Frequently asked questions (FAQ)
The oceans cover more than two thirds of the world's surface. For over one hundred years GEBCO has been involved in producing maps and digital data sets of this terrain.
Find out more about GEBCO, our data sets and products, and how ocean depths (bathymetry data) are measured, collected and used.
- What is GEBCO and what do we do?
- GEBCO's data sets and products
- GEBCO's grids and vertical datum
- How can I access GEBCO's data sets?
Bathymetry data (ocean depths)
- What is bathymetry?
- Why do we need to measure depths in the oceans?
- How do you measure depths?
- Acoustic methods — using sound waves
- 'Measuring' bathymetry from space, why and how
- Creating a bathymetric grid
- Using bathymetric soundings and satellite-derived gravity data to create a bathymetric grid
- What does map and data set 'resolution' mean?
- what are grid artifacts
- How can we know what areas show measured depths of the seabed and what areas are 'predicted' depths?
Useful links for further information on bathymetry data and projects studying our oceans
GEBCO is an acronym for the General Bathymetric Chart of the Oceans. We are an international group of geoscientists and hydrographers who work on the development of a range of bathymetric data sets and data products. Our aim is to provide the most authoritative publicly-available bathymetry of the world's oceans.
GEBCO operates under the joint auspices of the Intergovernmental Oceanographic Commission (IOC) (of UNESCO) and the International Hydrographic Organization (IHO).
Our work is directed by a Guiding Committee and supported by sub-committees on ocean mapping and undersea feature names plus ad hoc working groups.
We are involved in training a new generation of scientists in bathymetric mapping through the GEBCO/Nippon Foundation training programme.
Prince Albert I of Monaco initiated the first GEBCO chart series at the beginning of the 20th century. More information about the early years of GEBCO can be found in the History of GEBCO book.
GEBCO makes available a range of bathymetric data sets and products. Find out more
- global grid files
- GEBCO_2014 Grid — global grid at 30 arc-second intervals. The grid is largely based on ship track soundings with interpolation between soundings guided by satellite-derived gravity data. However, in areas where they improve on the existing grid, data sets generated by other methods have been included. The grid is accompanied by a Source Identifier (SID) Grid which identifies which grid cells are based on soundings and which have been interpolated.
- GEBCO One Minute Grid — one arc-minute interval grid, largely based on the bathymetric contours contained in the GEBCO Digital Atlas
- Global set of digital bathymetric contours, contained within the GEBCO Digital Atlas
- GEBCO Digital Atlas
- GEBCO Gazetteer of Geographic Names of Undersea Features
- GEBCO world map
GEBCO's global elevation models are generated by the assimilation of heterogeneous data types assuming all of them to be referred to mean sea level. However, in some shallow water areas, the grids include data from sources having a vertical datum other than mean sea level. We are working to understand how best to fully assimilate these data.GEBCO's grids are not to be used for navigation or for any other purpose involving safety at sea.
GEBCO's gridded data sets can be downloaded via the internet. Select to download the complete global grid files or data for a user-defined area. Find out more about the grids and how to access the data.
The grids are made available in netCDF form, which can be used by software packages such as Generic Mapping Tools (GMT). Free software is made available for viewing and accessing data from the grids in ASCII as well as netCDF.
GEBCO's gridded data sets, along with a global set of digital bathymetric contours, are included as part of the GEBCO Digital Atlas (GDA). The GDA includes software for viewing and accessing the data sets and for exporting the data in a number of formats.
GEBCO makes available a global data set of geographic names of undersea features. It is maintained and updated through the work of the GEBCO Sub-Committee on Undersea Feature Names (SCUFN). The gazetteer is available to download from the web along with information about how to submit name proposals for newly-discovered features on the seafloor.
The GEBCO world map shows the bathymetry of the world's ocean floor in the form of a shaded relief colour map. It can be accessed from the web in JPEG form.
Bathymetry is the study or representation of the land or other solid surface beneath a layer of liquid. As such, bathymetry can include the depths of oceans and lakes.
GEBCO's focus is on the bathymetry of the world's oceans.
The oceans cover nearly two thirds of the earths surface and have a great influence on our climate and our everyday lives. Far below the ocean's surface, the ocean floor is a varied terrain consisting of deep trenches and long mountain chains — formed by the geological processes that shape the Earth.
This terrain helps to steer ocean currents and influences circulation and mixing patterns that in turn affect our climate and influences hazards facing coastal communities (like approaching tsunamis).
