Nautical charts have many layers of important information, which can be understood fairly easily with a bit of study. First of all, a nautical chart will depict water depth. National Oceanic and Atmospheric Administration (NOAA) charts base this number on the mean lower low water (MLLW) level, which is the average height of the lowest tide recorded at a tide station each day during the recording period. Also depicted in different colors on the chart are sand bars and reefs, which may or may not be visible, depending on tide levels.
Other key information on the nautical chart concerns buoys, which are aids to navigation placed in waterways to show vessels safe routes, so as to avoid running aground. Red buoys are called nuns because of their conical shape. Green buoys are called cans because they are cylindrical. These buoys are labeled on the charts as either GC (green can) or RN (red nun). The labels have numbers, which generally increase as you head to port and decrease as you head to sea. Red nuns have even numbers while green cans have odd numbers. A handy way to remember the difference is the phrase “7UP cans are green.” Another phrase to remember is “Red Right Returning”. As you head into a harbor, returning from the sea, the red nuns will be on your right (and the green cans will be on your left).
There is information about light markers. Useful details listed on the chart will include how often a light flashes, its height, distance visible, and ID #. For instance, Fl R 6s 30ft 2M “3” means that particular light flashes red every six seconds, is 30 feet in the air, can be seen for two miles, and has the number “3” on it. The chart will also tell you whether or not the buoy or light makes a sound.
A chart will also depict hazard warning symbols. This is valuable information. At high tide, things that are dangerous to you (such as rocks, which are marked with an X) might be hiding underwater, so it’s important to know where they are. Even if a particular rock may not pose an immediate navigation hazard, it could cause a disruption in water flow that could affect your course.
Nautical charts use coordinates to describe where things are located. Latitude divides the Earth from top to bottom into parallel horizontal lines, which start at the equator, at zero degrees (0°) of latitude, going up to the North Pole at 90 degrees North (90°N) and down to the South Pole at 90°S. Longitude lines run vertically, from Pole to Pole. The 0° line runs through Greenwich, England, and the lines are labeled “east” and “west” from there, meeting at the other side of the globe, at 180°. Each degree (of both latitude and longitude) is broken into 60 minutes. Each minute is broken into 60 seconds, allowing for the pinpointing of exact locations.
There is always an equal distance between lines of latitude, and therefore they are able to be used for measurement. One minute of latitude equals one nautical mile, or 1.1508 land miles. But longitude cannot be used to measure distance, as the lines of longitude converge as they approach the Poles.
Charts come in a variety of scales. The larger the scale of the chart, the more detailed the information. So if you want an overall look at a long passage, you’ll want a small-scale chart, while if you’re looking to navigate through small passages or into a harbor, you’ll want the largest scale chart you can find. Every chart will have a graphic scale that you can use with a divider tool to measure distances between locations anywhere on the chart.
All charts have a compass rose, which shows the directions of true north and magnetic north for that particular chart. The outer circle of the compass rose shows true north and the inner circle shows magnetic north, and the variation between the two is shown in the center of the rose. Knowing this variation enables you to measure your boat’s direction with a compass (handheld or binnacle) and plot your course accurately on your chart.
Because true north and magnetic north are at two different locations on the globe, the difference between the two directions varies, depending on your location on the Earth. There are local geological conditions that alter how your compass reads the direction of magnetic north, and the actual location of magnetic north changes from year to year. It is important to always have up-to-date charts that show the current direction of magnetic north so you can accurately plot a safe course to your destination, wherever that may be.
Sailing is a discipline based on a foundation of knowledge and there’s no maritime skill that’s really more valuable than the ability to navigate your boat safely and accurately through waters both familiar and strange.
Getting to grips with nautical charts is an important asset to any new seafarer and familiarising yourself to the basics – from chart symbols to water depths, obstructions to buoys – will put you in good stead for developing the keen mind of a seasoned navigator.
To help you out, allow Seachest to shed some light on a few key aspects of reading nautical charts, making you a better sailor when you revisit the open ocean.
The General Information Block
Your centre of knowledge and the first place you look on the chart – in the general information block you’ll find the title of the chart, the name of the waterway or sea that’s suitable to navigate as well as water depth measurements, or soundings, represented in either feet or fathoms.
In the block notes, you’ll also find explanations on chart abbreviations, warning information, anchorage points and everything else needed to make sure you make safe passage.
