Ocean Depth Map help

Not for navigation. The depths are approximate values from a global seafloor model, not soundings. Use official nautical charts, your depth sounder and local knowledge to navigate.

What it shows

• The big number is the approximate water depth under your GPS position (or the elevation, when you're on land), in feet, meters or fathoms.
• The chart shades deeper water darker and draws labeled depth lines. Tap anywhere (or drag the marker) to read the depth at that spot. Back to me returns to your position.
• NOAA chart (Settings): in US waters you can switch on NOAA's official nautical chart display, with its soundings, buoys and channels. It's a viewer only, and NOAA says it is not certified for navigation either.

Where the depths come from, and how good they are

• Depths come from the GEBCO_2026 Grid, a free global model of the seafloor and land made by the GEBCO Compilation Group (GEBCO Compilation Group (2026) GEBCO_2026 Grid, doi:10.5285/4f68d5c7-45eb-f999-e063-7086abc036fa). Its cells are about 450 m (1,500 ft) across, so one number covers a whole cell: small reefs, rocks, sandbars and channels narrower than that don't show.
• Near coasts and on many banks, GEBCO's values are often estimated from sparse measurements or satellites, and can be off by meters or more. Shallow water may be shown deeper or shallower than it is. The app shows the cell size next to every reading so you can judge it.
• Depths are relative to mean sea level; tides are not included.
• Lakes and rivers: not covered, including the Great Lakes (the global model holds lake-bed heights there, not water depths). The app shows them flat and gives no depth.
• GEBCO, the IHO and the IOC have not reviewed or endorsed this app. GEBCO data is in the public domain.
• Place names and roads: OpenFreeMap © OpenMapTiles, data © OpenStreetMap contributors. US charts: NOAA Office of Coast Survey (not certified for navigation). Map engine: MapLibre.

Using it offline

• Every area you have looked at keeps working without a signal, at the zooms you viewed.
• Before you head offshore, open Settings → Save this area for offline while you still have a connection. It keeps the depths for the whole visible area at every zoom.
• Areas you haven't viewed or saved need a connection. Place names and the NOAA chart work offline only where you have already looked at them.
• On a website, add depth.hertzmc.com to your home screen so your phone keeps its saved areas.

Location and privacy

Your position stays on your phone: the depth is worked out on the device. The map downloads tiles for the area you look at. Details: privacy policy.

Contact

Questions, ideas or a wrong depth somewhere? Email depth@hertzmc.com.