The image below shows the curving feature of the Mariana Trench, which includes the deepest part of the oceans.
In shallower water regions, an accurate knowledge of seafloor depths is important for navigational purposes and for warnings of hazards to shipping. It is also important for offshore resource exploration and exploitation (such as fisheries and hydrocarbons), hazardous waste disposal, and for general marine spatial planning such as for Marine Protected Areas and Marine Conservation Zones. The ocean floor also holds a reservoir of natural resources such as oil and gas.
Bathymetry data was first collected in the deep oceans in the middle of the 19th century using simple plumb-lines (weighted twine or rope), marked at regular intervals to give the depth.
The introduction of acoustic measuring systems (single-beam echo-sounders) in the 1930s led to a huge increase in the volume of data collected. Today, multi-beam echo-sounder systems can map swathes of the ocean floor, collecting large volumes of data.
'The History of GEBCO, 1903-2003' book provides information about the early days of bathymetric mapping.
To store the increasing volumes of digital bathymetry data being collected and to make this generally available, the IHO setup the Data Center for Digital Bathymetry (IHO DCDB), hosted at the US National Centers For Environmental Information (NCEI), in June 1990.
The IHO DCDB operates a worldwide digital data bank of oceanic soundings on behalf of the member countries of the IHO. This digital data bank is maintained in several databases, including the GEODAS global marine geophysical database, and the Hydrographic Survey Data System.
The image below shows the coverage of ship-track data held in the GEODAS database at NCEI.
Even with the increase in the volumes of bathymetry data, there are still large areas of the ocean floor that have not been fully surveyed. Data are often concentrated along isolated ship tracks and there may be many miles between tracks.
In areas of sparse sounding data coverage, satellite altimetry data can help us map the ocean floor. This measures the height of the sea surface. Small variations in sea height can be related to changes in the earth's gravity field. This in turn can be related to the shape of features on the ocean floor. Find out more.
These systems measure broad swathes of the seabed (up to 10 kilometers across in very deep water) to accuracies of better than a metre or two. Very high accuracy systems measure narrower swathes and shallower water but have a depth accuracy of up to a couple of centimetres.
Since the early 20th century the ocean depths have been mapped using acoustic methods, i.e. using sounding waves.
This is done by sending a sound pulse from a vessel and listening until the echo from the seabed is heard. The depth is then found by dividing the speed of sound (approximately 1,500 meters per second) by half of the time it takes for the echo to be heard (recorded). We use half the time as the total time would include the time from the ship to seabed and back again, and we just want to find the one-way distance.
These early depth measuring systems give the depth at a single point, usually below the keel of the vessel, and are called single-beam echo-sounders.
Below you can see the theory of a single-beam echo-sounder.
From the late 1970’s onward, multibeam bathymetric mapping has become more and more common. This differs from single-beam acoustic soundings in that a 'swath' of sound is transmitted below and to either side of the survey vessel. This means that rather than a series of points being recorded just below the vessel as seen in the picture above, a swath or broad corridor of the seabed is mapped. The depths across this corridor are automatically logged onto a computer for data processing, cleaning and ultimately map making.
The image below shows the theory of multibeam or swath bathymetric mapping systems — note that they give a complete picture of the seabed rather than the series of single points of a single-beam echo-sounder.
Information about how single-beam and multibeam echo-sounders operate can be found at the Woods Hole Oceanographic Institution's web site.
An important component of measuring depth is positional information for the soundings, i.e. recording the longitude and latitude of where the soundings were measured. There have been great advances from the early days of recording positional information from astronomical observations to radio navigation and then to the present day and systems such as the Global Positioning System (GPS) which can record positions to within a few metres.
Errors in position information can be one of the causes of errors or 'artifacts' in bathymetric maps.
The world’s oceans are vast and it would take many years to systematically survey the global ocean floor using single-beam or multibeam echo-sounders. It has been estimated that a systematic survey of the oceans by ships would take more than 200 years of survey time at a cost of billions of U.S. dollars.
Using satellite-derived gravity data can help with mapping work in regions of sparse ship-track sounding data coverage. Although the satellites cannot 'see' the ocean floor they can observe gravity anomalies which can be correlated with ocean floor topography.
The process involved and advantages and limitations of 'measuring' bathymetry from space are discussed below.
