What is the difference between a raster chart and a vector chart?
Many CHS charts are available in digital form as either a Raster Navigational Chart (RNC) in the BSB format or as an Electronic Navigational Chart (ENC) in the S-57 vector format. Both kinds of electronic charts, when used with navigation software, relieve the navigator of many of the traditional paper chart routines, and contribute to safer navigation.
RNCs are simply an electronic image of the paper charts and as such provide no more information than that available on the paper chart. Raster charts are digitized by scanning the paper chart. Each tiny segment of each line on a chart is converted to a raster picture element or pixel. Similar to a television picture, when magnified they appear as dots on a grid. Beyond geo-referencing, there is no intelligence inherent in the raster image. Many recreational boaters have adopted raster electronic chart systems because the charts are less expensive and the navigation software required to use these charts has been tailored to meet the needs of the recreational boater.
ENCs are vector charts or "smart charts" and are coded with additional information not available in paper or raster charts. ENCs carry a wealth of geo-spatial intelligence through a database of information associated with them. On an ENC the user can click on different features, such as a light or buoy, and retrieve additional information about the feature. For example, a wharf appears only as an image on an RNC, but an ENC can identify it as a wharf and attach attributes to the wharf, such as height, length, age, ownership, number of berths – data that might otherwise be available only by consulting the relevant printed Sailing Directions.
ENCs also provide users with more control over the display of the chart, including the ability to turn different layers of information on and off. Because ENCs offer more powerful navigational flexibility and tools, they are typically used by commercial ships. When displayed on an Electronic Chart Display and Information System (ECDIS) and integrated with other data, such as GPS position, radar, planned route, heading, speed and draught, ENCs become part of a powerful system that allows mariners to know their ship’s position instantly and accurately and to be warned of dangerous situations.
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