How to read a nautical chart

A nautical chart is not a map of the sea. A map shows you what is there; a chart tells you what will happen to your boat. Everything on it exists to answer one of three questions: how deep is it, what will hurt me, and how do I know where I am.

Start with the title block

Before reading anything else, read the block of text near the chart title. It tells you three things that change the meaning of every number on the sheet:

• Units of depth. Metres, feet or fathoms. One fathom is six feet. Confusing them is the classic and expensive mistake.
• The chart datum. Soundings are measured from a reference level, usually a low-water datum. That means the real depth is normally more than printed — but on a strong negative tide it can be less.
• The horizontal datum (WGS84 on modern charts), which is what makes GPS positions line up with the paper.

Depths and the shape of the bottom

The scattered numbers are soundings: depth at that spot, at chart datum. The lines joining equal depths are depth contours, and they are the most useful feature on the chart because they show the shape of the bottom. Contours bunched tightly mean a steep drop; contours far apart mean a gentle shelf.

Learn to navigate by contour and you gain a real skill: with a depth sounder you can follow a contour line in poor visibility the way you would follow a road.

A sounding with a line under it, or in a different style, marks a drying height — that spot is above water at low tide.

The symbols that matter most

• Rocks. A cross, an asterisk and a dot inside a dotted circle all mean different things: rock awash, rock that covers and uncovers, or an underwater rock at an uncertain depth. The dotted circle means danger — stay outside it.
• Wrecks. Drawn differently depending on whether they are dangerous to surface navigation.
• Bottom type. S is sand, M is mud, R is rock, Wd is weed. This is the line that decides whether your anchor holds.
• Lights. A light is described in a code such as Fl(2)R 10s 15m 8M: two red flashes every 10 seconds, 15 metres above the water, visible for 8 nautical miles. At night, that description is how you identify one light among many.
• Buoys. Lateral marks tell you which side to pass, cardinal marks tell you where the safe water is relative to a danger. Note that lateral colour conventions differ between IALA region A and region B — Europe and the Americas are opposite.

The compass rose, and true versus magnetic

The rose has two rings: the outer one is true, the inner one magnetic, and the difference between them is variation, printed with the year and its annual change. Chart bearings are true; your magnetic compass is not. Failing to apply variation is one of the two classic navigation errors — the other is applying it the wrong way.

Measuring distance

Always on the latitude scale, at the side of the chart, and at the same latitude as the area you are measuring. One minute of latitude equals one nautical mile. The longitude scale along the top and bottom does not work this way, and using it produces distances that are quietly wrong.

Why this still matters with a plotter

Because the plotter draws the same symbols with the same meanings, and because electronic charts have known position errors in some areas. The skill that survives every equipment failure is being able to look at the screen and ask whether the number in front of you makes sense. That question only has an answer if you can read a chart.

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