There are typically 4 types of nautical chart : sailing charts, coastal charts, harbour charts and small craft (leisure) charts. All of these charts are important as they give a sailor different scales, ratios and information, depending on the distance of the sail and where you’re travelling to:
#1. Sailing Charts are really only used for long-distance sailing and navigating the open ocean.
#2. Coastal or general charts are great for sailing and navigating close to land – honing in on bays, harbours and other inland waterways.
#3. Harbour charts are great for detailing smaller waterways, anchorages and of course harbours.
#4. Small Craft or Leisure Charts are great for hobbyists and weekend sailors, typically printed in a practical and economical way for easy storage in your cabin.
The Compass Rose
A Compass Rose or Wind Rose is an important feature for plotting a course and is used to ascertaining true or magnetic bearings of north, south, east and west. Typically the ‘true’ orientation is displayed on the outside of the rose while magnetic bearings are found on the inside.
A modern compass rose typically has 8 principal winds – or points of the compass.
Latitude & Longitude
You can find where you are using the lines of latitude and longitude located vertically and horizontally on the chart. Latitude = vertical, signifying north and south, whereas longitude = horizontal, signifying east and west.
The zero points for latitude is the equator and the zero point for longitude is the Prime Meridian (Greenwich Meridian).
Soundings & Fathom Curves
What do the numbers mean on nautical charts?
Of all the various elements of a nautical chart, knowing the depth and understanding the underwater geography is probably the most fundamental. On a sea chart you’ll find numbers and colour coding, as well as wavy fathom curves that help you understand the seabed idiosyncrasies – the numbers themselves actually indicate water depth measurements or soundings, displaying the low tide depth area.
Bear in mind that soundings shown in white flag up deep water whereas shallow water is shown in blue.
The Distant Scale
Found near the top or bottom of a nautical chart, the distance scale is designed to help you measure the distance of a certain drawn course on a chart in miles, metres or yards. However, the latitude and longitude will also help you to determine distance as well.
Reading nautical charts is not the same as reading a map. It’s a bit more complicated because a map only shows you how to get from one point to another using roads while a nautical chart has a lot more information you can use to navigate safely on the open sea. Knowing how to read nautical charts is a skill that you must have if you enjoy spending time on your boat. Here are the things to know to be able to read nautical charts with ease.
Scale of the Nautical Charts
The scale of the chart you’re reading is the first thing you need to know when reading nautical charts. You can encounter a large scale chart like a harbor chart that is very detailed, or you can choose a small scale chart that covers a much bigger area on the earth’s surface. Large scale charts are usually between 1:20,000 and 1:40,000. This means that 1 nautical mile is about 4” of the 1:20,000 chart and 1.8” of the 1:40,000 chart.
To find out the scale of the chart you need to look for the designation on the chart which is a fraction(1/20,000). If the number after the slash is higher, that chart shows a larger area in a smaller scale which means it will be less detailed.
The water depth is indicated by numbers that are printed on the water areas of the chart. However, the essential part of chart reading is knowing which unit of measurement is used for creating the chart. It can be feet, meters or fathoms and it’s crucial for your safety. You can see the unit used on the face of the chart because it’s usually shown in a large print to avoid confusion. If the map uses fathoms and there is a number on the map like 04 it means the water at this spot is zero fathoms and 4 feet deep. So the first number shows the fathoms (1 fathom = 6 feet) and the second number shows the feet.
Aside from numerical chart symbols, water depths can be represented with contour lines. If you are reading a small scale chart you may encounter the 10-fathom contour line which means that the water depth within this line is not deeper than 60 feet. Larger scale charts have contour lines too but most of them are 3, 2, or 1 fathom lines. Large scale charts also have 3 feet contour lines and they have blue tinted areas so you can quickly identify the shoal water.
Symbols and Abbreviations
Nautical charts also incorporate other symbols and abbreviations to mark the location of various obstructions such as rocks, wrecks, anchorages, lighthouses, buoys, channels, reefs, submerged obstructions and tide rips. Most of the symbols used for them are intuitive and you’ll understand what the symbol means at first glance.
Nautical charts also include aids to navigation. You can find small triangles on a nautical chart which means there is a red daybeacon. If you see a magenta flare symbol, it means there is a lighted marker. The chart may have buildings, towers and hills marked if those locations may be of importance to the navigator.
