Chuck Husick: Techno-Talk, September 2005 from BoatUS Magazine - updated January 2010
Navigation Charts - Part II
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
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.