How Deep is the Ocean?
September 17, 2015
“How deep are Earth’s oceans?”
Satellite altimetry data is used to fill in gaps where multibeam echo sounder data is unavailable. Credit: NOAA
This may seem like a simple question, but it’s not. In fact, the surfaces of Mars, Venus and Earth’s moon are better mapped than our own planet.
These planetary bodies all have dry surfaces, making their topography easily mapped by laser and radar measurements. 71% of Earth’s surface, on the other hand, is covered by oceans. Because of the difficulty of mapping through water, we know far less about the shape of the ocean floor.
Bathymetry is the measurement of underwater depths of lake and ocean floors and is crucial for the study of Earth’s surface and processes. The direct measurement of these ocean depths is most accurately done by multibeam echo sounders carried aboard ships conducting surveys.
However, current survey lines sparsely cover the oceans, leaving substantial gaps between surveys and rarely reaching Earth’s remote oceans such as the Arctic and Antarctic.
In fact, it is estimated that it would take approximately 900 years for a single ship to obtain complete multibeam coverage of the world’s ocean floors.
Gravity anomalies derived from altimeter data are used to estimate bathymetric depths. Credit: NOAA
So how do scientists create a complete ocean floor map? They use satellites!
Seamounts 2 km tall or taller are easily detected in maps derived from satellite altimetry data.
Hypsometric analysis reveals that 50% of Earth’s surface is comprised of seafloor located 3,200 meters below mean sea level.
It would take approximately 900 years of surveying by a single ship to obtain complete multibeam coverage of the world’s ocean floors.
GEBCO_14 is a new digital bathymetric model of the world ocean floor merged with land topography from publically available digital elevation models. Credit: GEBCO
Satellite altimeters measure sea surface height. When there is a significant enough change in sea height, a gravity anomaly can be computed. Combined, these data provide estimated maps for the deepest and most remote parts of the world's seafloor. While not as accurate as multibeam echo sounders, altimetry bathymetry is vital for filling in the vast data gaps that currently exist.
To fully understand our Earth’s surface using all of this collected data the General Bathymetric Chart of the Oceans (GEBCO) has been working with partners around the world, including NOAA scientists and satellites, to provide the most authoritative, publicly-available bathymetry data sets.
In December 2014 GEBCO released the GEBCO_14 Grid. This grid is a new digital bathymetric model of the world ocean floor merged with land topography from publically available digital elevation models. It provides a significant update to the previous GEBCO_08, last updated in 2010, by incorporating new versions of regional measurements from a number of data sources.
Approximately 33% of the ocean grid cells (not ocean area) have been updated. The grid is the result of an international collaboration of bathymetric data providers and grid developers at institutional, national, and regional levels.
The Source Identifier (SID) grid can be used to idetify the source of data used to generate the GEBCO_14 grid for individual grid cells. C.F.: A new digital bathymetric model of the world's oceans doi: 10.1002/2015EA000107.
The IHO-IOC GEBCO Cook Book provides beginner information on how to produce a grid for xyz data, bathymetric fundamentals, and advanced topics for experienced users.
For more on satellite altimetric bathymetry visit NESDIS’ Laboratory for Satellite Altimetry page here!
Also visit GEBCO’s home page to learn more by clicking here!