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.
The Office of Coast Survey conducts hydrographic surveys to measure the depth and bottom configuration of water bodies. We use the data to update nautical charts and develop hydrographic models; increasingly, the hydrographic data is used for multiple purposes, through the Integrated Ocean and Coast Mapping program.
Surveyors pay particular attention to acquiring the precise location of least (shoalest) depths, to warn mariners of dangers to navigation, and they record the precise location of aids to navigation. They also record tide or water level measurements to provide a vertical reference (mean lower low water) for water depths. Surveys can also determine sea floor material (i.e. sand, mud, rock), which is important for anchoring, dredging, structure construction, pipeline and cable routing, and fisheries habitat.
Survey vessels primarily use side scan and multibeam sonar. Sonar (which was originally an acronym for SOund NAvigation and Ranging) uses sound waves to find and identify objects in the water and to determine water depth. Some vessels may use single beam echo sounders. NOAA’s Remote Sensing Division and commercial hydrographic contractors use aircraft equipped with LIDAR (LIght Detection And Ranging) to measure water depths in areas with complex and rugged shorelines.
NOAA and commercial contractors usually survey between 2,000 and 3,000 square nautical miles each year.
NOAA’s hydrographic data is publicly available from the National Geophysical Data Center.