Hydrographic survey

A hydrographic survey measures the depth and shape of the seabed plus features and hazards, producing the bathymetry that updates nautical charts and supports dredging, channel design, and under-keel-clearance management. Quality is set by IHO Standard S-44, whose 6th edition (2020) classes surveys by order: Exclusive (the most demanding, for critical harbors and channels), Special, and Orders 1a, 1b, and 2, each fixing horizontal and vertical accuracy, feature-detection size, and bathymetric coverage (full 100% coverage for Special and 1a). Modern surveys use multibeam echo sounders for full-coverage swath bathymetry, with DGNSS or RTK positioning and motion sensors. Results reduce to chart datum (commonly lowest astronomical tide) so charted depths are conservative.