The following explanation and figures are excerpts taken from a report entitled 'Bathymetry from Space', a summary of a workshop held in La Jolla, California in October 2002. The full citation for the report appears at the end of these paragraphs.
The ocean’s surface has broad bumps and dips that mimic the topography of the ocean floor. The extra gravitational attraction of seafloor features such as seamounts produces minor variations in gravity, which in turn produce tiny variations in ocean surface height. As tiny as they may seem, these bumps and dips can be mapped using a very accurate radar altimeter mounted on a satellite. In the deep ocean basins, where sediments are thin and seabed geology is simple, space radar data may be used to predict bathymetry. See the following pictures.
An Earth-orbiting radar in space cannot see the ocean bottom, but it can measure ocean surface height variations induced by ocean floor topography. The image above (A) shows how sea surface height can be measured from space.
A mountain on the ocean floor adds to the pull of Earth’s gravity and changes its direction subtly, causing extra water to pile up around the mountain. For example, a mountain on the ocean floor that is 2000 m tall produces a sea surface bump only 20 cm tall. Though small, this is measurable from space. The ultimate resolution of this method is limited by regional ocean depth. The schematic image below (B) shows the induced sea surface slope produced by a seamount on the ocean floor.
The tilt in the direction of gravity, called a 'deflection of the vertical', is equal to the slope of the sea surface, and is measured in microradians. One microradian of deflection appears as a 1 mm change in sea surface height per 1 km of horizontal distance.
However, there are some limitations in the correlation between gravity and bathymetry data. This can be influenced by sub-seafloor geology and variations in sediment thickness, i.e. with the original shape of the seafloor has been buried by the accumulation of sediment. Areas may be draped in sediment and gravity data may be capturing the 'buried' terrain.
Longer wavelength bathymetry is limited by isostasy and shorter wavelength to upward continuation.
Correlation is therefore stronger over rough topography in deeper ocean where sediment cover is thinner and weaker on continental margins and abyssal plains.
Report Citation: Sandwell,D.T., Gille,S.T., and Smith, W.H.F., eds., Bathymetry from Space:Oceanography, Geophysics, and Climate, Geoscience Professional Services, Bethesda, Maryland, June 2002, 24pp.,
For more information:
- Global seafloor topography from satellite altimetry from web pages at the US National Geophysical Data Center
- Oceanography magazine, volume 17, March 2004, Special issue: Bathymetry from Space
Traditionally this was done by drawing up a blank worksheet that covered the area of interest and then plotting the depth points at their appropriate locations in terms of latitude and longitude.
Until the 20th century there were so few depths recorded that almost all ocean depths were recorded as strings of numbers stretching across otherwise almost blank charts. With the advent of acoustic data recording in the 1930's the volume of soundings (as depth measurement are most commonly called) has increased phenomenally such that with increasing frequency survey tracks began to cross each other. Bathymetric contour lines, i.e. lines of equal depth, could then be drawn from the plotted depths.
Below is an example of a traditional plotting sheet covered in numbers (depths) at the locations where they were measured.
From these points a bathymetric contour map was produced. GEBCO used such sheets for the compilation of all its maps up to and including the 5th Edition which was published between 1975 and 1982.
Find out more about how bathymetry data are collected.
Using digital bathymetry data a 3D grid or terrain model can be made from the data. As discussed above, in regions of sparse data coverage satellite-derived gravity data can be used to help the guide the interpolation of depth values between the measured sounding points.
Bathymetric grids consist of data arranged in a 'grid' of cells. The majority of grids are structured so that all cells have exactly the same dimensions. However, some grids have cells of varying size and/or shape.
GEBCO's gridded data sets consist of grid cells at equally spaced intervals of longitude and latitude.
The images below show the arrangement of cells for a regular grid. The data values refer to the grid 'nodes', shown as black dots. Grid registration refers to the location of these nodes. Grid line registration means that nodes are centred on the intersection of the grid lines. Pixel centred registration means that the nodes are centred in the grid cells.
Ideally each cell would be based on a measured depth value. However, as discussed above, the world's oceans have not been fully surveyed by conventional echo-sounders. Therefore some of the grid cells may contain 'interpolated' values, i.e. a computer program has estimated what a reasonable value would be to assign to the cell based on the nearest measured depth values. These computer programs use 'gridding algorithms'.
From the gridded data sets, computer software packages can be used to model the terrain in 3D.