The compass rose is an indicator for true north – the outer circle and magnetic north – the inner circle. If you encounter a group of three concentric circles on the chart you can identify it as a compass rose. The more common compass people use is a magnetic one so the inner circles will be the ones of greater importance. Those circles are usually marked with degrees between 0 and 360.
Plotting A Course
When you’re first learning how to plot a course, it might be intimidating, but if you get the feel of it, it’s not that tough. You’ll need a few tools including a pencil, a few dividers and a parallel rule.
To start plotting a course you will need 2 points on the map. Starting point (A) and ending point (B). When you have these 2 points, draw a straight line between them using the parallel rule. You want to know the distance and direction from point A to point B.
To learn the direction and distance you will need to align one leg of the parallel rule to the line you drew from point A to point B. Holding that leg pressed on the chart, move the other leg until the outside edge of the leg is at the very center of the cross in the closest compass rose. The point where the edge of the parallel rule meets the center of the magnetic compass rose is your magnetic course from point A to point B. This is your steering course.
Measuring distance on a nautical chart is a bit easier. For this you will need to place one divider on point A and another on point B. Without moving the position of your dividers, place the points of the dividers on the latitude scale. Count the minutes on the lines of latitude scale between the dividers and you will get the distance in nautical miles because one minute equals one nautical mile.
Learning how to read nautical charts may be intimidating for beginners but it’s an essential skill you must have if you want to safely navigate your boat from one point to another. Even though it seems difficult at first glance, the symbols and other markings are quite intuitive and it doesn’t take long for people to get the hang of it and start plotting their course to enjoy their days off with plenty of fun activities.
Nautical charts are an important navigation tool, even for the lake you live on. A nautical chart helps you figure out which way to go, how deep the water is, and the location of harbors. You will also know about underwater obstructions that may not be visible and overhead bridges and power lines that could get in your way.
Scales are expressed as ratios that differ from map to map and are found in the top right of the chart. When purchasing a nautical chart for a specific waterway, you will typically have an option of the scale of either a smaller portion that covers in greater detail or a larger chart that covers less detail. The scale of a harbor chart, for example, might be 1:20,000, meaning that 1″ on the chart represents 20,000″ of the earth’s surface, or about 0.27 nautical miles. That makes a mile on the water a little less than 4″ on the chart. Harbor charts will be very detailed. When you are sailing in open waters, you’ll probably use a small-scale chart (1:100,000 or higher) that covers a large area. When you are close to shore, and more at risk from shoals and other dangers, your large-scale chart (1:20,000 to 1:80,000) will provide more detail over a smaller area.
If you look in the upper right corner of the map near the scale, you will see the unit of measurement printed in big letters. You need to know this because charts use all sorts of different measurements. Your chart could include feet, fathoms, or meters.The small numbers on a nautical chart are water depths at “Mean Lower Low Water,” which is the average depth at the lower of each day’s two low tides. Measurements at this level help boaters determine the closest underwater clearance possible for their boat.
Contour lines are there to give you an idea of what is happening beneath the water’s surface. Contour lines, often red, connect similar water depth listings. This has the effect of dividing areas into shallower and deeper waters. Use the contour lines as a reference for the overall water depth in an area. Shoals and other shallow areas are blue. These spots tend to be no more than 18 ft (5.5 m) deep, although this varies from chart to chart. Dry land will usually be inside a contour line, but not always. Pay particular attention to any tan spots in the middle of water channels. Sometimes these spots are submerged and can damage your boat, especially in shallow water.
Aids to Navigation:
Red day beacons are shown on your chart as small triangles. The green ones — any color but red, actually — appear as small squares. Lighted markers have a magenta flare symbol — like an exclamation mark. A variety of symbols are used for buoys. Lighthouses are generally shown as a dot inside a magenta disk, sometimes with the flare symbol.
The compass rose is a series of 3 circles surrounded by numbers. The outer circle is to help you find true north, represented by a star or 0. The inner circle points to the magnetic north pole at the time of the chart’s printing. You can use a magnetic compass to navigate with the compass rose as a guide.
To avoid cluttering the parts of the chart showing navigable water, cartographers show details about certain features, dangers, restrictions, and warnings in “chart notes” elsewhere on the chart. Some notes are about the entire chart; others relate to a specific feature or portion of the chart that uses a nearby label.
It is extremely important to know how to read a nautical chart. Even if you have a GPS chartplotter, many things can disable the electronic system that most of the time is out of your control. Knowing at least the basics on how to read a nautical chart will help you in the most dire situation and will make you and your passengers feel much safer out on the water in your new lakefront house!
