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Chuck Husick: Techno-Talk, September 2005 from BoatUS Magazine - updated January 2010

Electronic Navigation Charts - Part II

Years ago numerous recreational boaters on the East Coast navigated using Texaco Oil Company Marine Maps. Although by no stretch of the imagination a nautical chart, these maps were free and even in the absence of Loran C or GPS many voyages were safely completed. These mariners may have been particularly skilled and sensitive to the marine environment or just plain lucky. Regardless of their luck or ability, taking a voyage without accurate charts is, at the least, inadvisable, if not foolhardy.

The need to carry the appropriate charts has not been diminished by the availability of GPS. Without an accurate chart, the precise latitude and longitude, often to three decimals provided by the GPS, is just a set of numbers. We need accurate charts and normally have to pay for them. However all United States raster navigation charts (RNCs) and S-57 format charts — Electronic Navigation Charts (ENCs) — can be downloaded at no cost from a U.S. government Web site ( and viewed on chart plotters equipped with the appropriate software.

Before we get too excited at the prospect of free cartography, let’s take a look at the paper charts we have been using. Even though they may not be free, we may choose to continue to use them. Many of us still rely upon “paper” charts, although these days the paper may be waterproof plastic. You can buy charts from the government or from any number of commercial sources who precisely reproduce the “official” government printed charts, waterproof versions singly or in chart books that include much useful ancillary information. The software used in the plotter will dictate the type of cartography we need since most plotters work with only one brand.

Perhaps confusingly, since we paid a handsome price for the plotter and the software, the initial screen we see when we turn on the plotter advises us that what we are about to use for navigation is not an “official” and approved source of navigation information. There are official and fully approved electronic charts, however until recently these were available only for use on the plotters of ECDIS (electronic chart display and information systems) used on large vessels.

The new ENCs mentioned above are an exception. These charts are in vector format, consisting of an encoded file that identifies every line and point on the chart. The charts can be viewed using software specifically designed to decode the files, such as the Fugawi Chart Plotter and MacENC software. ENCs are “official” charts, however they don’t look like the charts we are familiar with. For example, the details of buildings, roads, railroads and elevation contour lines shown on land adjacent to water areas are missing, replaced with a color tint and perhaps the words “inhabited area”. Information about bridges, type of structure, horizontal and vertical clearance is not shown on the chart. You will have to “click” on the chart (in precisely the right place) to open a dialog box that may contain the word “bridge,” which when queried will disclose the clearances, most likely in meters (since the database that makes up the chart is metric, however some software will convert metric to English). The MacENC chartplotter software provides a very useful option, a raster chart (RNC) can be overlaid on the corresponding vector (RNC) chart. The resulting screen image displays the familiar RNC or "paper" chart, however clicking on an object such as an aid to navigation will bring forth the complete ENC data base. From a navigation information standpoint it's the best of both worlds.

The advent of the free for the downloading ENCs and reasonably priced software with which to view and use the charts might be expected to spell the end of the familiar paper charts and the digital versions of those charts, the RNCs (raster navigation charts). However, it is likely that the familiarity and convenience of the “paper” product will keep it in use for a long time. Current producers of digital charts will continue to innovate, providing “value added” additions such as instant access to tide data, bathymetric data, satelite and photo images to their versions of the official charts. The larger screens used in some high-end systems allow the simultaneous display of information from other, non navigation sources including propulsion systems.

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