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Frequently Asked Questions

What is a nautical chart?
A nautical chart is a two-dimensional graphic of a 3-D world. It represents part of the spherical earth on a plane surface. It shows water depth, the shoreline of adjacent land, prominent topographic features, aids to navigation, and other navigational information.

The navigator uses the chart to plot courses, ascertain positions, and view the relationship of the ship to the surrounding area. It helps the navigator avoid dangers and arrive safely at the desired destination.

What is the difference between a map and a chart?
A nautical chart presents the coastline taking into account varying tidal levels and water forms.  A map, on the other hand, emphasizes landforms.

Where do I get chart updates?
The United States Coast Guard issues Local Notices to Mariners, providing up-to-date important navigational information. The National Geospatial-Intelligence Agency issues weekly the U.S. Notices to Mariners for larger vessels, i.e. those operating in 12 feet of water or more.

What is hydrography?
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.”

What is the purpose of a hydrographic survey?
Think of hydrographic surveying as “looking” into the ocean to see what the seafloor looks like. A survey supports 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. The primary information obtained by hydrographic surveys is water depth. They also help determine the nature of the sea floor material (i.e. sand, mud, rock), which is important for anchoring, dredging, structure construction, pipeline and cable routing and fisheries habitat.

Where can I find bathymetric and hydrographic data?
NOAA’s hydrographic and bathymetric data is available from the National Centers for Environmental Information.

Where can I find hydrographic and bathymetric information for inland lakes and rivers?
NOAA's Office of Coast Survey primarily surveys U.S. coastal waters. The U.S. Geological Survey and the U.S. Army Corps of Engineers survey inland rivers and waters.

Where can I find information on tides and currents?
NOAA's Center for Operational Oceanographic Products and Services predicts and measures tides, water levels, and currents.

Where can I find topographic sheets (T-sheets) or shoreline information?
Shoreline information and data is available from NOAA's Office for Coastal Management  and the National Geodetic Survey.

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