Head to the NOAA Coast Survey website for the latest charting products.
NOAA’s Office of Coast Survey is the nation’s chartmaker, with over a thousand charts covering 95,000 miles of shoreline and 3.4 million square nautical miles of waters within the U.S. Exclusive Economic Zone. Here’s a quick overview of the nautical charts that NOAA produces.
- Paper nautical charts are printed on demand by NOAA-certified agents.
- NOAA PDF nautical charts are digital formats of the traditional paper charts, and are up-to-date to the day they are downloaded. PDF charts are free.
- NOAA Raster Navigational Charts (NOAA RNC ® ) are digital images of NOAA’s entire suite of paper charts, updated continually with critical corrections. They can be used in many electronic charting systems and offer advanced functionalities such as real-time positioning. Available for free download in BSB format.
- NOAA Electronic Navigational Charts (NOAA ENCs ® ) are NOAA’s most powerful electronic charting product. These layered vector charts, available for free download, can be used in Electronic Chart Display and Information Systems (ECDIS).
- BookletCharts ™are free, print-at-home products to help recreational boaters locate themselves on the water. BookletCharts, in letter-sized format, contain all of the information on full-scale nautical charts.
- Historical Maps & Charts Collection contains over 35,000 images, including some of the nation’s earliest nautical charts.
- United States Coast Pilot®is nine volumes of supplemental information important to navigation. PDFs are updated regularly and are free for download.
Search Our Facts
President Thomas Jefferson founded Coast Survey in 1807 and tasked it with creating charts of the nation’s coastal waters so America’s young shipping industry could thrive. Reflecting Coast Survey’s beginnings, the Historical Map & Chart Collection contains over 35,000 images — charts, maps, sketches, and more — that are available free to the public.
Last updated: 02/26/21
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Nautical charts show all the features necessary for your safe navigation from here to there. To get that information, however, you must know how to read a chart.
Several outstanding boating safety organizations teach people how to use nautical charts, and every boater should use those opportunities. Before you take a class – or even if you’re a seasoned sailor plotting your next cruise – it helps to familiarize yourself with common chart elements.
Depths and dangers: The small numbers on a nautical chart are water depths at “Mean Lower Low Water,” which is the average depth at the lower of each day’s two low tides. Measurements at this level help boaters determine the closest underwater clearance possible for their boat. Depths (also called soundings) are shown in either feet, fathoms (6 feet), or meters (3.28 feet). To see which unit of measure the chart is using, check the large magenta letters at the top right corner of the chart. Subscripts, such as 52, show depths in fathoms and feet (5 fathoms and 2 feet) or decimal meters (5.2 meters). Dangers like rocks, reefs, wrecks and other obstructions are depicted with a variety of symbols, depending on the type of danger, its depth, and other characteristics. Dangers in less than 66 feet of water are surrounded by a ring of black “danger dots,” and have a blue interior.
Aids to navigation: U.S. waters have more than 40,000 buoys, beacons, and lights that mark dangers, and show safe water and limits of dredged channels. NOAA nautical charts depict them all, using different symbols that indicate the purpose of each.
Scale: Nautical charts come in different “scales,” the ratio of a given distance on the chart to the actual distance. When you are sailing in open waters, you’ll probably use a small-scale chart (1:100,000 or higher) that covers a large area. When you are close to shore, and more at risk from shoals and other dangers, your large-scale chart (1:20,000 to 1:80,000) will provide more detail over a smaller area.
Distance: Charts of the Great Lakes, other inland lakes, and the Intracoastal Waterway show distances in statute miles, the unit of measure you use on land. Most other charts use the “nautical mile” (1.15 statute miles), the length of one minute of latitude. Traveling one nautical mile in one hour is a speed of 1 knot.
Chart notes: To avoid cluttering the parts of the chart showing navigable water, cartographers show details about certain features, dangers, restrictions, and warnings in “chart notes” elsewhere on the chart. Some notes are about the entire chart; others relate to a specific feature or portion of the chart that uses a nearby label, such as “(see Note B).” Around the outside of the chart, in the lower left corner, is the chart’s edition number and publication date. In the lower right corner is the title and scale of the chart. The full chart title, chart notes and other information are stacked underneath the NOAA logo.
A nautical map, plotter and dividers
Whether you’re going on an offshore adventure or doing a course in nautical navigation, it’s useful to have a basic understanding of how to use a nautical chart. These tips will help you find your way.
