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Maps versus Charts

By Tom Watson

Have you noticed that land features represented on a flat plane are called topographic maps while features represented on the ocean are called nautical charts? While the two may seem to be interchangeable, you'll never be accepted as the salty dog you think you are if you ever refer to a nautical chart as a map. While maps are generally thought of as graphical representations of an area of land, a "chart" typically means that specific information about the ocean is provided in a specific format. Ironically, you could have a 'map' of the ocean floor. Go figure.

As far as the differences go for kayakers navigating along the oceanic coastline, knowing the difference in what information is represented and how each is symbolized is much more important than what terminology you use - salty dog status notwithstanding.

Pull out a topo' map and lay it along side a nautical chart so you can easily see these main differences. It really helps if you have a map and a chart of the same area, at the same scale. Your topo' map may have just contour lines (lines of equal elevation that form irregular loops on your map) or even a surface printed in graphic relief (the mountains appear almost three-dimensional). The nautical chart shows relatively few distinctive land features as contours typically disappear a very few units of distance inland from shore. The water surface, usually blank on topo' maps is otherwise covered with various shades of blue denoting differences in depth and a cluster of numbers throughout the water surface indicating depth in fathoms (one fathom = six feet, remember?) at mean tide.

For those navigating along a coastal route it may seem irrelevant which map is used since both generally have the same degree of detail at the same relative scale. A topo' shoreline throughout a bay or around a point is expressed almost exactly the same way on the nautical chart. However, the chart may include submerged rocks or troublesome kelp beds that could hinder your progress. A topo' may be more informative if you are trying to determine exactly which bay it is you are traversing. Since the topography beyond the shoreline is usually not detailed on a chart, you may have a hard time determining exactly what area of an indented coastline you've come upon. The chart won't show a prominent mountain peak or ridgeline a mile or three inland. That's where the topo' is handy, it will help you determine which peak or ridge is which - presuming you know how to read your map.

One instance where a topo' would help you make a course determination while on the water would be where there is a chance you might encounter a williwaw wind. These are winds caused by cold air that settles into a bowl high in the mountains. Since its cold air, it's denser and heavier and sinks to the bottom of the basin. If that air keeps on rising without disturbance, pretty soon it flows up and over whatever was restraining it - the rim of a bowl or a confining ridge across an upper valley - and this mass of cold, heavy air comes flowing down the side of the mountain and out across the water as a straight line, boat upsetting, gust of wind. Reading the topo' map to reveal such likely williwaw makers may cause you to either alter your course or pick another time of day. It should certainly cause you to be continually observant throughout your crossing.

Very seldom will a topographic map show shoals or underwater rock outcroppings, just two of many underwater structures that, depending upon the tide depth, can create serious currents and other challenges for unwary or unskilled boaters.

A sad case in point: several years ago in Alaska's Prince William Sound -one of the most idyllic spots in North America for kayaking - a group was preparing to return to town after a few days of kayaking in the Blackstone Glacier area, southeast of Whittier. I believe there were four in the party, two singles and a double. I believe all the kayakers were of average skill levels. Upon awaking on the morning of their departure, winds were unusually strong for that early in the day. Upon checking their topo' map (they had no nautical chart), they could see that it would be much shorter to scoot in between an island and the mainland.

The group reached this narrow constriction in the channel and soon realized that lying across the floor of this channel was a ridge of submerged rocks - rocks that created a fierce wall of standing waves as the strong winds pushed the water up against the constriction caused by the rocks - a shoal that did not appear on their topo' map. Sadly, as I remember it, at least two of the foursome suffered a fatal capsize trying to negotiate this section. A nautical chart would have revealed both the shoal and the mean depth of that water giving a prudent navigator fair warning.

The obvious lesson here is to learn to utilize both topo' maps and nautical charts if you are going to be navigating on the ocean. Inland waters rarely are represented in such detail unless you are on one of the Great Lakes or have one of those "fishing" maps of a lake - in which case they can be most helpful when seeking out submerged hazards (not to mention great fishing spots).

One last area of specialized charts might be those used to navigate larger river systems, especially those with heavy commercial traffic (the Mississippi River is a classic example). Wing dams, stump fields and other submerged hazards are often identified on those charts - and for good reason. As dams are manipulated to regulate flowage, water levels can really fluctuate - like the tide on the ocean. Knowing how to read that changing river via these charts could be a real lifesaver.

Carry the appropriate maps and charts with you and know how to read them. There are several good chart cases on the market that easily fasten to your foredeck. The old saying, "the right tool for the right job" can certainly be applied to maps and charts as well.


Tom Watson is an avid sea kayaker and freelance writer. He also posts articles and thoughts on his website www.wavetameradventures.com. He has written 2 books, "Kids Gone Paddlin" and "How to Think Like A Survivor" that are available on Amazon.com.

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