Spend a reasonable amount of time boating in one particular area and you begin to know the waters intimately, from the unmarked sandbar outside the channel to the height of nearby bridges. It's called "local knowledge."

What if you are not a local? You can gain a lot of instant local knowledge from a proper nautical chart – if you know how to read it.

While most boaters have had some experience reading charts, ask yourself if you've really taken the time to study everything these charts provide. A chart is a trove of information about the water, including depths, bottom characteristics, landmarks, navigation markers, shipping lanes and hazardous areas. Many of us could stand to learn more about how to use them.

The best navigational charts for American waters are the ones put out by the National Oceanic and Atmospheric Administration, often called NOAA, so we will use these charts for our discussion. Most chart books and electronic charts are based on NOAA's library, so the skills are transferable.

GENE BJERKEOn the chart and in the water.A quick look at any of these charts will show a dense collection of numbers and symbols printed in a variety of colors, all of which have meaning if you know how to read them. There are over 900 symbols and abbreviations in use, and the key to them is known as Nautical Chart #1. This publication can be downloaded from the NOAA web site (see link below), or purchased online for about $10. The same information can also be found in some books, such as Chapman Piloting and Seamanship, and Nigel Calder's How to Read a Nautical Chart. It's not necessary to know all 900 symbols and abbreviations, just the most common. For the rest, you can keep a reference onboard.

FIVE COLORS

The most obvious thing about a chart is that it is printed in several colors, most of which are fairly intuitive. Water is either white or blue. The blue areas show shallow water, with dark blue being shallower than the light blue. The demarcation between deep and shallow follows a contour line. The contour lines depend on the scale of the chart. Small-scale charts, meaning those showing smaller areas, often use shallower contours, say six feet. Large-scale charts may consider anything below 18 feet, or even 30 feet to be shallow. There are usually additional contour lines, which will give you a sense of the shape of the bottom. Contour lines close together show a sharp drop-off. Those more spread out indicate a gradual change in depth. Land areas are also fairly simple: they are drawn in tan or green. The tan is dry ground and the green indicates marshes, which are areas of vegetation that may be flooded – though not necessarily navigable – at high tide.

Also fairly intuitive are the numbers printed on the water portion of the chart. These refer to the soundings, or depth, of the water at that point. Depending on the source and scale of the chart, the numbers may refer to feet, fathoms, or meters. This will be indicated in the margin of the chart. Of course, in tidal waters, those depths will vary from time to time. The depths indicated are at the "datum level," which is printed just beneath the chart title in the lower margin. Chart datum is usually Mean Low Water (MLW) or Mean Lower Low Water (MLLW). That means there will usually be more water than shown and only rarely less, depending on the tides. Charts always show the most conservative situation. That means that vertical clearances, such as under bridges or utility wires, are measured from Mean High Water (or Mean Higher High Water).

Scattered among the soundings will be abbreviations that tell you the character of the bottom, such as S for sand M for mud. In earlier days, when depths were measured with a lead-line, the lead brought up a sample of the bottom, which helped navigators figure out where they were. Most depth sounders today do not show the character of the bottom, but knowing the bottom characteristics is useful when anchoring.

BOUYS, DAYMARKS AND LIGHTS

Less intuitive are the navigation symbols on a chart, the most common of which are buoys, daymarks, and lights. An unlighted buoy is shown by a dot (indicating the location of its anchor) and a diamond shape that is the same color as the buoy. Lighted buoys and other lighted aids have either a magenta circle around the dot or something that looks like a magenta exclamation point. Daymarks, which are shaped boards on a piling, are shown as green squares or red triangles, depending on the shape and color of the board. Each mark has some abbreviations next to it, which will tell you color, number (in quotes), and any other identifying characteristics (see photo). Some common abbreviations are listed in the accompanying table. If a light has something that looks like flower petals around it, that means the light is protected by rip-rap, a pile of large rocks that protects the light from wave erosion. Since rocks can sometimes fall into the surrounding water, you need to keep well clear.

GENE BJERKEThe flower-looking symbol indicates rip-rap, rocks that protect against wave erosion. Keep clear.

You will also see some symbols on the land portion of the chart indicating landmarks, objects you can see from the water that help you know where you are. They include such things as chimney stacks, church steeples (called spires), water towers, tall buildings or anything else that can be readily seen from the water. The position is indicated by a dot in a circle, with a description written alongside. Some charts also include contour lines on the land, when that might be helpful.

In addition to things you can see above the water, charts also show the location of dangers that lurk below the water. Rocks, reefs, wrecks (immersed or awash), snags and other hazards are plotted – often with special symbols – and labeled as to what they are (see table).

