How to read a nautical chart: five first checks for beginners

A nautical chart is a map built for navigation on water. It shows depths, the shape of the coast, aids to navigation and potential hazards. Before interpreting any symbol, identify the chart's area, scale, source and update status. The details you can use for a harbour approach may not appear on a chart intended for a broad coastal overview. NOAA explains that overview, coastal, approach and harbour charts serve different purposes.

1. What area and scale does this chart cover?

Start with the chart title or electronic chart information. Locate your departure point, destination and the waters between them. A larger-scale harbour chart shows more local detail than a small-scale regional chart. Zooming into a screen does not create surveyed detail that the source chart lacks. Choose an official chart at the appropriate scale for each part of a route and make sure it is current.

2. What do the numbers in the water mean?

The numbers are usually soundings, or measured water depths. Lines connecting depths are depth contours; they help show the shape of the seabed. NOAA's beginner explanation describes both. Check the units: charts may use metres, feet or fathoms, and a fathom equals six feet.

A sounding is not a promise of the water depth beneath your keel at this moment. Charted depths use a stated chart datum, a reference water level. Actual water level varies, and a safe depth assessment also depends on the vessel's draft, tide or water-level information, and local conditions. NOAA's Chart No. 1 is a reference for chart notation; use the chart notes and hydrographic authority for the specific area you are sailing.

3. Which marks need a closer look?

Scan for shallow areas, rocks, wrecks, obstructions, restricted areas, channels, lights, buoys and beacons. Do not guess a mark's meaning from its colour or shape alone: chart portrayal can differ between products and jurisdictions. Open the legend or the responsible hydrographic office's symbol guide. For US products, U.S. Chart No. 1 explains the symbols used on paper and electronic charts.

4. Is the chart current?

Coasts, depths and navigation aids can change. NOAA says critical chart corrections may cover new rocks, wrecks and shoals or changes to aids to navigation. Check the latest official chart updates and local notices before relying on the chart for a passage. An attractive map or an old downloaded image can be useful for orientation while still missing a current hazard.

5. What can MARLOG help with?

MARLOG's Atlas helps you explore coastal places, with depth contours and seamarks in its map view. Its Academy includes a course on nautical-chart symbols and abbreviations. These are useful ways to learn the vocabulary and investigate an area. MARLOG Atlas is a discovery and reference instrument, not an official navigation chart. Use current official charts and local notices for navigation decisions.

If a chart symbol is unfamiliar, the best first action is to look it up in the chart's official key rather than infer what it means. Explore MARLOG Academy or get MARLOG for iPhone.

Sources: NOAA, what chart numbers mean; NOAA, U.S. Chart No. 1; NOAA, electronic chart scales; NOAA, chart updates. MARLOG feature descriptions were checked against its live Atlas and Academy pages on September 22, 2026.