Beginning in the 13th century, sailors used simple navigational charts to plot their journeys across the sea and follow known routes between trading ports. As knowledge of the oceans grew and technology advanced, nautical charts and marine chart plotting techniques became more complex and more accurate. Today, nautical charts still play a critical role in marine navigation. Navy vessels always carry these valuable tools on board, and skilled modern navigators can sail on unfamiliar seas simply by reading nautical charts.
With a little practice, you can join the ranks of expert navigators and learn how to plot your course across any sea. In this piece, we will get you started with the skills you need for marine chart plotting and reading nautical charts.
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Many boaters rely on GPS to track their location and determine their bearing. However, if the electronic navigation on your boat fails, you’ll be stuck on the water with no idea where to go. Even worse, if visibility is obscured by a storm or nightfall, you risk collision with rocks or the shore. By learning to navigate without GPS, you will feel more comfortable on the water and will be prepared in an emergency.
Nautical charts provide all the information you need to navigate your boat without GPS. These detailed maps of the water can steer you clear of underwater hazards and guide you to your destination. While all boaters should carry nautical charts and a compass on board, it is essential to know how to use these tools when necessary. By learning to read navigational charts, you can always steer your way home safely.
Nautical charts are maps that depict water areas rather than land areas. These essential navigational tools depict the physical features of oceans, seas, rivers, lakes and coastlines. Nautical charts also describe underwater features, list water depths, show navigational aids and provide many other details that assist in navigating the ocean safely.
Mariners rely on nautical charts to plot the most efficient route between harbors and navigate busy waterways without collisions. Nautical charts guide seafarers through treacherous passages or to remote islands in the open ocean. Because nautical charts play a critical role in safe navigation at sea, they hold more authority than traditional maps. While maps show predetermined roads and routes, navigational charts allow mariners to chart their own course across any waterway, sea or ocean.
Nautical charts contain a lot of information, which may seem intimidating to new boaters. However, once you become familiar with the basic features of nautical charts, you can read navigational charts like a true mariner. Here are some important nautical chart terms and symbols to know:
Nautical charts come in different scales, depending on the area of water they cover. Charts depicting large areas of water have a smaller scale and show less detail, while charts depicting small water areas have a larger scale and show more detail.
Scale is listed on nautical charts as a fraction. The first number represents one inch on the chart, and the second number represents the equivalent number of inches on the earth. For example, on a nautical chart with a 1/40,000 scale, one inch on the chart represents 40,000 inches on the water. This is equivalent to about 0.55 nautical miles per inch on the chart.
Nautical charts range from very detailed harbor charts with scales around 1/20,000 to general charts depicting hundreds of miles of open ocean with scales as small as 1/1,200,000. When selecting a navigational chart for your journey, choose an appropriate scale that covers the area you plan to traverse, while providing enough detail to get you there safely.
Depth is marked on nautical charts as numbers printed on different water areas. Depending on the scale and age of the chart, depth numbers may represent feet, fathoms or meters. The unit of depth on a nautical chart is typically listed in the top right corner in large print. Here are a few necessary terms for reading nautical chart symbols for depth:
In addition to depth numbers, nautical charts also depict depth with contour lines. Depth contour lines connect areas of the same depth to provide a clearer picture of the shape of the seabed below your boat. Contour lines are particularly useful when traveling near coastlines. On some nautical maps, shoal areas are marked with thicker contour lines or shaded areas for safer navigating in shallow waters.
The distance on nautical charts is calculated in nautical miles, abbreviated as M or NM. Nautical miles are longer than statute miles on land, measuring 6,706 feet. However, one nautical mile is equal to exactly one degree or one minute of latitude. This makes it easy to measure distance on nautical charts using the latitude scale on either edge of the chart.
Nautical miles also make it easy to calculate speed in knots. A knot is a unit of speed used for boating, abbreviated as KN or KT. One knot equals one nautical mile per hour.
Navigational aids include a variety of objects used to mark dangers in the water and waters that are safe for travel. The two primary types of aids to navigation include beacons and buoys. Beacons are fixed structures attached to the land or the seabed, and buoys are floating objects that are anchored in place. Both buoys and beacons can be lighted or unlighted.
Navigational aids are marked on charts with unique symbols so mariners know what each marker means and how to navigate around it. Here are a few common symbols for aids to navigation in U.S. waters:
These are only a few of the hundreds of symbols used to mark navigational aids on nautical charts. The National Oceanic and Atmospheric Administration (NOAA) provides a full list of symbols for aids to navigation in U.S. Chart No. 1.