NAVIGATION AND OTHER USES

Of course, charts also have other uses. They allow you to make notes, such as indicating an unmarked obstacle, recording navigation information or anything else that may be handy. If you are planning a trip, the chart is your primary tool. It gives you an overview of the area, which you can use to learn distance, time to destination and to plot your courses ahead of time by drawing lines on the chart (preferably with a #3 pencil, which erases cleanly) .

You can also make some measurements using chart tools. Parallel rules, for example, are a pair of straight-edges that can be pushed together or spread apart but are always parallel to each other. You can use them to transfer your course to the nearest compass rose on the chart, which allows you to determine a course (or bearing) either relative to true north or magnetic north. Some people prefer a course plotter, which is a wide straight-edge with a protractor attached that has lines you can align with either latitude or longitude lines to give you a true course (or bearing). Then, using your dividers, which are similar to a grade-school compass, you can measure distances on the chart using either the mileage scale in the margin or the latitude scale on the sides (one minute of latitude equals one nautical mile.) Similarly, you can determine the latitude and longitude of any point on the chart for entry into a GPS.

Charts are also helpful in determining where you are once you are out on the water. If you know approximately where you are, getting the number off a nearby buoy will give you an exact position. If you can take bearings, either exact or approximate, from a landmark, a light, or other charted object, you can plot your position, either exactly or approximately. Crossing two bearings gives you your position. Comparing your depth sounder information with the chart can also tell you where you are, especially if you are crossing an area of rapid change.

CHARTS FOR ALL BUDGETS

Any story on charts should contain a few words in defense of the good old-fashioned paper variety. Electronic charting tools are marvelous additions to a boat, and there are a number of chart plotters and PC-based navigation software packages available. These can make a trip safer, easier – even cheaper. Official government charts can be downloaded for free in digital format, called "Raster" nautical charts, from the NOAA site. There are also many commercial vendors who sell chart information on CD or other media. These electronic charts often cover a large geographic area, such as the entire Chesapeake Bay and the dozens of rivers that feed it. They can be cheaper than buying the many paper charts it would require to cover that area with even a moderate level of small-scale detail.

However, electronic tools should not replace traditional paper charts. Electronics can go down and computers can glitch. When they do, it's nice to know that you have a set of paper backups for your area onboard, stored in a waterproof tube and ready to use as a fail-safe fall-back. Paper charts can also be more efficient. It is sometimes easier to glance at a chart in your hand than it is to page around a plotter or a PC with a cursor. And when it comes to planning a trip, nothing beats rolling that big old chart out on the dining room table and getting a good overview of the situation.

Official NOAA charts – the gold standard – cost about $20 each, a bargain considering the amount of information they contain. They are available at marine supply houses, marinas, travel stores and you can also order them online (see links below). However you obtain them, be sure the charts you are buying are current. NOAA updates charts periodically, and you want the latest information available (it is also a good idea to replace your paper charts every few years for this same reason). For the latest version of any NOAA chart, various vendors such as West Marine can dowload Print-On-Demand (POD) charts and print them out on waterproof paper. These will have all the latest corrections.

SMALLER IS SOMETIMES BETTER

NOAA charts do have some drawbacks. While an individual chart is inexpensive, if you plan on cruising a large area, the cost of all the charts required can add up to serious money. Also, a typical NOAA chart measures about 3 by 4 feet, too big to have handy in the cockpit of the average recreational craft unless you fold it.

GENE BJERKENote the weath of information: depth, channels, landmarks and more.

There are alternatives. The government puts out a number of Recreational Charts, which are spiral-bound books, and Small Craft Charts, which are smaller sheets printed front and back, folded, and contained in a cardboard cover. These are aimed at the recreational boater, and contain most of the information on larger charts, plus additional elements of interest, such as more harbor detail, locations of marine facilities and ramps, courses and distances, basic tide and current data and information on rules of the road. These are obtainable wherever you get regular charts, either online or at a marine store.

There are also commercial chart books available for most boating areas from vendors such as ADC, MapTech and, for the Intercoastal Waterway, Dozier. Some focus on a particular area, and those chart books are available in that area. Chart books contain copies of government charts, arranged in a spiral-bound book format and priced well below what the total cost of government charts for that area would be. For example, the ADC Chesapeake Bay chart book replaces dozens of government charts and costs about $60. These books and "chartlets" also include additional information, such as the common courses and distances, coordinates of principal marks, the location of fuel docks and the phone numbers for local marinas and restaurants. Some of these books are even water resistant, or come packaged together with electronic charts

A good navigator may choose to combine several of these options, using a chart plotter and a chart book for piloting and keeping a roll of paper charts in the cabin just in case. You need to know a lot about the waters you boat in, and there is no quicker or easier way to learn than with a traditional navigational chart. And with all the choices available, there is no excuse for not having one on your boat.