Chart No. 1 · I 15
Drying height
Depths & contours
An underlined figure marks ground that covers and uncovers, standing that height above chart datum.
What it means
A sounding-style figure with a line underneath is a drying height: the spot is above chart datum and uncovers at low water. 2₄ underlined means it dries 2.4 units above datum.
Why it matters
Drying areas are usually tinted green. At high water there may be plenty of depth here; near low water it is dry land. The underline is the giveaway that the number means height, not depth.