Depth contour line

A depth contour line, or isobath, joins points of equal depth on a nautical chart, drawn from soundings reduced to chart datum. Contours give the shape of the seabed at a glance: closely spaced lines mark a steep gradient, widely spaced a gentle one. Selected contours, often the 5, 10, and 20 metre lines, are emphasized for safety, and the navigator sets a safety contour in ECDIS so the system alarms when the planned track crosses water shoaler than the chosen depth, per the IHO S-52 presentation rules. Depths between contours are confirmed by the printed spot soundings.