How to Read Marine Charts and Navigation Aids

Marine charts are the fundamental tool of navigation. Whether on paper or a chart plotter screen, a chart shows water depths, seabed type, coastlines, hazards, and navigation aids — the information a boater needs to travel safely from one point to another. This guide covers how to read a marine chart, the meaning of common chart symbols, the buoyage system, and how navigation aids guide you through channels and past hazards.

Chart basics: projections and scales

A marine chart is a two-dimensional representation of a three-dimensional surface. The most common projection for nautical charts is the Mercator projection, which preserves angles and shapes — meaning a straight line on the chart corresponds to a constant compass bearing. This line is called a rhumb line and is the basis of most recreational navigation.

Chart scale determines how much detail is shown. A small-scale chart (e.g. 1:1,000,000) covers a large area — an entire sea or ocean — but shows little detail. A large-scale chart (e.g. 1:25,000) covers a small area — a harbour or bay — but shows great detail including individual rocks, buoys, and depth soundings. Always use the largest scale chart available for the area you are navigating, especially in coastal waters and harbour approaches.

Depth and sounding data

Water depth is the most critical information on a chart. Depths are shown as soundings — individual depth measurements printed as numbers on the chart. The datum (reference level) for soundings is usually Lowest Astronomical Tide (LAT) in Europe, meaning the depth shown is the minimum depth expected under normal tidal conditions. In areas with significant tide, the actual depth at high tide will be greater.

Depth contours are lines connecting points of equal depth, shown at regular intervals (e.g. 5m, 10m, 20m). They give a picture of the underwater topography — closely spaced contours indicate a steep slope, while widely spaced contours indicate shallow, gently sloping ground. Shallow areas are often shown with a blue tint, and drying areas (that uncover at low tide) are shown with a green tint.

Chart symbols and abbreviations

Marine charts use a standardised set of symbols defined by the International Hydrographic Organization (IHO). The most important symbols to recognise include:

• Rocks: An asterisk (*) indicates a rock that covers and uncovers with the tide. A plus sign (+) indicates a rock that is always submerged. A dot with a ring indicates a dangerous rock.
• Wrecks: Shown as a fishbone symbol or a dotted outline with "Wk" abbreviation.
• Weed/Kelp: Indicated by a wavy line pattern or "Wd" abbreviation.
• Seabed type: Abbreviations like "S" (sand), "M" (mud), "R" (rock), "Sh" (shells) indicate the nature of the bottom — important for anchoring.
• Lights: Shown as a magenta dot or star with a light characteristic description (see below).
• Restricted areas: Shown with a dashed boundary and a note indicating the restriction (naval exercise, nature reserve, etc.).

A complete chart symbol reference (Chart 1) is published by national hydrographic offices and is an essential reference for every boat.

The IALA buoyage system

The International Association of Marine Aids to Navigation and Lighthouse Authorities (IALA) maintains a standardised buoyage system used worldwide. There are two regions: IALA Region A (used in Europe, Africa, Asia, and Australasia) and IALA Region B (used in North and South America, Japan, and the Philippines). The key difference is the lateral (port and starboard) mark colours.

Lateral marks (IALA Region A)

Lateral marks indicate the port (left) and starboard (right) sides of a channel when entering from the sea. In Region A:

• Port marks: Red, can-shaped (cylindrical) buoys or red pillar beacons. Top mark is a red can. Light (if fitted) is red.
• Starboard marks: Green, cone-shaped (conical) buoys or green pillar beacons. Top mark is a green cone. Light (if fitted) is green.

In Region B (Americas), the colours are reversed: port marks are green and starboard marks are red. The shapes remain the same.

Cardinal marks

Cardinal marks indicate the safe water side of a hazard relative to the compass cardinal directions (north, south, east, west). A North Cardinal mark is placed to the north of a hazard — safe water is to the north of the mark. Cardinal marks are black and yellow, with topmarks consisting of two cones. The cone orientation indicates the cardinal direction:

• North: Both cones pointing up
• South: Both cones pointing down
• East: Cones pointing away from each other (base to base)
• West: Cones pointing toward each other (point to point)

Cardinal marks use white lights with specific quick-flash patterns that correspond to clock quadrants — for example, a North Cardinal flashes continuously, while an East Cardinal flashes 3 times every 10 seconds.

Isolated danger marks

Isolated danger marks indicate a danger with navigable water all around it — a single rock or wreck in otherwise deep water. They are black with red horizontal bands and have two black spheres as a topmark. The light (if fitted) is white, flashing two times.

Safe water marks

Safe water marks indicate that there is navigable water all around — often used at the start of a channel approach. They are red and white vertically striped, with a red sphere topmark. The light is white, flashing, occulting, or isophase.

Special marks

Special marks indicate a special area or feature — such as a sewage outfall, military exercise area, or data collection buoy. They are yellow with a yellow X topmark and a yellow light.

Lights and light characteristics

Many navigation aids carry lights for night-time and low-visibility navigation. Each light has a characteristic — a specific pattern of flashes that allows you to identify it from a chart. The characteristic is described using a standard notation:

• Fl (Flashing): The light is on for less time than it is off. Flashes at regular intervals.
• Q (Quick flashing): Flashes 50-79 times per minute.
• VQ (Very quick): Flashes 80-159 times per minute.
• Occ (Occulting): The light is on for more time than it is off — the flash is actually a brief dark period.
• Iso (Isophase): The light is on and off for equal durations.
• F (Fixed): The light is on continuously — no flashing.

The period (the time for one complete cycle of the characteristic) is given in seconds. For example, "Fl 3s" means a flashing light with a 3-second period. The colour is also specified — most navigational lights are white, but some are red, green, or yellow. By matching the observed light characteristic to the chart, you can confirm your position at night.

Leading lights and transits

Leading lights (or range lights) are pairs of lights that, when kept in line (one above the other), indicate that you are on the safe centre line of a channel. The front light is lower and the rear light is higher. If the lights are not aligned, you are off the centre line. Transits work the same way but use natural or artificial objects rather than lights — for example, two church spires or two beacons. Leading lines are shown on charts as dashed lines with the bearing.

Electronic charts and chart plotters

Electronic charts (ENCs and raster charts) display the same information as paper charts on a chart plotter or navigation app. Electronic charts offer advantages including real-time GPS position, automatic route planning, and depth alarms. However, they should never completely replace paper charts — a chart plotter can fail, and electronic charts can contain errors. Always carry paper charts for the area you are cruising and know how to navigate with them. For more on electronic navigation tools, see our boating apps guide.

Practical navigation: plotting a course

To plot a course on a paper chart, draw a line from your current position to your destination. Measure the bearing of this line with a parallel ruler or plotting compass — this is your course to steer. Check the line for hazards (shallow water, rocks, restricted areas) and adjust the route as needed. Estimate the distance using the chart scale and calculate your estimated time of arrival based on your boat speed. In tidal waters, you will need to account for tidal set and drift — see our tides and currents guide for more on this.

Marine charts and navigation aids are the foundation of safe boating. Every boater should know how to read a chart, recognise buoyage, and interpret light characteristics. For those new to boating, consider taking a navigation course — see our sailing certification guide for recommended courses.