The International Hydrographic Organization defines hydrography as “the branch of applied science which deals with the measurement and description of the physical features of the navigable portion of the earth’s surface [seas] and adjoining coastal areas, with special reference to their use for the purpose of navigation.”

Hydrographic surveying “looks” into the ocean to see what the sea floor looks like.
Hydrographic surveys support a variety of activities: nautical charting, port and harbor maintenance (dredging), coastal engineering (beach erosion and replenishment studies), coastal zone management, and offshore resource development. Most surveys are primarily concerned with water depth. Of additional concern is the nature of the sea floor material (i.e. sand, mud, rock) because of the implications for anchoring, dredging, structure construction, pipeline and cable routing and fisheries habitat.

The Office of Coast Survey (OCS) conducts hydrographic surveys to measure the depth and bottom configuration of water bodies. We use the data to produce the nation’s nautical charts and ensure safe navigation in U.S. coastal waters and the Exclusive Economic Zone.

Surveyors pay particular attention to acquiring the precise location of least (shoalest) depths on dangers to navigation and depths significant to surface navigation. They record the precise location of aids to navigation. Tide or water level measurements are also recorded to provide a vertical reference (Mean Lower Low Water) for water depths.

OCS conducts hydrographic surveys primarily with side scan and multibeam sonar. SONAR (SOund NAvigation and Ranging) uses sound waves to find and identify objects in the water and to determine water depth. Most survey vessels are equipped with side scan and multibeam sonar systems. Some vessels use single beam echo sounders, divers least depth gauges, and lead lines. Some contractors employ LIDAR, which uses light to determine objects in the water and water depth.

NOAA and outside contractors usually complete about 70 to 80 hydrographic surveys each year.

The data is collected and processed digitally with specialized computer systems that store the data in digital form. Chartmakers use the data, with shoreline information, to update nautical charts and generate graphic displays in both digital and hardcopy form.

Hydrographic data collected by and for OCS is publicly available to the broader scientific and oceanic community. Contact NOAA’s National Geophysical Data Center for more information.

Resources
NOS Education Discovery Classroom:
Seafloor Mapping