Bathymetric Contour

A bathymetric contour is a line on a chart or model joining points of equal water depth, the underwater equivalent of a topographic contour; an individual depth contour is also called an isobath. Contours are referred to the chart datum, usually lowest astronomical tide on modern charts, so the charted depth is near the shallowest the tide produces. The contour interval and the selected depth curves (such as a 5, 10, and 20 m safety contour) convey seabed slope and shoal extent to the mariner. Bathymetric contours are derived from gridded soundings and are graded for position and depth against IHO S-44.