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| What do Chart Numbers Mean?
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April 1, 2003
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I recently bought your cruising guide. When it describes certain areas it says, "see chart number such & such." What do these numbers refer to? Are they from your waypoint guide? or a certain book of nautical charts?
--Joe
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Response
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Hi, Joe,
You've asked a very good question. Once again I am reminded that nobody is born knowing these things, and that I shouldn't assume our readers have been hanging around the waterfront all their lives. Thank you for having the courage to ask for an explanation.
"Chart" means nautical chart. Nautical charts are specialized maps of the waters, and are essential for safe navigation. Nearly all charts are published by government agencies, which keep them up to date with changes and sell them through authorized chart dealers. U.S. charts come from the NOS (National Ocean Service) branch of NOAA, the National Oceanographic and Atmospheric Administration. Canadian charts come from CHS, the Canadian Hydrographic Service.
Each chart is identified by a number and a title. U.S. charts of Puget Sound and the San Juan Islands are 184xx series, such as 18421, Strait of Juan de Fuca to Strait of Georgia. British Columbia charts are identified by 34xx, 35xx, 36xx, 37xx, 38xx or 39xx series, depending on location. The 34xx series covers the southern Strait of Georgia, including the Gulf Islands. The 35xx series covers the northern Strait of Georgia, Desolation Sound, and the waters to the north tip of Vancouver Island. The 37xx and 39xx series cover the central and northern B.C. coast, the 36xx series covers the West Coast of Vancouver Island, and the 38xx series covers the Queen Charlotte Islands. On the central and northern B.C. coast, the 37xx series are the older charts; the 39xx series are the new generation of charts for the same waters.
Charts show water depths, called soundings, with contour lines drawn at intervals connecting certain soundings. Thus, if you studied your charts and saw that on a particular section of coast all the rocks, reefs and other hazards were inshore of the 20-fathom line, you could run down that coast on the 20-fathom line and be safe. Charts also show the location and characteristics of buoys and beacons, the location and shape of rocks and reefs, the location and depths of shoals, the makeup of the bottom (rock, sand, mud, etc.), the location of cable and pipeline crossings, the clearances under bridges, the location and characteristics of other navigation aids, and many, many other things. The more you learn about charts the more amazing they become.
Nautical charts are overlaid with horizontal lines, which show latitude, and vertical lines, which show longitude. Using dividers, you can measure the latitude and longitude of any place on the chart. The chart also has one or more compass roses, each made up of two rings of 360 degrees. The outer ring is true north. The inner ring is offset, and shows magnetic north. In the Seattle area magnetic north is about 19 degrees east of true north. You can draw a course on a chart, then run your parallel rules or plotting rule from the course to a nearby compass rose and determine the heading of that course, either magnetic or true.
Charts come in different scales, from small scale to large scale. A small scale chart shows a large area, such as all of Puget Sound. Of necessity many details are left out, but for an overview the small scale chart is extremely useful. A large scale chart shows only a small area, such as a harbor. But boy, is it detailed! It will show the location and even the shape of individual docks. When the day comes that you're making your way through a narrow, tricky passage, with rocks on one side and shoal depths on the other, you will want the largest scale chart available.
The wise and prudent skipper carries a complete set of charts -- small scale, medium scale and large scale -- for whatever waters he is on. This can seem like overkill, but from personal experience I can tell you there is no substitute for having all the charts you can get.
Navigating on water is not like driving a car down the road. Although I had not been in the Northeast before, with no road map I once drove a rental car from Hartford, Connecticut to Boston, Massachusetts, for a meeting at the Sail Magazine office at 38 Commercial Wharf. I got there without a hitch, parked the car, and walked into the Sail Magazine office five minutes before the appointed hour. With no map! Crazy? Sure. But I did it. I had road signs all along the way, and the freeway was free of hazards.
Coastal waters are different from the road from Hartford to Boston. Coastal waters have no road signs, and are dotted with rocks, reefs and shoals, areas of strong tidal currents, dangerous tide-rips, restricted navigation areas, and all kinds of other little surprises, none of which may be obvious. You need charts to navigate safely.
Privately produced chart books are available, which combine many charts into one handy package. Even though the chart books cost much less than buying individual chart sheets, I'm not a fan of them. The charts in the books tend to be out of date, sometimes decades out of date. It's hard to plot a course from one page to another, and much information from the original chart sheet can be deleted. The more I'm on the water, the more I realize that I need all the navigation help I can get. Individual, up-to-date chart sheets are what I want.
I use individual paper charts even though we absolutely love our Nobeltec electronic navigation program. The problem is, you can't squeeze a 36"x48" chart sheet onto a 15" laptop computer screen and have it mean anything. We use both the paper chart and the laptop when we're underway. The laptop is tied into our GPS, and shows exactly where we are. The paper chart provides a larger perspective and shows details that are lost on the screen.
Enough. I've gone on much too long here, and there's so much more that could be covered. I realize that I went far beyond your question, but I suspected that if you didn't know what chart numbers were, perhaps you were not familiar with other things about charts. Forgive me if I assumed incorrectly. Feel free to write with further questions.
--Bob Hale
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