The first thing that may strike you when you see a chart is all of the little numbers everywhere. These small numbers are the “Mean Lower Low Water” which is the average depth at the lower of each day’s two low tides. These measurements help boaters determine the closest underwater clearance possible for their respective boat.
Depths (or soundings) are shown in either meters, feet, or fathoms (6 feet).
If you’re unsure which measurement your chart is using, look for a large magenta letter in the top right corner of your chart. Subscripts show depths in fathoms and feet (52 being 5 fathoms and 2 feet) or decimal meters (5.2 meters).
Rocks, reefs, wrecks, and other dangerous obstructions are depicted with a large variety of symbols depending on the type of danger, its depth, and a few other characteristics. Dangers in less than 11 fathoms of water are surrounded by a black ring of “danger dots” and have a blue interior.
Now the other big dangers are these brown areas right outside the land. These are places where sandbars might appear and could hinder your path. Plan accordingly, as finding yourself beached on a sandbar doesn’t make for a fun trip.
Worried about your ability to navigate around dangers? Luckily, U.S. waters have a massive system of more than 40,000 buoys, beacons, and lights. These telltale signs are there to mark dangers, show safe water, and demonstrate the limits of dredged channels. Nautical charts by the NOAA depict all of these buoys and beacons using different symbols that show the purpose of each.
This can be a massive lifesaver, since you can also use those buoys as a way to mark your location when navigating. Sailors can hop from buoy to beacon and back again for longer trips, helping to prevent them from losing their way.
Now, there is another little trick for buoys. On charts you’ll often seen green and red diamonds showing these buoys and that are labeled GC 6 or RN 3. This is because buoys often come in two shapes: Cans (cylindrical) or Nuns (triangular). If you’re trying to return back to port you can always use “Red, Right, Return.” Basically, if you keep the red buoys to your right you’ll be returning to port via the channel.
For those of you familiar with land maps, this is all going to sound very familiar. However, for those of you who aren’t too familiar, this might be the most important section. On land maps, scale indicators are used to show distance. Scales are almost always expressed as ratios, and they differ pretty heavily from map to map.
The scale, printed most often in the top right corner of the map, will look like this: 1:10,000. This specific ratio means that every 1 inch on the map is equivalent to 10,000 inches in the real world. Oftentimes, if you’re looking for a more detailed map with a close up view, you can get one with a smaller scale. This will help you when navigating around specific markers or checking out area details.
There are most likely going to be some arrows with little numbers over them on your chart. The direction of these arrows indicates the direction of the current flow. The numbers also tell you the water’s speed in knots. This is something that needs to be from a recently charted map, as currents and tides change quickly and unpredictably. An old map can provide you with inaccurate information that could potentially harm you, your boat, or ruin your experience. Even if you do have a relatively recent map, don’t be afraid to watch the water for anything out of the ordinary.
The little anchor shape? Exactly what you would think it means. It’s a familiar symbol for a safe anchorage and is carried true here. These anchors often have some clarification associated. An anchor marked DW often means deep water, or a number like 24 will notify you that you can only keep your boat here for 24 hours at a time.
It’s really good to take notice of anchorages on the way to and around your destination before you set sail. You don’t want to accidentally spend extra hours sailing that you should have been resting because you plotted a course with no anchorages available.
Latitude and Longitude
Latitude and longitude are the way we tell exactly where we are on the globe. For those who might not remember this from high school, latitude is the division of earth from top to bottom in parallel horizontal lines. These lines start from the equator with the equator equaling 0 degrees. It eventually reaches the poles at 90 degrees north and south.
Longitude lines are the opposite. They are vertical lines that connect the poles. Zero degrees is located at Greenwich, England. They go east and west, meeting on the other side at 180 degrees. Every degree is broken down into 60 minutes each. Every minute is further broken down into 60 seconds. You can use these smaller divisions to focus down and give a more exact location.
Magnetic North Vs. True North
Magnetic north changes. As time passes, magnetic north changes ever so slightly. This means that every compass rose will have a magnetic compass rose inside of it. Since your compass reads magnetic north, this allows it to be more useful. Now, you can line yourself up with magnetic north and be able to tell true north off of it.
This magnetic rose will also have some tiny text showing you how much variation there is, as well as the annual increase and decrease of that variation. This means that older charts can still be useful as you can perform the math for the increase or decrease of variation to chart out where you’re going. It’s also good to note that sometimes this deviation can be tiny. If it is not relatively small, it may be easier to simply purchase a new chart.
Chart notes are important. Oftentimes, a lot of really interesting or helpful information can’t fit on the water itself. In this case, there is often text elsewhere on the chart that is linked to either a feature or portion of the chart that uses a specific label. However, in some cases, the note is about the entire chart. That information could be something about how the tides change or specific information about a dangerous feature. Do not ignore it. It is definitely worth the read.
Regardless of where you start with chart reading, it can be important to remember the small details listed here. It’s crucial to look back at the basics and keep a strong foundation of knowledge. This will help maintain your safety and the safety of your crew. Next time you pull out a map, make sure you can identify all of these details before you set sail. Whether you’re a new skipper or an old salt, it’s always good to keep yourself on your toes and be prepared for the worst-case scenario.