Where Rhumb's depths come from

Rhumb shows depth shading, contour lines, and soundings on the free chart, everywhere in the world. This page explains where these depths come from, how Rhumb draws them, and what they cannot show. Please read it before you rely on a depth in Rhumb.

What you see on the chart

• Depth shading in four shades of blue, for 0–2 m, 2–5 m, 5–10 m, and 10–20 m. Deeper water keeps the plain sea colour.
• Contour lines at 2, 5, 10, 20, and 50 m, labelled with their depth.
• Soundings in metres, in the style of a paper chart, with the tenths in a small figure below 21 m. They appear as you zoom in. Soundings deeper than 50 m are spaced further apart and show whole metres.

Rhumb shows no soundings shallower than 1 m. Close to the shore the grids hold only an estimate that slopes down to the coastline, so the 0–2 m shading and the drying areas show these shallows instead.

Where the depths come from

• EMODnet Bathymetry (Digital Terrain Model 2024) covers the seas around Europe, including the Mediterranean, the Baltic, and the Black Sea, and the eastern Caribbean. Its grid cells are about 115 m across. It combines surveys and national depth models from many countries. Its depths refer to Lowest Astronomical Tide where its sources allow, which is the level most tidal charts in Europe use for their soundings.
• GEBCO (2026 Grid) covers the rest of the world. Its grid cells are about 460 m across. Its depths refer to about mean sea level, which is higher than the datum of a tidal chart. At low tide the water can be shallower than Rhumb shows, by up to about half the spring tidal range.

Both are public scientific datasets. EMODnet is published under the CC BY 4.0 licence, and GEBCO is in the public domain. Both say that their data must not be used for navigation, and Rhumb labels its depths in the same way.

How Rhumb chooses a sounding

Rhumb divides the sea into cells about 460 m from north to south and shows the shallowest grid value in each cell. Soundings from 1 to 50 m appear only in cells where the grid has measured data. In EMODnet that means cells built from its own sources, mostly surveys and national depth models. In GEBCO it means ship surveys, lidar, and chart soundings, and in some clear, shallow waters also depths measured from satellite images.

The shading, the contour lines, and the soundings deeper than 50 m use all the data. Where nobody has surveyed, that includes depths that GEBCO estimates from satellite measurements of the sea surface.

Each number sits at a fixed spot inside its cell, so the soundings do not line up in rows. The number gives the shallowest value of its cell, but its position inside the cell is approximate.

What the depths cannot show

• Each grid cell holds one value for an area 115 or 460 m across, usually an average of the survey data inside it. A rock, a wreck, or a narrow shoal inside the cell can be much shallower than that value.
• Harbours, marinas, pontoons, and dredged channels are too small for these grids. Use the harbour chart and the pilot book there.
• Sandbanks move. A survey can be many years old, and the bank may lie somewhere else today.
• Some coasts have no measured shallow data in either grid. There you see shading and contour lines, but no shallow soundings.
• Water mapped inside the coastline, such as rivers, lakes, and some lagoons, has no depth data. Rhumb shades it like 0–2 m water.
• Rhumb does not add the height of tide. Use the tide tables for the depth at a given time.

How to use them

Use the depths in Rhumb to see the shape of the seabed: where the water gets shallow, where a bank lies, and which side of a bay is deeper. Before you go close to anything shallow, check the official chart, the pilot book, and your depth sounder. Rhumb's routes avoid land and do not check depth.

What comes next

Next to the average, EMODnet also publishes the shallowest survey value of each of its cells. A coming update of the depths will use it for soundings, so that a sounding leans to the shallow side, as a chart sounding does. GEBCO publishes a new grid every year and EMODnet every two years, and Rhumb rebuilds its depths when they do.

Credits

Depth shading, contour lines, and soundings are derived from EMODnet Bathymetry (DTM 2024 and Caribbean DTM 2024, CC BY 4.0, doi:10.12770/cf51df64-56f9-4a99-b1aa-36b8d7b743a1) and the GEBCO 2026 Grid (GEBCO Compilation Group 2026, doi:10.5285/4f68d5c7-45eb-f999-e063-7086abc036fa).