Nautical Charts: How to Read, Understand and Navigate Safely

The nautical chart is the fundamental tool of every navigator. Whether printed on paper or displayed on a screen, it contains a wealth of essential information: depths, hazards, buoys, currents, harbours, and much more. Yet many recreational sailors exploit only a fraction of this wealth. This comprehensive guide will help you read, understand and use nautical charts like a true mariner.

What Exactly Is a Nautical Chart?

A nautical chart is a specialised cartographic representation of the sea and coastal areas. Unlike a road map, it focuses on what happens both below and above the water surface: seabed topography, shoals, wrecks, rocks, currents and navigational aids. In France, the SHOM (French Naval Hydrographic and Oceanographic Service) produces and updates official charts. In the UK, the UKHO publishes Admiralty charts. These paper charts serve as the official legal reference for navigation.

Nautical charts come in two main families: traditional paper charts and electronic charts (ECS or ECDIS depending on the system). Each format has its advantages, and every prudent navigator uses both in combination.

Understanding Chart Scale

Scale is one of the first things to check on a chart. It determines the level of detail available and therefore how you can use it:

• Large scale (1:10,000 to 1:50,000): ideal for harbours, channels and coastal approaches. Highly detailed, it shows every buoy, every rock and every critical depth.
• Medium scale (1:50,000 to 1:200,000): perfect for daytime coastal navigation, giving a good overview of the route between two ports.
• Small scale (1:200,000 to 1:5,000,000): reserved for offshore passage making and planning long voyages. Coastal details are minimised.

A golden rule: the closer you are to a coast or harbour, the larger the scale chart you should use. Navigating a channel with a 1:500,000 chart is dangerous — hazards are not represented with sufficient accuracy.

Depth Contours and Reading the Seabed

Depth contours (isobaths) — lines connecting points of equal depth — are the alphabet of the nautical chart. They draw the underwater relief and allow instant visualisation of the seabed topography. The figures on the chart express depth in metres below chart datum, corresponding to Lowest Astronomical Tide (LAT). In practice, the actual depth will therefore always equal or exceed the charted value.

Colours also play an important role: light blue generally indicates shallow areas (less than 5–10 metres depending on the publisher), medium blue indicates intermediate depths, and white indicates deep water. Areas that dry at low tide (foreshores) are often shown in green or with specific hatching.

Spot Soundings

In addition to contours, the chart is scattered with spot soundings: individual numbers indicating a specific depth at a precise point. An underlined figure means the seabed dries at low tide — a potential hazard never to be ignored. Permanently submerged rocks are indicated by a dot or asterisk with a very shallow depth figure.

Essential Symbols to Know

The international standard INT 1 lists over 500 symbols. No need to panic — mastering the most common ones is sufficient for safe coastal sailing.

Hazards

• Drying rock: upward-pointing triangle + underlined figure (height above chart datum)
• Submerged rock: cross with central dot + depth figure
• Dangerous wreck: ship symbol with depth indication
• Shoal: contoured area with minimum depth figure

Navigational Aids

• Lights and lighthouses: magenta star on paper charts, with their characteristics (colour, period, range)
• Buoys: coloured symbols following the IALA buoyage system (Region A in Europe)
• Cardinal buoy: double triangle indicating the safe side to pass
• Lateral buoy: red to starboard (entering port), green to port

Paper Charts vs Electronic Charts: Which to Choose?

The question is not which to choose, but how to use both. Regulations in most countries require carrying appropriate nautical charts for the intended voyage. Electronic charts offer considerable advantages in terms of ergonomics and safety — but they should never be your only reference.

Paper Charts

Official paper charts are the legal reference documents. They are updated via Notices to Mariners (NtM) published weekly by national hydrographic offices. Check these notices regularly and apply corrections to your paper charts — this is a legal requirement for professional vessels and a sound practice for all sailors.

Electronic Charts (ECS)

Electronic chart systems have revolutionised recreational navigation. They display your position in real time on the chart, automatically calculate routes, distances and bearings, and can generate alarms when approaching a hazard. The most common formats are CM93, S57/S63 (official vector format) and rasterised scanned charts. Vector charts allow zooming without loss of detail and display customisation — a considerable advantage underway.

"The electronic chart is a remarkable tool, but it does not replace the navigator's judgement. A power failure, a GPS fault or an out-of-date chart can put you in danger if you cannot navigate without them."

How to Plan Your Route on a Nautical Chart

Route planning is the navigator's fundamental exercise. It involves tracing a safe path between departure and destination, avoiding all hazards. Key steps include selecting appropriate scale charts for each section, identifying all dangers along the route, placing waypoints that give adequate clearance from hazards, and calculating distances and times. Remember that distances on a chart are measured on the latitude scale — 1 minute of latitude equals 1 nautical mile.

Keeping Your Charts Up to Date

An out-of-date chart can sometimes be worse than no chart at all, as it creates a false sense of security. Seabeds shift, buoyage changes, new hazards are discovered and restricted areas are created. Always verify when your charts were last corrected before any voyage, especially in unfamiliar waters. Apps like YachtMate include automatic updates for vector charts, eliminating this concern for electronic navigation.