What does the ringed figure, with a line under it, tell you?
A chart extract: land in the top left corner, a green area along its edge, then blue water. Depth figures in the water read 2.3, 5.8, 6.4 and 12. Ringed in gold, on the green area, is the figure 1.4 with a line drawn under it.
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That spot dries: it is 1.4 m above chart datum. An underlined figure is a drying height: the seabed there is that far above chart datum, so it uncovers as the tide falls. Here you need more than 1.4 m of tide before there is any water over it at all.
