Most sailors today rely on GPS to find their way at sea. While GPS has helped to make finding your way at sea far easier than in the past, sailors still need to study charts before they embark on a passage and to use them en route to check and record their progress.

Failure to study and understand charts covering their proposed course could potentially put a sailor’s vessel in danger. For example, while a GPS might automatically pinpoint a sailor’s position it will not alert them to potential hazards along a route, such as shallow water or hazards beneath the surface, such as rocks.

Nautical charts are therefore a crucial tool for any sailor to navigate safely and effectively. Here is a reminder of some of the basic steps involved in reading a nautical chart:

• Understand the symbols: Nautical charts are full of symbols that represent different features and hazards on the water and below its surface. Before you start using a chart, you should familiarise yourself with these symbols and what they mean. Most charts will have a key that explains the symbols used but there are also books and apps available that explain all the symbols and abbreviations used.
• Determine the scale: The scale of a nautical chart refers to the ratio between the size of the chart and the size of the actual area it covers. For example, a chart with a scale of 1:50,000 means that one unit of distance on the chart represents 50,000 units of distance in the real world. Knowing the scale is important for calculating distances and planning your route.
• Locate your position: Once you understand the symbols and scale, you can start to locate your position on the chart. Take bearings of visual landmarks and navigational aids such as buoys, lights and shapes to help determine where you are on the chart.
• Plan your route: Nautical charts show depths, shoals, rocks and other hazards, so use this information to plan a safe route. Look for deep channels and avoid shallow areas. Be aware of navigational hazards such as rocks, reefs, and wrecks.
• Check tides and currents: Nautical charts often include information about tides and currents, which can have a significant impact on your route and speed. Make sure to factor in these variables when planning your journey.

Overall, reading a nautical chart takes practice and familiarity with the symbols and conventions used. It’s important to approach chart reading with care and attention to detail to ensure a safe and successful voyage.

Variation

Variation is an important factor to consider when using a nautical chart for navigation. Variation is the difference between true north (which is the direction of the North Pole) and magnetic north (which is the direction of the earth’s magnetic field).

Magnetic north is not fixed and can change over time due to shifts in the earth’s magnetic field. This means that the magnetic north pole is constantly moving and can be different from true north at any given time and location.

When you’re using a compass for navigation, it’s important to account for this difference between true north and magnetic north. This is where variation comes in – variation is the angle between true north and magnetic north at a particular location.

On a nautical chart, variation is indicated by lines of isogonic variation, which show the difference between true north and magnetic north for a particular year. For example, a chart might show a line labeled “2023 Variation 6°W”, which means that the variation for that location in 2023 is 6 degrees west of true north.

To use a compass effectively for navigation, you need to adjust for variation by either adding or subtracting the appropriate amount from your heading. This will ensure that you are actually travelling in the direction you intend to go, even if your compass is pointing slightly off of true north due to magnetic variation.

It’s important to note that variation changes over time, so you should always consult the most up-to-date chart and navigation information to ensure accuracy in your calculations.

Compass deviation

Another factor that needs to be considered when using a compass for navigation is compass deviation. Compass deviation refers to the error in a magnetic compass caused by the magnetic fields of a vessel’s own structure and equipment. In other words, a compass will not necessarily point to true north or south because of the magnetic influence of the vessel itself.

This deviation can be caused by a variety of factors, such as the presence of steel or iron in the vessel’s hull, or by the presence of electrical equipment that creates a magnetic field. Deviation can also vary depending on the vessel’s orientation with respect to the Earth’s magnetic field, so it may be different when the vessel is heading north compared to when it is heading east or west.

To correct for compass deviation, a vessel will typically use a compass deviation card or table that lists the deviation values for different headings. This information can be obtained by swinging the compass, which involves turning the vessel in a complete circle while comparing the compass reading to the known true heading at each point. Once the deviation is known, the compass readings can be adjusted accordingly to provide more accurate navigation.

Summary

These are just a few reminders of how to read a nautical chart. There is no mention of the traditional methods and instruments used to plot a course. It is also a reminder that it is not prudent to place blind faith in electronics without having a good understanding of navigation skills, as well as seamanship and meteorology.

Take a look here at our Chart Symbol apps:

For all sailors who use paper nautical charts or electronic ones – with these apps you’ll have all the chart explanations to hand!

Boating App – Chart Symbols & Abbreviations

Boating App – Electronic Chart Symbols (ECDIS)