just a rough guide.
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SIKSUKR
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