Tracing The ICW

By Bill Parlatore
Published: October/November 2013

It's the yellow brick road for thousands of boaters along the Intracoastal Waterway, but the ICW's magenta line is sorely in need of an update.

Photo of a Grand Banks 46 Classic tied up on the ICW in North Carolina
Photo: North Carolina Tourism
A Grand Banks 46 Classic tied up on the ICW in North Carolina.

We begin on an unsettled day in May, apparently a bit too soon for summer's warmth, but late enough that most snowbirds have taken flight. It was clearing for the first time in a week, the break in relentless wind and driving rain raising our hopes that we might finally see some nice weather. It had been a wet, windy trip up from Florida, forcing us inside the ICW after offshore conditions made life lumpy, even on our supremely capable Grand Banks 46 trawler. She's a beautiful and rugged vessel, so crew comfort dictated our decision, rather than the ultimate safety of the boat. My shipmates were two retired professionals, both former Coast Guard officers, well-versed in things of the sea. No newbies.

We'd all hoped for a mostly offshore passage up the coast to Annapolis, a chance to travel in company with dolphins and sea turtles ever present at the 12-mile territorial boundary. But that plan was short-lived, and we diverted into the protected waters of the ICW at Florida's northern border. From there we'd follow the charted magenta line that showed the route along this inside waterway.

The ICW connects rivers, canals, sounds, and cuts into a continuous navigable waterway that stretches along the Atlantic and Gulf of Mexico coasts from New England to the Florida Keys and west to Texas. It offers commercial, military, and recreational mariners a mostly protected alternative to venturing offshore when conditions turn ugly. The ICW also happens to pass through some very beautiful areas of the United States.

Our trip had been mostly uneventful, though we did have to seek refuge when steady 30-plus-mph winds forced the closure of several swing bridges along the North Carolina waterway. That was a first for me, as was seeing how sheets of green water over the boat can get through a full enclosure to soak everything on the flybridge aboard a 46-foot trawler, on a protected waterway! Imagine that.

ICW travel is fairly straightforward. For the most part, a skipper just follows the marked magenta line on the chart, which is especially helpful when a boat exits a confined channel into a wide sound with no obvious direction to go next. The magenta line is also handy through a busy harbor or inlet, such as in St. Augustine, where it is easy to get confused. An early-morning departure found us motoring north on calm seas in the Alligator River, northbound to the Albemarle Sound. With any luck, we'd make Annapolis in a couple of days. We reached the Alligator River Swing Bridge midmorning, and I began to relax. The early departure meant we'd be across Albemarle Sound before the winds came up. Good plan.

Then a curious thing happened. Within moments of leaving the swing bridge behind, we were hard aground, stopped dead in our tracks. We then noticed we were in good company. A large sportfishing yacht lay a hundred yards out of the channel; it had been traveling fast when it ran out of water. This made no sense. We were more or less on the magenta line, and I'd been through this area dozens of times before, although not recently on a boat that drew four-and-a-half feet.

Our collective situational awareness had been on autopilot for days as we'd dutifully followed the magenta line. Now, after grounding, we took a closer look at the plotter and iPad and paper chart. What we saw only confused us more. The magenta line squirreled around buoys in a way that had us wondering if the rules of the road had suddenly switched directions. There was something wrong. We powered off the bottom at a snail's pace, still trying to figure out what had happened while looking for a way back into the main channel. We found our way out, thankfully, mostly by looking at the water, and were soon back on our way. We made it to Annapolis without additional drama, and, as if to either mock or celebrate our arrival, the bright sun finally came out as we passed the seawall of the U.S. Naval Academy, a reminder of the offshore passage that might have been. But that is boating, isn't it?

Our grounding experience still bothered me. Was the magenta line meant to be the yellow brick road by those who created and maintained it or not? I decided to find out more about how the magenta line comes to be, and what it means. I always assumed some all-knowing Ph.D. sat in a big comfy chair, surrounded by huge displays, spending each day updating chart information, massaging the magenta line to fit the curves and bends of current typography, using the latest technology and multi-quad core processing speed. I couldn't have been more wrong.

Photo of the author on his boat in the ICW
Despite carefully following the magenta line, the author was surprised
to run aground in North Carolina.

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Keeping Our Charts Accurate

The magenta line isn't the only line on our charts that requires updating. With 500,000 square nautical miles of navigable waters in our country, and 95,000 miles of shoreline, NOAA's Office of Coast Survey has an enormous job. These days, underwater mapping has become more complicated than ever, with rising water levels, shifting sediments, shoaling, and the increasing number of extreme storm events. All these changes significantly impact the underwater landscape, making NOAA's job more crucial than ever.

NOAA has four primary survey vessels — two on the East Coast, two on the West. Together with contract vessels, they survey approximately 3,000 square nautical miles of U.S. waters each year. In 2013, that number will be greater given the additional survey needs in the Northeast due to the altered shoreline from Superstorm Sandy. Data is collected and processed aboard the ship, around the clock, then sent to one of two processing facilities, in Norfolk, Virginia, and Seattle, Washington. In areas with rugged shorelines where survey vessels can't operate safely, aircraft determine near-shore water depths through bathymetric light-detection-and-ranging technology.

At the processing centers, staff sort through the millions of soundings to determine those most significant to cartographers. For example, if an area is 30 feet deep and then becomes 29 feet, 8 inches, does that need to be recorded on the chart? While a four-inch difference may not be that important to recreational boats, inches determine how much can be loaded onto a ship in a shipping channel and can have an enormous economic impact.

Data is then sent to the Marine Chart Division of NOAA in Silver Spring, Maryland, and transferred to paper, raster, and electronic nautical charts. In bygone days, data was hand-etched in reverse onto copper plates! Now it happens digitally. For paper charts, data is sent to the Federal Aviation Administration, which prints them. There are also print-on-demand charts, new booklet charts, and electronic navigation charts that are free for download or may be purchased through a provider ( In all, it takes about a year from the ship survey to inclusion in NOAA's database of electronic charts. For urgent changes, such as a hazard to navigation, NOAA survey vessels submit "Danger to Navigation Reports" as soon as practicable after discovery and ahead of the complete survey package for incorporation into "Notice to Mariners." Every paper chart has an edition number and date (month and year of printing) in the lower left-hand corner. To see if NOAA has updated your chart, go to Dates of Latest Editions at


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