HOW TO READ A NAUTICAL CHART
Chart reading is a basic skill every boater and sailor needs to develop. With today’s modern technology, it’s tempting to skip the theory and just jump on a boat, hoping that our smartphone will take care of everything.
However, as we’ve mentioned before, apps like Google Maps simply don’t display enough information to help you make the right decisions on the water. They don’t show Aids to Navigation or depth data, which are essential to navigate any stretch of water. In order to stay safe, you really need access to the details contained in a nautical chart.
What's the difference between a nautical chart and a map?
A nautical chart represents hydrographic data, providing very detailed information on water depths, shoreline, tide predictions, obstructions to navigation - such as rocks and shipwrecks - navigational aids, and more.
A map, on the other hand, is a reference guide showing man-made routes, like roads and highways.
Let’s see what a nautical chart shows and how to read it.
SCALE
First, check the scale. This is printed rather big near the top of the chart and looks something like “1:10,000”. The scale is the ratio of the distance on the chart to the distance on land or water. For example, in a 1:10,000 scale, every 1in (2.5cm) on the map equals 10,000in (25,000cm) in the real world.
You can choose between small-scale and large-scale charts. Small-scale charts show a smaller area in more detail; while large-scale charts show a much bigger area, offering less information.
Understanding the scale will allow you to work out distances. For example, if a chart has a 1:20,000 scale, 1in (2.54cm) on the chart corresponds to 20,000in (50,800cm, or 508m) of the Earth’s surface, which is the equivalent of 0.27 nautical miles. This means that a nautical mile will be about 4in (10.16cm) on the chart.
On a paper chart, you can use the latitude scale on the sides of the chart and dividers to quickly measure distances. To understand how to do this correctly, you’ll need to study some basic navigation.
UNIT OF MEASUREMENT
Next, you’ll need to figure out what unit of measurement the chart uses to indicate depth. It’s usually indicated in feet, fathoms, or meters.
This is especially important to check if you’re navigating foreign waters, as you may automatically use the unit of measure you’re used to and your navigation may be way off. You can find the unit of measurement used in the chart near the scale information.
DEPTH
The numbers dotted around the water represent the depth, expressed in the unit of measurement found by the scale. The soundings showed on the chart usually represent the depth at “mean lower low water” (MLLW), which is the average depth at low tide.
For this reason, you’ll need to know the tide state and height to work out the current depth. You can find this information on the Deckee app, in the weather forecast section.
If you see a negative number, it means that the water depth in the area is usually less than what the chart says.
CONTOUR LINES
On a chart, you’ll notice some lines on the water, which shade from white, light blue, and dark blue. They are called depth contour lines.
They are represented in the unit of measurement you found at the top of the chart. Don’t worry too much about working out the correlation between a colour and a depth - you will always have the numbers as a reference. Pay attention to how close or far the contour lines are - this shows how steeply the seabed changes in depth. In shallower areas, if the contour lines are close to each other, it’s best to slow down.
CHART SYMBOLS AND AIDS TO NAVIGATION
Chart symbols mark dangers, Aids To Navigation (ATONs), and structures in and out of the water, such as rocks, wrecks, and bridges.
ATONs are buoys and markers which signal hazards or indicate a route to be followed. If you’re not familiar with ATONs, head over here. On the Deckee app, you can look up each aid to navigation on the map to find out what it means.
Nautical charts are produced by different organisations around the world. They usually offer a free downalodable booklet explaining all the symbols and abbreviations used on their charts. For example, the Australian Hydrographic Office issued this guide, while NOAA have a dedicated page on their website. If you’re in doubt about what a symbol means, check these resources before heading out on your boat.
Electronic charts are easier to use, as the software on which they’re loaded will show you where you are. If you’d like to work out your position on a chart without electronics, you’ll need to take a boating or sailing course to learn how to read a chart in-depth and navigate. It’s an extremely rewarding skill to develop and it will help you keep your boat and crew safe.
Always carry a spare GPS device, handheld compass, and waterproof charts on your boat - electronics, such as a chart plotter or tablet, can break. In some countries, carrying paper charts is mandatory. Check the requirements for your area.