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).
