For your own safety and wellbeing, learning how to read a nautical chart should be at the top of your pre-sailing priorities. Whether you use paper or electronic one combined with the marine radar, it represents one of the pieces of navigational equipment that are essential for every journey you’ll take.
In this guide, we’ll show you how to properly read marine chart so once you’re out on the water you know how to properly gather all the needed information that will safely and without any problems get your from your starting point to the desired destination, even if you’re a sailing beginner.
The distance can be tracked by the scale indicator. Check for the upper right corner of your map and you’ll see some numbers that look something like 1:100,000. This means that every inch on the map is equal to 100,000 inches. So, if the distance between you and your targeted location is 5 inches, it means you’re about 500,000 inches away from it. The scale numbers could vary; the smaller scales provide a closer view with more details. Also, check to make sure what metric system it uses, as it can also be in meters, but this should be mentioned on the map itself. If you’ve ever had contact with land maps, navigation maps shouldn’t be too hard to understand.
The second useful information nautical chart provides is the depth of the specific area you’re currently at, but there is a catch. Check for the unit used for your map, as it can be in feet, fathoms, or even meters. Most of the time the manufacturers made in the USA use feet of fathoms, but it is not unusual to run into one with a metric system.
The black numbers you see represent the “mean lower low water” (or MLLW – the average of the lower of the two low water heights), the average water depth when the tide is low. This basically means that the number you’re seeing is the lowest one recorded, unless the number is negative, which means the water depth is less than listed.
Unlike most parts of the world, the US uses feet, so when you run into centimeters, meters, kilometers, it may be confusing at first, but it is actually rather simple. If you know how the metric system works, great, but if you don’t simply multiply your meter number by 3 and you’ll get roughly amount of feet. For more precise information, multiply number 3.28084 by the number of meters and you’ll get the right calculation. The negative part is the fact that not all of us know how to multiply in our head, so you may need a calculator within a hand reach.
Fathoms may sound complicated, but they are rather simple to understand and use. They look different than meters and feet and you’ll have to try to miss them. Most of the time fathoms have two numbers. The first one is several fathoms, and the second number, written as a subscript, represents additional feet. To turn fathoms in feet, multiply the fathom number by 6 and add the feet that remained. Or, simply said, one fathom is equal to 6 feet.
Colour Contour Lines
Another important aspect of map reading colors. All of them have their meaning and represent something important.
On the vast majority of nautical charts, water is represented using a white color. This way, if there is something important that has to stand out on the map, it will be easier to spot when you look at it.
Red Dotted Lines
When you look at your map, you may have spotted that some areas are connected with most often red dotted lines. This is used to connect areas with a similar depth, so you can easier spot the places with shallow water and the area where the water is deeper. This way you’ll have a better perspective on the overall depths in your near or far surroundings.
Light Blue Colour
The shallow water is marked with a blue color. What is considered as shallow water could differ from chart to chart, but some medium value is 18 feet. Anyway, if you’re drawing near the blue area, pay special attention.
Yellow or tanned areas are land or sandbars. Remember, nautical charts do not show your current situation, so sometimes these areas could go under the water and you won’t be able to see them from your boat. To prevent any misfortune, be especially careful when you find yourself near the tanned or yellow area, but you can’t see it. It didn’t move, it is not a mistake, it just went under the surface and can cause real damage to your vessel.
Besides dots, lines, and colors, charts have many other symbols used to describe a particular thing.
Compass rose, also known as Wind rose or Rose of the winds is a term used to describe three circles with numbers around them. The biggest circle is used to point to true north and it usually has a star or zero. The inner circle is used to point out to the magnetic north pole. Because of the magnetic north movement, the real situation could be a bit different, so make sure your map is updated, or simply use your magnetic compass, combine it with the compass rose and you’ll have a great guide.
Buoys And Circles
On maps you’ll find colored circles, mostly red or green. They represent water channels, while the color of the circles is the same as the color of buoys used to mark that area. Buoys are numbered, so it is easy to follow your place.
Red markers are used to mark the right side of a channel and green markers are located on the left side. This way your orientation will be easier.
Anchors are used to locating the points where you can safely anchor your vessel. It is important to know that not all places are intended to anchor every boat. If you see the “DW” mark beside anchor, it means “deep water”, so only that kind of boat can be anchored there.
If there is also a number near the anchor, it represents the number of hours allowed to stay at that place. Check out our article about the best anchors.
Dangers And Obstacles Along The Way
Nautical charts also provide information about rock formations, shipwrecks, or any other obstacles that could potentially be dangerous if you sail over them. Here are some vital chart symbols:
Plus sign circled with dots means there is a basic rock formation beneath the surface all the time. If the symbol is an asterisk, rock formation will become visible when the tide is low, and if the sign looks like a plus with dots in corners, that’s the rock just under the surface.
These small islands are visible all the time and are marked by a shaped circle with the number in the middle used to describe the height of the rock above the high water.
Breakers occur when the wave runs into a sea bottom only up to two times their height. They are dangerous for smaller vessels, so if you see the symbol of a few half-circles connected, avoid that area. It can also be described by the words “Br” instead of the symbol.
Rock or islet symbols combined with “Co” means that there is a coral reef.
If you see a fish-bone mark, that means there is a shipwreck (and a possible amazing place for diving). Fish-bone without dots means it is perfectly safe to move over it, but don’t cast any fishing nets or anchor your ship there. If it has dots around it, or it is represented as a sunken hull, it is better to go wide and not to risk anything.
Most often marked as “Spoil area”, “Fish heaven” or “Dumping ground”, spoil areas are areas you should avoid at all cost, or else you’ll risk damaging your hull.
Arrows are used to describe the currents and tide’s direction and are combined with the numbers that represent water speed in knots.
Sometimes these are combined with symbols, sometimes they come alone, so it is important to learn them so you can recognize them even if there are no symbols:
- Rocks – Rk, R o Rks
- Wreck or Hulk – Wk or Hk
- Obstructions – Obst
- Coral – Co
- Foul Ground – Foul
If you see any of these, avoid the area:
PA – Position Approximate
PD – Position Doubtful
ED – Existence Doubtful
Rep – Reported
SD – Sounding Doubtful
Gather All The Information
Most navigational charts have an instruction manual or a legend that will help you read it easier, but if they don’t, you can always check their symbol meaning online by looking for that specific map, or you could find the written version.
Also, it is recommended to check the dates of the last update. The fresher the update, the better.
Globo Surf Overview
Learning how to read a marine chart is an essential piece of knowledge for everyone who tends to sail away. It is not complicated, and it will keep you and all of your crew along with your boat safe and sound during your trip.
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