liminfoGEBCO Reference
Free reference guide: GEBCO Reference
About GEBCO Reference
The GEBCO Bathymetry Reference is a comprehensive guide to the General Bathymetric Chart of the Oceans, covering the GEBCO 2023 Grid at 15 arc-second resolution (~450m), the TID (Type Identifier) Grid for data provenance tracking, and the SRTM15+ satellite altimetry-based bathymetric model. It provides detailed explanations of survey technologies including multibeam sonar (Kongsberg EM122/EM710, Reson SeaBat), singlebeam echosounders, airborne LiDAR bathymetry (ALB), and satellite altimetry from Jason, Sentinel-6, and CryoSat-2 missions.
This reference covers essential data processing workflows for ocean scientists, hydrographic surveyors, and marine engineers. You will find guidance on working with GEBCO NetCDF-4 data in WGS84 (EPSG:4326), converting to GeoTIFF with GDAL, generating bathymetric contours, and performing slope/aspect analysis for seafloor habitat mapping and submarine cable route planning. Visualization examples use GMT (Generic Mapping Tools), QGIS, and Python with xarray and matplotlib.
The reference also documents major ocean mapping initiatives including the Nippon Foundation-GEBCO Seabed 2030 project aiming to map 100% of the ocean floor, the IHO Data Centre for Digital Bathymetry (DCDB), and the Crowdsourced Bathymetry (CSB) program. Seafloor feature terminology follows the SCUFN (Sub-Committee on Undersea Feature Names) IHO B-6 standard, covering seamounts, guyots, trenches, mid-ocean ridges, and abyssal plains.
Key Features
- Complete GEBCO 2023 Grid specifications including 15 arc-second resolution, NetCDF-4 format, and WGS84 coordinate system details
- Survey technology breakdowns for multibeam sonar, singlebeam sonar, satellite altimetry, and airborne LiDAR bathymetry with equipment examples
- TID Grid interpretation guide mapping type identifiers (0-70) to data sources such as singlebeam, multibeam quality grades, and satellite-predicted depths
- Step-by-step data processing workflows covering GDAL conversion, area extraction, slope/aspect analysis, and profile cross-section generation
- Visualization tutorials for GMT grdimage/grdcontour commands, QGIS NetCDF raster loading, and Python xarray/matplotlib bathymetric plotting
- Seafloor feature classification following IHO SCUFN standards including seamounts, guyots, trenches, ridges, continental shelves, and abyssal plains
- Seabed 2030 project details including four regional mapping centers (Stockholm, Lamont, NIWA, AWI) and current global mapping coverage statistics
- Hypsometric analysis methods for depth-based area and volume calculations useful for sea-level change scenario modeling
Frequently Asked Questions
What is the GEBCO 2023 Grid and what resolution does it provide?
The GEBCO 2023 Grid is a global bathymetric dataset at 15 arc-second resolution (approximately 450 meters) in NetCDF-4 format. It uses WGS84 (EPSG:4326) coordinates with an elevation variable where negative values represent ocean depth and positive values represent land elevation. The full global grid is approximately 11 GB in size.
What does the GEBCO TID Grid tell you about data quality?
The Type Identifier (TID) Grid assigns a numeric code to each grid cell indicating its data source: 0 for land, 10 for singlebeam sonar, 11-17 for multibeam sonar quality grades, 40 for satellite altimetry-predicted depths, and 70 for directly surveyed depths. This allows users to assess the reliability and provenance of each bathymetric measurement.
How do I convert GEBCO NetCDF data to GeoTIFF for use in GIS?
Use GDAL with the command: gdal_translate -of GTiff GEBCO_2023.nc gebco.tif. To extract a specific region, add the -projwin flag with coordinates: gdal_translate -projwin 124 38 132 33 gebco.tif east_sea.tif. The output retains the EPSG:4326 coordinate system and is compatible with QGIS, ArcGIS, and other GIS software.
What is the difference between multibeam and singlebeam sonar for bathymetric surveys?
Multibeam sonar fires 100-800+ simultaneous acoustic beams at frequencies from 12 kHz (deep water) to 400 kHz (shallow water), covering a swath width of 2-6 times the water depth with resolution of 1-2% of depth. Singlebeam sonar uses a single beam to measure depth along a track line using the formula D = (c * t) / 2, where c is sound velocity (~1500 m/s). Multibeam provides full-coverage seafloor maps while singlebeam only measures along survey lines.
How can I visualize GEBCO data using GMT (Generic Mapping Tools)?
Use GMT 6 modern syntax: gmt grdimage @earth_relief_15s -JM15c -R120/150/20/50 -Cgebco -png map. Key commands include grdimage for raster visualization, grdcontour for bathymetric contour lines, grdinfo for metadata inspection, and grdcut for extracting sub-regions from the global grid.
What is the Seabed 2030 project and how much of the ocean floor has been mapped?
Seabed 2030 is a Nippon Foundation-GEBCO collaborative project aiming to map 100% of the world ocean floor by 2030. Currently approximately 25% of the seafloor has been mapped with direct measurement data. The project operates through four regional centers: Arctic/North Pacific (Stockholm), Atlantic/Indian Ocean (Lamont), South/West Pacific (NIWA, New Zealand), and Southern Ocean (AWI, Germany). It also accepts crowdsourced bathymetry contributions.
How does satellite altimetry predict ocean depth without direct measurement?
Satellites such as Jason, Sentinel-6, and CryoSat-2 measure sea surface height variations. These variations correlate with gravitational anomalies caused by underwater topography. Large seafloor features like seamounts and ridges produce detectable gravity signals that can be inverted to estimate bathymetry. The method achieves approximately 3-5 km horizontal resolution but has limitations with small features and sediment-covered regions, with prediction errors of 100-300 meters.
What is a Sound Velocity Profile (SVP) and why is it important for bathymetric surveys?
A Sound Velocity Profile measures how the speed of sound changes with depth due to variations in water temperature, salinity, and pressure. Without SVP correction, depth measurements can have 1-3% error. SVPs are measured using instruments like XBT (expendable bathythermograph), CTD sensors, or dedicated sound velocity probes. In multibeam surveys, SVP data is critical for correcting beam refraction to ensure accurate depth calculations across the full swath width.