When creating a bathymetric grid, the first task is to carry out quality control work on the digital bathymetric sounding data sets to check for errors in positioning and depth measurement, which may show up as 'spikes' in the data set. An iterative process may then take place of gridding the data, carrying out checks on the resultant grid and then making any necessary corrections to the source data (e.g. removing 'bad' soundings) before re-gridding the data.
Please see the IHO-IOC GEBCO Cook Book for further information on developing bathymetric grids.
In areas of sparse sounding data coverage, satellite-derived gravity data can be used to help model the shape of the ocean floor, creating a grid of 'predicted' bathymetry.
The following details the method used to create a gridded data set using a database of ship-track soundings with interpolation between the soundings guided by predicted bathymetry. This process has been referred to as 'polishing'.
Initially, the predicted bathymetric grid is used to quality control the ship-track soundings database, looking for obvious errors in the data set, such as data 'spikes'.
To generate the combined predicted bathymetry and ship-track sounding grid, the value of the predicted bathymetry at the position of each measured sounding is subtracted from the sounding value, the difference is interpolated using a gridding routine. The value of the predicted bathymetry is added back to the interpolated difference. The result is a polished grid that passes smoothly through each median sounding and has the value of the predicted bathymetry far from the sounding.
- Becker, J. J., D. T. Sandwell, W. H. F. Smith, J. Braud, B. Binder, J. Depner, D. Fabre, J. Factor, S. Ingalls, S-H. Kim, R. Ladner, K. Marks, S. Nelson, A. Pharaoh, R. Trimmer, J. Von Rosenberg, G. Wallace, P. Weatherall., Global Bathymetry and Elevation Data at 30 Arc Seconds Resolution: SRTM30_PLUS, Marine Geodesy, 32:4, 355-371, 2009 DOI: 10.1080/01490410903297766
- W.H.F. Smith and D.T. Sandwell, Global Sea Floor Topography from Satellite Altimetry and Ship Depth Soundings, Science, VOL. 277, 26 September 1997
Please see the IHO-IOC GEBCO Cook Book for further information on developing bathymetric grids.
In ocean bathymetry, we use the term resolution to describe the level of detail that can be seen using various types of acoustic sounding.
There two components of resolution that are of prime importance, these are vertical and horizontal. The vertical component of bathymetric measurement deals with the absolute depth values that are recorded, while the horizontal component deals with the spatial separation between recorded depth soundings.
In terms of bathymetry, artifacts are distortions or other types of error that may appear in the gridded set. They may be produced by some form of processing error. This may occur in the primary data collection when the data are being processed to remove extraneous noise or during the gridding process.
A list of reported bugs for GEBCO's gridded data sets is maintained at the British Oceanographic Data Centre.
When viewing the GEBCO_08 Grid, there will be some areas that seem to show a much higher detail, and other areas that are relatively 'smooth'. Generally speaking, it is the smoother areas that are predicted whereas the areas of greatest detail are usually well-constrained by data.
Above is a section of the Reykjanes Ridge to the southeast of Iceland, note the apparent 'sharpness' of detail in the region running NE-SW, (which shows sub-sampled multibeam bathymetry data) compared to the much smoother areas to the southeast. There are a few thin 'stripes' or lines over this southeastern area and they represent single-beam echo-sounding lines that were used to constrain the satellite-predicted bathymetry.
Here is the same area as above, but showing the 'control' or real data points that were used. The depths of the areas between the lines were predicted using sea-surface deflections as described above in the section describing how you can measure the depth of the ocean from space.
The GEBCO Source Identifier (SID) Grid accompanies the GEBCO_2014 Grid. It identifies which grid cells in the GEBCO_2014 Grid are based on soundings or gridded data sets and which have been interpolated. The grid can be downloaded from the web.
Use the links below to find out more information about the bathymetry data and the ocean floor.
- Backscatter measurements by seafloor-mapping sonars - Guidelines and Recommendations
- The Jason Project
- Dive and Discover from Woods Hole Oceanographic Institution.
- Discover Polar and Marine Research from the Alfred Wegener Institute for Polar and Marine Research
- Educational resources from the US National Geophysical Data Center
- Learning and teaching from the UK National Oceanography Centre
- Outreach from the Center for Coastal and Ocean mapping at the University of Durham, New Hampshire
- JAMSTEC Kids - explore the wonders of the sea and earth from the Japan Agency for marine-earth science and technology (JAMSTEC)
- NOAA educational resources - from the US National Oceanic and Atmospheric Administration (NOAA)