Navigation: Read a Marine Chart Part 1

Learning how to read a marine chart is an important part of learning to navigate. A chart, like a map, represents the real world projected onto paper. It helps you figure out where you’ve been, where you’re going, where you’re at and what to expect at each point along the way. There are lots of symbols on a chart, but, for novice kayakers and canoeists, knowing the main features is most important. After learning the basics, the rest come easily with some study. In this two-part article, part one covers the basics like finding the chart’s name, number, scale, variation and other important items to discover at first glance. Part two examines specific symbols and aids to navigation shown on the chart. These two articles will help you learn chart reading.

Marine chart vs. topographic map vs. other maps

For canoeing and kayaking, both marine charts and topographic maps have uses. For the time spent on the water, a chart details all the features you might need to navigate, but it lacks many of the features you might need when determining where to land and camp. For those features, a topographic map excels. For example, while a chart may show a river mouth, the topo shows the mouth and it might show the public campground located there. In addition to topographic maps, other regional maps may have information that you need to help plan a trip. For example, a park map showing all the landings and campsites. When planning a trip, transfer any info you think you might need from a topo or regional map to your chart.

Chart name, number, edition and year

Each chart has a name and a unique number. The name describes the area the chart covers, and the number designates the specific chart. When ordering or downloading a chart, you specify the chart’s number. NOAA offers an online viewer to help you decide if a chart meets your needs. You can also find a chart’s coverage and number by using the online chart catalog. For example, here’s the Great Lakes’ chart catalog. Each chart also has an edition and year designation. Use the edition and year to get the latest version. An example chart for the Munising, MI area reads Munising Harbor and Approaches, 14969, 22nd Edition, April 2005. The name is “Munising.” The number is “14969.” The edition and year is “22nd Edition, April 2005.”

Latitude and longitude grid lines

Latitude and longitude grid lines overlay each chart. You use these to name your position and to reference the cardinal directions. Longitude lines run north and south and latitude lines run east and west. Latitude and longitude scales frame the map. To read a position, first find the latitude of your position following an east/west line to the latitude scale on the side of the map. Do this for longitude by following the north/south lines. Latitude and longitude is divided into degrees, minutes and seconds. Each degree has 60 minutes, and each minute has 60 seconds. You give a position by saying the degree of latitude first, then the minutes and seconds. Then follow this with the longitude. For example, the Grand Portage buoy on Chart 14968 is at 47° 50′ 42″ N 49° 30′ 48″ W. Instead of giving the minutes and seconds separate, it’s acceptable to give the seconds as a decimal point. The buoy would then be read 47° 50.7′ N 49° 30.8′ W.

Chart scale – distance scale – latitude measurements

The scale of a marine chart refers to how much distance it covers. If it covers a large distance, it’s consider a small-scale chart. If it covers a small distance, it’s consider a large-scale chart. The larger the scale, the more detail the chart shows in a smaller area. Charts show scale using a ratio of one to a consequent, where one inch on the map equals the number of inches stated in the consequent. For example, read 1:120,000 to mean 1 inch on the chart is 120,000 inches in real life. When looking for charts of Lake Superior, the smallest-scale chart is 1:600,000. The scale for most coastal navigation is 1:120,000. The largest scale is for harbor areas at 1:15,000.

To measure distance, use the linear distance scale. It’s a graphical representation of distance shown in yards, meters, miles and nautical miles. For a paddler, using a length of string drawn from your position to your destination to measure the length on the map and then aligning the string with the linear scale is a quick way to determine the distance between you and your destination. You can also use the latitude scale to measure distance. One minute of latitude equals 1 nautical mile.

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Note: Because the distance between longitude lines decreases as they get closer to the poles, you can’t measure distance using them.

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Compass rose and variation

The chart’s compass rose shows all 360 degrees of a compass in two rings. You use the compass rose to help determine course bearings and translate bearings to the chart. If you like to think in directions instead of degrees, you can mark the directions on the chart. North is at 0 degrees, south is at 180 degrees, east is at 90 and west is at 270. The rose’s outer ring shows the true directions and the inner ring shows magnetic directions. Typically, the rose’s outer north/south and east/west lines align with the chart’s grid lines. There are multiple roses across the map for convenience and accuracy.

Because the magnetic north pole is offset from the true north pole, the inner ring and outer ring may not align. If they don’t align, they have “variation.” Variation is shown in a number of degrees either west of north or east of north. Each year, the variation increases. The increase and the year last measured is shown inside the inner compass ring. To account for the increase, you add the amount of increase for each year that has passed. For example, if the map shows 15 degrees west variation in 2007 with an annual increase of 33 minutes, then in 2010 — three years later — the variation is 15°99′ or 16 degrees.

When taking or shooting bearings, you have to account for the variation. For westerly variation you add the degree of variation to true course to arrive at the magnetic course. You subtract for east variation. For example, if your true course on the map shows 150 degrees and the variation is 10 degrees east, then your magnetic course becomes 140 degrees (150-10=140). When marking a bearing you took in the real world onto your chart, you reverse the math and subtract for west and add for east. For example, if you took a magnetic (from your compass) bearing of 250 degrees and the variation is 10 degrees east, you end up with a true bearing of 260 degrees (250+10=260).

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Because paddlers don’t have the luxury of space to use conveniences, like dividers, we have to account for variation using addition and subtraction. The easily way to remember which to do is by remembering a rhyme.

When going from map to real;
compass west is best and compass east is least.
When going from real to map;
stay found by turning the rhyme around.

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More Reading

The most comprehensive book on canoe and kayak navigation is the Fundamentals of Kayak Navigation, 4th: Master the Traditional Skills and the Latest Technologies (How to Paddle Series). Every paddler should own a copy.