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RNC – ENC Comparison

Raster Navigational Chart (RNC)

Electronic Navigational Chart (ENC)

RasterInformation is stored as raster data — rows and columns of color pixels. VectorInformation is stored as vector data — pairs of coordinates that define the position and shape of points, lines, and area features.
Each RNC pixel is assigned one color to display and is georeferenced to a particular latitude and longitude. Pairs of latitude and longitude coordinates define nodes that form points, lines, and areas within a database. Each of these is encoded to represent one or more features. Each feature is further encoded with attributes that describe the characteristics of a specific instance of a real world feature.
Each RNC pixels “knows” what color it is and its location on the Earth, but nothing else about what it represents or its position relative to other features. Each ENC feature “knows” what it is and other information about itself, such as buoy shape or clearance height. It also knows its location on the Earth, as well as its topological relation to other features. For example, an international boundary feature will know which country is on each of its sides.
All the information available in an RNC is presented in its symbolization. Thus, some portions of raster charts are cluttered with symbols and text to present all the needed information. Users may “query” the ENC data by clicking the cursor on a feature to obtain more information about it. Information, such as light characteristics need not always be displayed in ECDIS, because it can be retrieved by a cursor query.
RNC systems do not provide automated alarms or other indications. ECDIS systems use the information in ENCs and ship information provided by the mariner, such as the ship’s draft, speed, and direction to alert mariners with visual and auditory alarms when dangerous conditions may exist.
The depiction of features in raster charts is through the arrangement of pixels to form symbols when the data is compiled. The symbology cannot be changed. NOAA paper nautical charts and RNCs are created from the same digital chart images so RNCs are also displayed using “paper chart’ symbology. ENC data is rendered in real-time by ECDIS navigational systems in which standardized IHO ENC symbology is stored. Mariners may choose between two ENC symbology sets to display ENCs, “simplified” and “paper chart.” Non-ECDIS systems that can display ENC data may use other symbology developed by the manufacturer.
Although similar, RNCs and paper nautical charts produced by different countries will have variations in colors and some of the symbols. Any ECDIS will portray all ENC data with the same colors and symbols, regardless of which country produced the ENC.
The symbols on an RNC enlarge and become “pixelated” as one zooms into the chart. The symbols on an ENC remain the same size as one zooms into the chart.
The selection of the features to display on RNCs is set when it is compiled and cannot change. The display of certain features in ENCs may be turned on or off, based on the scale of the chart display or by settings selected by the mariner.
Most RNCs are compiled in a “North-up” orientation. RNCs can be rotated to a “course-up” orientation in a chart display system, but the symbols and text will become tilted as the chart is rotated. ENCs have no inherent orientation. The symbols and text displayed in an ECDIS will always be displayed facing up correctly.
NOAA RNCs conform to the International Hydrographic Organization S-61 Product Specification for Raster Navigational Charts. NOAA ENCs conform to the International Hydrographic Organization S-57 Transfer Standard for Digital Hydrographic Data.

Revised: 10/2/2017