An NOAA electronic navigational chart, or NOAA ENC®, is a database of chart features.

The structure and format of an NOAA ENC® is defined by the International Hydrographic Organization S-57 standard, which means that the NOAA ENC® is in a non-proprietary, publicly available format. The NOAA ENC® contains information about the location of each chart feature, as well as attribution information such as color, shape, depth, and so forth. This information is used by navigation system software to draw a display that resembles a nautical chart, but 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 is a file of information that can be thought of as dots, or pixels, on the computer screen. The raster file tells the computer what color to draw at each pixel. Thus, the computer "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 symbol.

The NOAA ENC®, as described above, stores what the actual feature is, its actual latitude and longitude location and descriptive information. A navigation system using an NOAA ENC® would "know" that the dangerous wreck in the above example is a dangerous wreck, as well as how much water is over the wreck. A navigation system using an NOAA ENC® can use this information to check planned routes to see if they cross or pass too near dangers, sound alarms if a vessels projected course will carry it close to dangerous features, and inform the mariner about various warnings and regulations that relate to areas that the vessel is transiting. Further, the mariner can control the display of NOAA ENC® data and remove features from the display that are not important to the safety of the vessel and declutter the screen. This allows the important features to be more easily viewed.

Revised Friday January 31 2003by OCS Webmaster