How to read a nautical chart in 5 steps

A nautical chart is every sailor's indispensable roadmap. Even though modern vessels are equipped with GPS plotters and digital displays, a solid understanding of paper nautical charts remains compulsory and the cornerstone of maritime safety.

Whether you are casting off from the Abra of Getxo or working towards your skipper certification, mastering these 5 steps ensures you can plan any passage with complete confidence.

1. Orientation and Coordinates Frame (Latitude & Longitude)

Nautical charts are oriented with True North at the top.

The vertical margins (left and right) display latitude (North/South of the Equator). Remember the golden rule of maritime navigation: 1 minute of latitude arc equals exactly 1 nautical mile (1,852 meters).
The horizontal margins (top and bottom) display longitude (East/West of the Greenwich Meridian). Never use the longitude scale to measure distances, as meridians converge toward the poles.

2. Depth Soundings and Seabed Composition

The numbers across water areas represent soundings or depths in meters referenced to Chart Datum (Lowest Astronomical Tide).

Alongside soundings, depth contour lines connect points of equal depth (e.g., 5m, 10m, or 20m isobaths).
Charts also specify bottom characteristics with international abbreviations: A (sand), F (mud), P (rock), C (gravel), or Al (seaweed). This data is vital for assessing holding ground before dropping anchor.

3. Navigation Hazards and Obstacles

Recognizing submerged and exposed hazards prevents running aground:

Drying rocks: Rocks uncovered at low water and covered at high water (marked with an asterisk or cross with dots).
Submerged dangerous rocks: With known or unknown clearance under 2 meters.
Wrecks: Marked as partially or completely submerged ship hulls.
Restricted areas: Submarine cables, pipelines, and prohibited anchorage zones.

4. Aids to Navigation: Lighthouses, Buoys, and IALA System

Along the Basque coast and European Atlantic waters, IALA Maritime Buoyage System - Region A applies:

Port-hand marks: Red color, cylindrical topmark, red flashing light.
Starboard-hand marks: Green color, conical topmark, green light. When entering a harbor or estuary (such as entering the Abra of Getxo from open sea), keep red marks to port and green marks to starboard.
Lighthouses and sector lights: Listed with light characteristics, rhythm, period, and visual range in nautical miles (e.g., Fl(3) W 15s 23M).

5. Plotting True Courses and Correcting Magnetic Variation

To plot your passage (e.g., from Getxo Marina toward Plentzia or Castro Urdiales):

Use a parallel ruler to draw a straight line from origin to destination: this represents your True Course (TC).
Read the angle against the nearest meridian using a nautical protractor.
To steer by magnetic compass, correct the course for local magnetic variation (Var) and vessel deviation (Dev): Compass Heading (CH) = True Course (TC) - (Var + Dev)

With these 5 steps and regular practice on our training sails at Getxo Bela Eskola, reading a nautical chart becomes second nature.