Paddle Smart - Nautical Charts

Have you ever been out on the water in a small boat when the visibility suddenly changed for the worse? 

Such changes are often caused by a change in the weather; i.e., a warm or cold front moves through the area. Fronts can be accompanied by thunder and lightning. Or, fog will envelop an area due to various weather phenomena. A type of fog known as advection fog can persist for several days, until a significant change in the local weather occurs. Another cause of restricted visibility is rain obscuring the landscape and familiar landmarks.

In any event, it is very easy to get disoriented on the water, especially if there are a number of islands in the area and you must thread your way through them to get back home. This is the time when a basic knowledge of nautical charts and maps and how to use them comes in very handy. On the water, charts and maps are different from road maps. They show you where you cannot go as well as where to go. 

As a boater, you must always know where you are and the best way to reach your destination safely. Charts are pictures of parts of the earth's surface. They provide a variety of information to aid you in the navigation of your boat and are vital to your safety. Nautical Charts: come in various sizes and scales. Typical charts for small boats are referred to as coastal, lake, harbor, or small-craft charts. Nautical charts are produced for virtually all coastal regions, large lakes, and major rivers. Topo maps provide terrain profiles of the land and outlines of shorelines. They may be the only form of quality information for areas not covered by nautical charts. They can help you locate your position, but generally tell little about conditions on and under the water. Topo maps are produced by the United States Geodetic Survey (USGS).

The scales differ so that the area of interest is portrayed on the size of paper being used. Nautical charts can be obtained from local marine stores, from marinas or directly from the National Oceanic and Atmospheric Administration (NOAA). Topo maps can be obtained in outdoor stores or from the USGS. Nautical Chart No. 1 is a booklet that explains the nautical terms and symbols used on charts. Nautical Charts Show: many items of interest to paddle boaters, such as the depth of the water. 

Charts contain four colors to describe the makeup of the earth's surface: ¨ White for deep, safe water ¨ Blue for shallow water ¨ Green for tidal areas covered at high water ¨ Gold for dry land Direction is another vital piece of information that may be obtained from a chart. Direction is measured in degrees from 000º to 360º, clockwise from geographic, or true, north. 

A Compass Rose, colored magenta, has circles marked in degrees like those on a compass. The Compass Rose shows the difference between true north and magnetic north. The angle between the directions of geographic north and magnetic north is called variation. Grid lines on charts and maps generally are oriented toward true north-south and east-west. If your map has not compass rose, you will need to determine your local magnetic variation to convert between your compass reading and your chart. GPS units can be set to display either true or magnetic headings.

A long-distance paddler should be able to determine from the chart the true course to the desired destination and be able to establish the compass course to steer to get there. 

Finding Your Position: on the water and on the nautical chart can be established by use of the latitude (L) and longitude (Lo) grid system. This system makes it possible to locate any point on the earth. It consists of two sets of imaginary lines. Parallels of L run east and west and are parallel to the equator. They are numbered from L 0º at the equator to L 90º at the poles. Meridians of Lo run north and south to the geographic poles. Lo 0º passes through Greenwich, England. Lo lines are numbered east (E) from Greenwich toward Europe and Asia, or west (W) from Greenwich toward the United States. 

Your position is described in degrees and parts of degrees of L and Lo. GPS units can readily readout your position in degrees of L and Lo and display additional data valuable to a voyaging paddler. 

To Measure Distance: on coastal charts, distance is measured in nautical miles (6076 feet); and in inland waters, distance is measured in statute miles (5280 feet). Using dividers, you can find distance on a chart in several ways: ¨ The distance scale will be near the chart title block ¨ The latitude scale on the side of the chart provides a means for measuring distance. One minute of latitude is equal to one nautical mile. ¨ You will find latitude scales on small-craft charts along certain meridians of longitude. Dividers are useful at home while planning a trip but most paddlers leave their dividers at home and estimate distance while underway. Two fingers may be used as a crude set of dividers. Also, if you calibrate the span from the tip of your thumb to your fingertip, it is possible to determine reasonable estimates of distance. 

Aids to Navigation (ATONs): for boaters, are like street and caution signs for drivers. Each gives you the information you need to know to locate and move your vessel safely. ATONs include lights, buoys, daymarks, lighthouses, foghorns, etc. Buoys are floating objects anchored at specific locations. The shapes and colors of marks make it easy to identify them. Numbers and letters further identify many marks and help you find them on charts. 

Always Use Up-to-Date Charts and Maps

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Page updated July 8, 2016