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 from 2 m down to 5000 m, labelled with their depth. In US waters the chart shows NOAA's own contours.
• 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 sources 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.
• EMODnet high-resolution DTMs (HR-DTMs) give finer detail in some European waters. They are CC BY 4.0, © EMODnet Bathymetry and their data providers. Contains Irish Public Sector Data (Geological Survey Ireland & Marine Institute) licensed under a Creative Commons Attribution 4.0 International (CC BY 4.0) licence.
• Kartverket depth data covers Norwegian waters. It is licensed under CC BY 4.0. © Kartverket.
• NOAA chart data covers US waters. Source: NOAA Office of Coast Survey ENC data. These are not official NOAA charts. See the NOAA data agreement.
• Regional survey grids give finer detail in Brittany (Litto3D), the German federal waterways (DGM-W), the US Virgin Islands (NOAA CUDEM), New Zealand coasts (LINZ) and the Great Barrier Reef.
• Satellite estimates fill the Mediterranean shallows, down to about 8 m, where no survey reaches them. Rhumb estimates these depths from the colour of the water in Copernicus Sentinel-2 images and fits them to NASA ICESat-2 laser depths. They are estimates: they stay below every survey and give no 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. Credit: GEBCO Compilation Group 2026.

These are open datasets. EMODnet and Kartverket are published under the CC BY 4.0 licence, and GEBCO is a public domain grid. Rhumb uses the finest source that is available for each area. None of the sources is official chart data, and Rhumb's depths must not be used for navigation.

How Rhumb draws depths

Outside US waters, contour lines and bands come from one smooth surface for each area. This surface joins the finest sources that are available there. In US waters the chart shows the depth areas and contour lines of NOAA. The colour of a NOAA area follows the shallow edge of that area, so a colour edge and a contour line can differ.

Each sounding sits on the real shallowest measured cell or survey sounding inside its cell (in US waters, on a NOAA sounding), with no random offset. Its position on the chart is the position of that measurement.

Soundings shallower than 50 m appear only where a source measured the depth. Where nobody has surveyed, the shading and contour lines can use depths that GEBCO estimates from satellite measurements of the sea surface, but Rhumb shows no shallow soundings there.

What the depths cannot show

• Each grid cell holds one value for an area up to 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 the seabed 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 any source. 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

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). Finer detail comes from the EMODnet high-resolution DTMs (CC BY 4.0, © EMODnet Bathymetry and their data providers), Kartverket (CC BY 4.0, © Kartverket) and NOAA (Source: NOAA Office of Coast Survey ENC data. These are not official NOAA charts.).

Mediterranean shallows are partly estimated from satellite water colour (contains modified Copernicus Sentinel data 2023–2026), fitted to NASA ICESat-2 ATL24 laser depths (NSIDC).

Regional grids: Litto3D Bretagne (Shom – IGN, 2024, doi:10.17183/LITTO3D_BZH_2018_2021, Licence Ouverte / Etalab 2.0); DGM-W (Wasserstraßen- und Schifffahrtsverwaltung des Bundes, GeoNutzV); Great Barrier Reef 30 m bathymetry (Beaman, R.J. 2020, Geoscience Australia and James Cook University, CC BY 4.0); New Zealand coastal LiDAR DEMs (Toitũ Te Whenua Land Information New Zealand and regional councils, CC BY 4.0); US Virgin Islands CUDEM (NOAA NCEI, public domain).

Contains Irish Public Sector Data (Geological Survey Ireland & Marine Institute) licensed under a Creative Commons Attribution 4.0 International (CC BY 4.0) licence.