Hydrography

Hydrography is the applied science that measures and describes the physical features of seas, coasts, and rivers, above all the depth and shape of the seabed, for the safety of navigation. A hydrographic survey fixes soundings to a position and to a vertical datum, maps the bottom with echo sounders, multibeam, and side-scan sonar, and records tides, currents, and the nature of the bottom. The results feed the nautical chart, where depths are reduced to a low-water chart datum so the charted depth is the least the mariner should expect. The International Hydrographic Organization sets the survey standard, IHO S-44, and the chart-product standards S-57 and S-100. Hydrography underpins every depth, contour, and danger on the chart.