2 min read · Updated 25 June 2026
A nautical chart is a map of the sea, made for navigation. Where a road map shows roads and towns, a chart shows what a sailor needs: water depths, the coastline, hazards, and the navigation marks that guide vessels. Learning to read one is a foundation of seamanship — and thanks to a standard international symbol set, a chart can be read anywhere in the world.
Depths and datum
The most striking feature of a chart is the numbers scattered across the water — the soundings, or depths, at each point. Crucially, these are measured relative to a low-water level called the chart datum, chosen so the real depth rarely falls below what the chart shows. To find the actual depth at a given moment, you add the height of tide to the charted sounding. Depth contours join points of equal depth, drawing a picture of where the water shoals — invaluable for staying in safe water.
Hazards and marks
Charts warn of the things that can end a voyage:
- Rocks (above and below water), wrecks, shoals, reefs and obstructions, each with its own symbol.
- The navigation marks — buoys, beacons and lights — that guide vessels and warn of dangers, explained in our guide to buoyage.
The coastline, harbours, prominent landmarks and a compass rose for taking bearings complete the picture.
Reading the symbols
A chart's information is packed into a standard set of symbols and abbreviations — internationally agreed so a chart is legible worldwide. Depths, seabed type, light characteristics, mark shapes and colours all have their own notation. Learning the common ones (published in the reference known as "Chart No. 1") turns a chart from a jumble of marks into a clear picture of the sea.
Paper and electronic
Modern sailors mostly navigate with electronic charts on a chartplotter or app, which show the same information and add the boat's live GPS position from its instruments. That makes navigation far easier — but the underlying skills are identical, and prudent sailors still know how to read a chart and cross-check rather than trusting the screen blindly. For the rest of the vocabulary, see the sailing terms glossary.
Frequently asked questions
- What is a nautical chart?
- A nautical chart is a specialised map of the sea, made for navigation. Where a road map shows roads, a chart shows water depths, the shape of the coastline and seabed, hazards such as rocks and wrecks, and the navigation marks — buoys, beacons and lights — that guide vessels. It uses a standard international set of symbols so it can be read anywhere in the world.
- What do the numbers on a nautical chart mean?
- The numbers scattered across the water areas are soundings — the depth of water at that point, shown relative to a low-water level called the chart datum. Because they are measured to a low datum, there is usually a bit more water than the chart shows, and the tide adds to it. Depth contours join points of equal depth, showing where the water shoals.
- What is chart datum?
- Chart datum is the reference level that charted depths are measured from — usually a low-water level, chosen so the actual depth rarely falls below what the chart shows. To find the real depth at a given time, you add the height of tide (from the tide tables) to the charted sounding. This conservative approach helps keep vessels from running aground.
- What hazards are shown on a chart?
- Charts mark hazards such as rocks (above and below water), wrecks, shoals, reefs and obstructions, along with the navigation marks that warn of them. They also show depth contours so you can see where the water shoals, and features like the coastline, harbours and prominent landmarks. Reading these together lets a navigator plan a safe route.
- Are electronic charts the same as paper charts?
- Electronic charts on a chartplotter or app show the same information as paper charts — depths, hazards, marks — and add the boat's live GPS position, which makes navigation far easier. But the underlying symbols and the need to understand depth, datum and hazards are the same, and prudent sailors know how to read a chart and cross-check rather than trusting the screen blindly.