Check the depth measurements (soundings) on your chart. Some charts show depths in fathoms and feet, but metres are more use to us in the UK. The depth measurement is usually calculated for the lowest possible level of tide, so you will normally have more actual water underneath you than it says on the chart.
Know the scale of the chart you’re using. It’s no use trying to enter Aberdeen harbour using a map showing the whole of the UK. Large scale charts show rivers and harbours; small scale charts are used to plan passage from port to port.
Use the colours to guide you. At Sea Cadets we say, “Sail on the blue or white and stay off the yellow.” The green bits are more tricky – sometimes they’re above water and sometimes they’re underwater.
To plot a course, use the compass rose drawn on the chart, but remember the difference between a ‘true course’ and a ‘magnetic course’ as a result of the Earth’s magnetic fields. Use a plotter to line up along your route and then read off the course.
To measure distance use a pair of dividers (like a drawing compass but with two points). Use the scale on the left or right side to convert your measurement to nautical miles (1 minute of latitude = 1 nautical mile).
To plot your position you need to know how far north or south of the equator you are and how far east or west of the Greenwich Meridian you are. We plot the latitude (north/south) first using the scale on the left or right of the chart, then the longitude (east/west) using the scale on the top or bottom.
After a three-month trial period, PDF versions of NOAA nautical charts will become a permanent product, free to the public. The free PDFs, which are digital images of traditional nautical charts, are especially valued by recreational boaters who use them to plan sailing routes and fishing trips.
The free PDF charts are part of a suite of new and enhanced navigational products designed to make NOAA’s data more accessible to the general public.
NOAA’s newest addition to the nautical charting portfolio is the new Portable Document Format (PDF) nautical chart. The image above is of the Gulf Coast – Key West to Mississippi River. (Credit: NOAA)
“Up to date charts help boaters avoid groundings and other dangers to navigation, so our aim is to get charts into the hands of as many boaters as we can,” said Rear Admiral Gerd Glang, director of NOAA’s Office of Coast Survey. “Within about 90 days of the product’s beta release, nearly 2.3 million charts were downloaded. To us, that represents more than two million opportunities to avoid an accident at sea.”
To help boaters who aren’t sure which chart they need, NOAA has launched a new interactive chart locator that allows people to select a chart from a map of the U.S. and choose their format. Whether downloading one of the new PDF nautical charts, selecting a chart to order from a “print-on-demand” vendor, or finding an electronic chart, the interactive catalog presents a highly integrated suite of navigation products.
NOAA is also making available for free the NOAA ENC Online, a new web map viewing application that shows chart data previously only available to users who purchased specialized viewing systems.
Also the digital charts’ image resolution will increase, from 254 dots per inch (DPI) to 400 DPI. This will improve image quality and legibility of the raster navigational charts (NOAA RNC®) that are used in chart display systems. Coast Survey asked users how the changes affected chart appearances on mobile apps. Initial compatibility issues and discrepancies that were identified have been resolved. The suite of RNCs will be upgraded starting April 3.
Printing PDFs may alter the chart scale, color, or legibility that may impact suitability for navigation. Printed charts provided by NOAA-certified Print-on-Demand (POD) providers fulfill a vessel’s requirement to carry a navigational chart “published by the National Ocean Service” in accordance with federal regulations.
The changes are part of a systematic reconstruction of NOAA’s nautical products, anticipating mariner needs as navigation transitions to a new digital age. Last year, NOAA announced the privatization of chart printing and distribution, with NOAA-certified vendors providing improved paper “print-on-demand” charts that fulfill a vessel’s requirement to carry a navigational chart published by the National Ocean Service in accordance with federal regulations.
NOAA’s Office of Coast Survey is the nation’s nautical chartmaker. Originally formed by President Thomas Jefferson in 1807, Coast Survey updates charts, surveys the coastal seafloor, responds to maritime emergencies, and searches for underwater obstructions that pose a danger to navigation. Follow Coast Survey on Twitter @NOAAcharts, and check out the NOAA Coast Survey blog at noaacoastsurvey.wordpress.com for more in depth coverage of surveying and charting.
NOAA’s mission is to understand and predict changes in the Earth’s environment, from the depths of the ocean to the surface of the sun, and to conserve and manage our coastal and marine resources. Join us on Facebook, Twitter, Instagram and our other social media channels.