U.S. Chart No. 1 also includes standard nautical chart symbols for underwater features, obstructions, anchorages, channels, prominent land features and more. Symbols for underwater dangers like reefs, rocks and wrecks also indicate the depth of the object and other significant characteristics. Although many of these symbols are self-explanatory, carrying U.S. Chart No. 1 onboard your boat allows you to identify any symbol you encounter on your nautical map.
No nautical chart would be complete without a compass rose. A familiar symbol to any seasoned mariner, the compass rose contains three concentric circles marked with hashes and labeled with degrees. The outer circle of a compass rose points to true north, and the inner circles point to magnetic north. If you are using a magnetic compass for navigation, refer to the inner circles when measuring your bearings.
Now that you are familiar with the basic elements of a nautical chart, it’s time to start plotting your course. Marine chart plotting involves calculating the direction and distance of travel from your starting point to your final destination. This course may be a straight shot or involve multiple turns. By carefully marking the distance, direction and travel time for each leg on your nautical chart, you can navigate to your destination with confidence.
Chart plotting is fairly straightforward once you get the hang of it. Follow these steps for how to use a nautical chart to plot your sea voyage:
You only need a few simple tools for marine chart plotting:
Using the parallel ruler, divider and pencil, you can measure the direction and distance of each leg of your course and mark it on the nautical chart. The stopwatch will come in handy during your journey to track how far you have traveled and help you stay on course.
Locate the starting and ending points of your journey and draw a straight line between those points using your ruler. More complicated routes may require several turns to reach your final destination. Choose the route you want to travel based on water conditions and hazards, then mark each leg of the journey in a straight line using the edge of your parallel ruler or plotter.
Measure your direction of travel using the parallel plotter and compass rose on your nautical chart. Align the plotter with the first leg of your course and press one side down firmly. Move the other side of the plotter until its outside edge meets the small cross in the center of the compass rose. The degree measurement on the magnetic compass rose is your bearing for that leg. Write the direction of travel in degrees magnetic next to that leg. Repeat this process for each leg of your journey.
If you cannot reach the compass rose with a single movement, you can walk the parallel plotter across your chart by holding down one side and moving the other. Because the two sides of parallel plotters always remain parallel, the angle will not change as long as you only move one side at a time. This ensures an accurate measurement of the direction of travel on even the largest nautical charts.
Use your divider to measure the distance of each leg by placing one point of the divider at the beginning of the leg and the other point at the end. Without changing the spread of the divider, move the divider to the latitude scale at the edge of your nautical chart. Count the degrees of latitude between the two points of the divider to find the nautical miles of each leg. You can also measure the distance using the distance graph on your navigational chart.
If the distance of your course is longer than the spread of the dividers, you can walk them across the chart like you did with the parallel plotter. First, set the dividers to a standard distance, such as five nautical miles, using the latitude scale or distance graph. Start at the beginning of a leg and walk the divider down the leg moving one side at a time by rotating the divider.
Continue to the end of the leg, counting each five-mile increment. When the divider rotates past the end point, squeeze the divider until it lines up with the end of the leg. Measure the final spread of the divider on the latitude scale and add this to the miles you already counted. For example, if you moved the divider seven times in 5-mile increments and had a final spread of 3 miles, the total distance for that leg would be 38 nautical miles. Record the nautical miles of each leg on the chart.
The final step before heading out on the water is to calculate the time it will take to complete each leg of the course. Begin by determining the cruising speed that you plan to maintain during the journey. You can then use a basic formula to calculate your travel time for each leg. Simply multiply the distance in nautical miles by 60 and then divide that number by your speed in knots. The answer will be the time in minutes it will take to complete that leg.
For example, if you travel 2.5 nautical miles at a cruising speed of 30 knots, it will take five minutes to reach your destination. The equation would read 2.5 NM X 60 / 30 KN = 5 minutes.
Use this formula to calculate the travel time for each leg and mark it on your nautical chart. While traveling, you can track the time elapsed using your stopwatch. If you maintain a consistent cruising speed and keep your bearings, you can navigate to your destination successfully.
Although chart plotting is an effective tool for marine navigation, it does not account for water currents, wind direction and other factors that may steer you off course. Luckily, you can use the other navigational clues and landmarks on your navigational chart to keep your boat on the right path. Here are a few more tips for navigating the ocean using nautical charts:
With careful chart plotting and good observation, you can navigate the ocean with confidence using nautical charts.
Navigating without GPS is a useful skill for any boater to have. If you are ever caught in a tight situation, you will be grateful that you know how to read nautical charts and plot a course to get you home safely.
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