What is hydrography?

Hydrography is the science of measuring and describing the physical properties of the ocean and aspects of the land areas bordering it. These measurements and descriptions are essential for creating accurate nautical charts, but many other kinds of data and information are necessary to create accurate charts as well, including topographic information from NOAA’s National Geodetic Survey or the U.S. Geological Survey. An example of topographic information is data about the elevation of the shoreline itself.

A NOAA navigation response team on a boat conducts surveys near the collapsed Francis Scott Key Bridge in Baltimore. — Physical Science Technician Michael Bloom sits in the foreground working with hydrographic data as Lieutenant Patrick Debroisse (left) and Lieutenant Junior Grade Mark Meadows (right) navigate the survey vessel and collect data. The wreckage of the Francis Scott Key Bridge and the cargo vessel Dali can be seen in the background.

Hydrographic data includes information about water temperature and salinity, which is how salty the water is; tides; currents; and waves; as well as measurements of the seabed, which is also known as the seafloor. The seabed is constantly changing due to storms, erosion, and even coastal construction. When hydrographers take measurements of the seabed, they focus on obstructions, such as rocks, shoals, shipwrecks, or reefs that could damage or sink a vessel. Information about the seabed also informs navigators about where they can anchor safely and effectively.

Hydrographic surveys may include the accurate position and representation of rocks and ledges; navigational aids like buoys; and parts of the shoreline, including hills, mountains, inshore lights and towers, which all help navigators determine their vessels’ exact position.

A 3-D sonar map of a volcano-like feature found by USCG Healy on Alaska's continental slope, 1,600 m below the surface. — Bathymetric data of volcano-like features discovered on the continental slope along the north slope of Alaska during the U.S. Coast Guard Cutter Healy’s October 2024 mapping mission. The feature rises from the seabed approximately 585 meters, residing approximately 1,600 meters below the surface at its shallowest depth.

Hydrography often includes the investigation of phenomena also studied in oceanography, such as currents, waves, and tides, but there are differences between the two fields. Hydrography focuses on information about the ocean and adjacent lands that affect safe navigation. Oceanography investigates the chemical, physical, and biological properties of the ocean and its interaction with coastal lands, the atmosphere, and the many ecosystems found within it.