|Electronic Charting Terminology|
Electronic charts free you from the tedious process of plotting positions and courses on paper charts. With the position from the GPS or Loran automatically updated on the screen, you can quickly see where you are, where you've been, where you're going and what lies ahead.
The differences between the ways computerized navigation programs use electronic charts are minor. However, the charts they use are not similar. Electronic charts are available in vector or raster format. Not all charting programs or plotters are able to use both formats. Basically, vector formats are layered graphic representations of the data presented on the paper chart, while raster formats are exact photographic images of the paper charts.
Vector charts can have up to 100 layers. You can have outlines of land masses, elevation contours, depth contours, spot depths, aids to navigation and written chart information, such as landmarks and place names, all as separate layers. But these examples are simplifications; vector charts actually have different layers for each depth and elevation contour and different types of aids to navigation. They have excellent color separation and often appear identical to a paper chart. Because vector charts are layered, you can choose to have as much or as little information as you want to have displayed. As you zoom in, increasing layers of detail become available, all without any sacrifice to image resolution or datum change. Vector charts are produced from the most up-to-date navigation information available. They may even contain additional information not found on paper charts.
The major hydrographic offices of the world, including NIMA in the U.S., have announced that their electronic charts will be in the vector format. Recently, the IHO selected the vector format as the standard for ECDIS equipped vessels and also approved the raster format for ECS use. (See sidebar on terms.)
Vector charts used in most GPS chart plotters are made by Navionics and C-Map. However, there are several newer models of GPS chart plotters that use raster charts on CD-ROM. Navionics also makes a floppy disk version of their plotter cartridges for use on PCs. Using the new IHO ECDIS format standards, Transas makes very sophisticated vector charts on CD-ROMs. Maptech recently signed a CRADA with NOAA to manufacture and distribute the new ENC vector charts for the 80 deep-water ports in the U.S. You can expect to see them in the market place before the end of the year.
The main advantages of vector charts are: they take up less storage space in your computer, they are easily updated, the various layers can be viewed separately and they are virtually seamless. The main criticism is that except for the more sophisticated (and more expensive than raster charts) vector charts, they don't look like paper charts.
Raster charts in various proprietary raster formats are currently available from four main vendors. Format differences are usually of little concern to charting programs, which are able to adjust to different formats. Because they are scanned off the paper charts or the mylar masters, raster electronic charts look identical to paper charts. Most sailors used to paper charts feel more comfortable with a raster format.
The main advantage of raster charts is that they are identical to the paper charts and thus include all the same information and markings. Because they are easier to produce, they are less expensive. Raster charts cover wider chart areas. For example, bathymetric, inland waterways and 3-D charts are only available in raster format.
The main disadvantages of raster charts are they take up much more space on your computer and are not as seamless as vector charts. Because raster charts are scanned individually, when you zoom in and out or reach the edge of one chart, you will automatically jump to the next chart. The new chart may have a different scale, or depth and datum format. If you are unaware of the change, it can be disorienting. Nobeltech has come out with a "quilting" procedure that joins the charts in a virtually seamless manner.
But as computer and plotter manufacturers use more powerful processors and more sophisticated software are developed, remaining disadvantages of vector and raster formats continue to fade away. With today's larger hard drives, increased RAM and faster CD-ROM drives, concern for the amount of space required by raster chart has dwindled, while navigation software programs becomes more and more seamless.
One bonus is there is more outside data available in the programs and chart formats. Now, for example, you can click on a marina shown on the chart and find out the services and facilities available-without even looking it up in the cruising guide. With a single click of the mouse, the tidal range and current vectors are shown on the chart. Formerly, a NOAA chart was incomplete without the companion coastal piloting book. Now this information comes right with the chart CD-ROM.
Which electronic chart format is best for you? It depends on several things. The size and type of your boat, GPS chart plotter, the navigation software you use and your personal preference all come into play. The formats are starting to blend as more and more navigation programs are able to use both. As more ECDIS quality vector charts come into use, I expect they will become more affordable for sailors.
Smaller boats may need to use the vector chart cartridges with a GPS chart plotter, while larger boats can choose between using a raster format GPS chart plotter or an on-board computer with a navigation and charting program. If your computer or monitor gets exposed to sea water, you may want to choose a GPS chart plotter or a marinized computer.
Today the type of charts you use may depend on the charting program you select, your budget and cruising area. But tomorrow it may depend solely on your personal preference.
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