CORPS OF ENGINEERS

At the port of Chicago, you enter waters surveyed and charted by the U.S. Army Corps of Engineers (USACE). Corps charting efforts focus on shallow draft inland waters, meaning 14 feet or less at minimum maintained depth. The Corps distributes its charts only in vector format. Inland Electronic Navigation Charts, called IENCs, are available for free on the USACE website (see link below).

"The Illinois, Mississippi, Black Warrior, Tombigbee, and Mobile Rivers are all completed, comprising 90 percent of inland river system usage," said Anthony Niles, IENCs Project Manager. "Over time, we'll complete work on an additional 2,500 miles of lower traffic areas, with a current priority of the Missouri and upper Tennessee Rivers."

From Chicago to the Gulf of Mexico, vector charts are the only electronic chart option. There are no government raster charts for this stretch. Raster charts may be purchased, but with a caveat. Since these files are not government-produced, they are created by scanning outdated paper chart books, making them expensive and of questionable accuracy.

Fortunately, the Canadian Government, NOAA and the USACE work together using what is called an edge match system. For example, NOAA produces a chart to the head of Mobile Bay. Their cell for that region, "Mobile Bay Alabama," abuts the USACE chart running from Mile 0 to Mile 88 of the Tombigbee River, "Mobile Bay to East Bassetts Creek." The two charts are relatively seamless.

So, practically speaking, the biggest hurdle for Great Loopers and other long-distance cruisers is not the availability or alignment of electronic charts, but gathering them from multiple agencies and sources. It can be frustrating, but having charts for your entire trip in one laptop, where you can experiment with routes and anchorages, will make planning much easier. You can then take that laptop onboard, interface it with your instruments and have a sophisticated navigation system that is capable of making onboard life easier.

ACCURACY ISSUES

Despite the high-tech format, electronic charts are no more precise or accurate than a chart printed on paper. Because electronic charts use the same data transcribed from ink to bytes, they are only as accurate as the paper charts from which they were created.

Ironically, GPS technology has leapfrogged electronic chart accuracy, creating a potential mismatch of information. In the past, a fix using visual bearings, radar or Loran C gave an approximation of your position. This position estimate was good enough, particularly given the survey accuracy of the charts. Now you can get a GPS fix of your vessel's position to within three to five meters but, depending on the region, that position may be plotted on digital charts based on old survey data.

This "discrepancy of accuracy" between the chart image and your GPS fix can result in some strange screen displays. Boaters who zoom in and expect to see their vessel in more detail on their screen may experience the "ship on the pier" problem: your vessel icon appears on land, but you're in plenty of water. Remember, the image on the screen is only an electronic image of a paper chart-not the real world!