An ENC chart is a vector based chart format that can be displayed using your marine navigaion software. NOAA and the Army Corps of Engineers provide offshore and inland waterway ENC’s for all of the United States. Although charts in the intenational s-57 standard are called ENC, the international standard for this format is designated s-57.

Raster vs. Vector Charts

The structure and format of a vector-based ENC is defined by the International Hydrographic Office S-57 standard. The ENC is in a non-proprietary and internationally recognized format. The ENC contains information about the location of each chart feature, as well as color, shape, depth, and the like. This information is used by your marine navigation software to draw a nautical chart display—one that has greatly enhanced functionality. A raster chart, by contrast, is a facsimile of a paper chart—that is, a picture of a chart displayed on a computer screen. It’s a file of information that can be thought of as dots. The raster file tells the software which color to draw at each pixel. The software knows what color is there, but cannot distinguish a black pixel that is part of a sounding from a black pixel that is part of a dangerous wreck. The ENC chart stores the feature, its latitude and longitude, and its descriptive information. When reading a ENC chart, the software knows the depth of soundings, and can display it in feet or meters depending on your preference. Furthermore, one can control the way the software displays the data by switching off the display of soundings, lights, or text as desired.

ENC Publishers

The Office of Coast Survey (OCS) has been involved in the development of the NOAA ENC chart suite to support the marine transportation infrastructure and coastal management for a number of years. Every NOAA ENC is in the International Hydrographic Office (IHO) S-57 international exchange format, and complies with the NOAA ENC Product Specification. These NOAA ENCs, which support real-time navigation, are now being released to the public at no cost. NOAA ENC updates will provide automatic corrections for Notices to Mariners and other changes. Once the updates have started, the NOAA ENC will be established as an official navigational product.

ENC Coverage Areas

While the current dataset does not yet have complete national coverage, the good news is that the catalog of charts is continually growing. Better yet, the data and subsequent updates are available free-of-charge from the NOAA website.

IENC Charts

The U.S. inland navigation system consists of 8,200 miles of rivers maintained by the Corps of Engineers. The shallow draft waterways have many unique characteristics and difficulties over coastal harbor and ocean navigation; river levels can change by over 30 feet in a seasonal cycle, the navigation channel can shift significantly within the river banks, and shifting yet ever present river currents pose constant challenges in these confined waterways. Following recommendations by the National Transportation Safety Board, the National Academy of Science and the American Waterways Operators, The United States Congress directed the Corps of Engineers to develop and publish electronic charts for the inland waterways. Development of Inland Electronic Navigation Charts (IENCs) to cover the Mississippi River and tributaries thus began in 2001 with pilot projects on the Atchafalaya River in Louisiana and Lower Mississippi River near Vicksburg, Mississippi. These projects involved the conversion of inland waterway data, commonly used for river and channel maintenance, into the international S-57 hydrographic data exchange. This highly structured data format is now used for Corps IENCs. Following the pilot project, development of initial IENCs to cover most of the Mississippi and Ohio Rivers began in 2002. IENCs currently under development are:

• Lower Mississippi River, Mile 220 AHP to 985
• Upper Mississippi River, Mile 0 to 480
• Ohio River, Miles 0 to 981
• Black Warrior, Tombigbee, Alabama Rivers
• Atchafalaya River (update of pilot IENC)
• Red River

ENC data for International Waters

International hydrographic offices and data producers around the world are also producing S-57 ENC data, although not all charts will be free-of-charge. The S-57 ENC Chart format has been ratified as the international standard for vector-based marine charts. U.S. produced NIMA vector charts for foreign waters are only available to the military or individuals with appropriate security clearance.