How to Read the Numbers on a Nautical Chart

Look at a nautical chart and you will see water covered in small numbers. Each one is a depth. Those numbers, called soundings, are how a chart shows a mariner what lies beneath the surface.

Soundings, or water depth measurements, appear as numbers on nautical charts. Credit: NOAA.

Soundings

Soundings are water depth measurements, usually collected by hydrographic surveys. They can be given in feet or in fathoms — the chart says which. A fathom is a nautical unit of measurement equal to six feet.

Depth contours

Depths on a chart may be joined by lines called depth contours, like the contour lines on a topographic map of land. Together they give the mariner a picture of the bottom.

Everything else on the chart

Besides depths, a nautical chart shows dangers to navigation, aids to navigation, anchorages and other features. U.S. Chart No. 1 explains every number and symbol on both paper and electronic nautical charts.

Sources

Based on "What do the numbers mean on a nautical chart?" from NOAA's National Ocean Service, a work of the United States government in the public domain.