Nautical Charts

My first boat was a 21’ bowrider which had no electronics, only a compass. Before I bought it, I read everything I could about charts and how to use them. I was to pick up the boat in Stamford and bring it home to Norwalk. I was familiar with the waters (sort of) from having grown up in the Shippan section of Stamford and working around the shoreline homes of Darien and Rowayton. One of the most interesting aspects of boating is learning how to navigate, that is, to find your way to the destination you are seeking and to always know where you are at any given moment while on the water. Essential to navigating are nautical charts, which are the marine equivalent of the maps used to find your way on land. Nautical charts, however, are far more complex and multi-dimensional than land maps. Each nautical chart is packed with information that will seem like Greek to a newcomer to boating navigation, but yields a wealth of information to knowledgeable mariners who can interpret all the markings. To add to the confusion, today there are more choices than ever with the advent electronic charts in all sorts of formats, from CD-ROMs to compact memory cards designed to operate in chart reading GPS units. To understand the basics of reading charts, however, we will stick to the old fashioned paper charts.

Most paper charts available for navigating in U.S. waters are produced by the National Oceanic and Atmospheric Association, known as NOAA. These official government charts are used by everyone from the U.S. Coast Guard to the most casual recreational boaters, and are widely available in marine supply stores, bait and tackle shops, or online directly from NOAA. NOAA has compiled hundreds of charts for all the waters bordering the United States and many areas beyond, and each chart is designated with a number and a descriptive name. To find the chart for the waters you wish to explore you can search through the NOAA catalog of charts and decide which one is most appropriate for your use. Charts are available in many sizes, or scales, with large-scale charts offering the greatest detail of small areas, such as harbor entrances, and small-scale charts offering much less detail, but providing the big picture of a large area, such as the entire Long Island Sound, for example.

If you are new to reading charts, a good first resource to seek out is the booklet published by NOAA that is known as Chart Number One. Chart Number One contains all the symbols, notations and abbreviations found on all NOAA charts, and is your key to learning to interpret what everything from colors to graphic and numeric symbols on the chart mean. Keep Chart Number One handy for reference, because even after you become well-versed in reading charts, you are sure to find some new symbol you haven’t come across before.

Unlike most land maps, the first thing you might notice about a nautical chart are the incremented scales printed on the borders, both across the top and bottom of the chart and along the left and right margins. These are latitude and longitude scales, with longitude at the top and bottom showing the position of everything on the chart in degrees west (or east) of Greenwich, England, which is the default reference point for zero degrees of longitude; and latitude scales on each side showing degrees north (or south) of the Equator, with is the default reference point for zero degrees of latitude. All the locations on the chart can be plotted in terms of latitude and longitude by intersecting lines drawn from these latitude and longitude scales on the chart margins. Distance in nautical miles between two points can also conveniently be taken from the vertical latitude scales, since one minute of latitude equals one nautical mile, and 60 minutes equals 60 nautical miles.

In addition to these latitude and longitude scales, another unique feature of nautical charts is the prominent compass rose printed on each chart. A compass rose shows direction on the chart in terms of north and south, but is much more precise than on land-based charts. The compass rose shows both true north and magnetic north, which is the direction compasses point. Magnetic north changes with longitude, so that there will be a slight difference measurable in degrees as you travel farther west or east of zero degrees longitude at Greenwich. For precise navigation, it is essential that you know the amount of this variation in magnetic north, which is known as declination. The compass rose on each chart provides the amount of declination in degrees for the area covered by the chart.

Perhaps the most important information given on charts for mariners is the depth of the water in the charted area. This information allows captains of all sizes of vessels to know where they can and cannot safely go, and how to avoid shoals, reefs and man-made obstructions such as wrecks. In addition, the chart shows the bottom composition, so that you can determine whether there is sand, mud, rock or grass under your hull and can choose your anchor accordingly.

This is just a glimpse of what you can find on a nautical chart. The best way to learn how to read them is to collect several and spend some time pouring over them with Chart Number One as your guide. You may find charts more fascinating than books as they have the power to generate dreams of adventurous cruises to faraway places.