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Understanding Nautical Charts


The ability to read and understand nautical charts is an important skill for all boaters. While sophisticated GPS and navigation systems can be found aboard even the smallest of boats, the charts that these navigation systems use has not changed much since the days of paper charts. In this blog post we will explain the colors, water depths and aids to navigation (ATON’s) that can be found on both paper and electronic charts. One could spend many hours studying and learning about nautical charts, but these are the most important skills for recreational boater to have. 

Nautical Chart Colors

The majority of nautical charts use the same color scheme, with some slight variations, so that boaters can easily go from viewing one chart to another. Or, from a paper chart to an electronic chart on a GPS or mobile application. The infographic below will help you to better understand the most common colors on the nautical chart.

The tan, and sometimes yellow colors on the chart indicate solid ground that is always above the waterline. Visually, your eyes should aways be able to see it while boating. The white areas indicate deep, safe and navigable waters regardless of the rise and fall of water levels during the tide cycles. The blue areas indicate less water depth than the white areas, however this does not mean that the water depth is unsafe for your vessel. The darker shades of blue indicate decreasing amounts of water depth, and in these areas more caution is advised when navigating. It is best to avoid the green areas on the chart, as these are shallows that may be exposed at low tide and hiding just below the surface at high tide. Generally these are not safe and navigable areas for most vessels. They may also indicate mangrove areas as shown and labeled below.


Water Depth on Nautical Charts

In addition to the colors, the estimated water depth is also listed on nautical charts to help you keep you in safe, navigable water. The numbers circled in yellow on the infographic below indicate the estimated depth at the average of the lowest low tides. This is know as MLLW or the Mean Lowest Low Water. These numbers are also used as a baseline when calculating tidal information. For example, if the tide is predicted to rise 2.5 feet, you would simply add 2.5 feet to the numbers circled on the chart below to know the estimated water depth at that time in the tide cycle.



ATON’s on Nautical Charts

The Aids to Navigation System, also known as ATON’s, is another critical component to your safety as a boater. Generally, the buoys and markers on the nautical chart are easy to identify but there are some that can be confusing or even mistaken at first glance. In the illustration below you will notice two ATON’s that look identical on the chart, but are very different when seen on the water or viewed on your GPS. The magenta exclamation point symbol indicates that both of these Aids to Navigation have a light attached to them. They do NOT indicate the color of the light or the ATON. One serves as a green marker and the other one serves as a red, even though the symbols are identical. The way to tell the difference is to read the abbreviated description on the chart. For example, “Fl G” is an abbreviation for flashing green, indicating that this would be a green ATON. Likewise, “Fl R” would indicate a flashing red ATON. Note that these ATONs will appear more obvious when viewed on GPS systems, as their respective colors will be shown. 

Another observation on nautical charts, both paper and electronic, are the quotation marks around the numbers associated with an ATON. For example, red number 12 below could easily be confused with a water depth estimate had it not been for the quotation marks. The surrounding depth estimates are 7 and 8 feet, so a 12 foot reading would not be out of the question. A simple oversight like this could lead to a navigational error and a vessel running aground if the depth were shallow. 



More Information About Nautical Charts

For more information about nautical charts and the Aids to Navigation System, we recommend two resources. The first one is called Chart Number One, and it is the key to all of the symbols that are found on nautical charts. You can download a free PDF copy of it from NOAA by clicking here. The other resource that we recommend is the US Coast Guard Aids to Navigation Brochure. You can also download a free PDF copy of it by clicking here. If you prefer an in-person class on navigation, we highly recommend taking our Coastal Navigation Course which will help you become a more knowledgeable navigator.