BATHYMETRY, TOPOGRAPHY, AND RELIEF DATA
Beyond the continental slope is the abyss. The abyss contains plains, long mountains ranges (i.e., ocean ridges), isolated mountains (i.e., sea mounts), and ocean trenches—the deepest parts of the oceans. In the centers of some ocean ridges are long rift valleys, where earthquakes and volcanic eruptions are common. Some volcanoes that rise from the ridges appear above the surface as islands. Some sea mounts (also known as guyots) are extinct volcanoes with flat tops. Scientists believe these underwater mountains were once islands, whose tops were worn away by pounding surface waves. In time, they become part of the sea floor as they settled deeper under the water (as much as thousands of meters), while the crustal plate on which they ride moved away from their source of hot magma. The image below shows examples of some of the main features found on the ocean floor, as well as their above-water continuations (i.e., the image below is a slice through a relief map of world. Courtesy of Peter W. Sloss, NOAA/NGDC)
Today's oceanographers use sonar instruments to generate a sound signal that is bounced or "echoed" off the sea floor and then recorded on board the ship. The speed of sound in water is approximately 1,500 meters per second (four times faster than the speed of sound in air). By carefully measuring the round-trip time of the sound waves (taking into account the effects of temperature, pressure and salinity on sound speed), the depth of the water and/or the distance to another object can be accurately measured. When pulses are sent out and received in quick succession, an almost continuous recording of the ocean depth (i.e., a bottom profile) may be obtained. Furthermore, multiple sonar beams arrayed perpendicular to the survey ship's track can produce a swath of data points that contains far more useful information than a "simple straight-down look." In recent years, radar measurements of the sea surface from an orbiting satellite have been interpreted to measure the effects of variations in the Earth's gravitational field on the shape of the sea surface. Stronger gravity from shallow masses (like sea mounts and ridges) attracts water toward the object and creates a "hump." Weaker gravity where the dense sea floor is farther from the surface attracts less water and leaves a "depression" over trenches and abyssal plains. Although these satellite data yield no absolute depth measurements, they can be used to fill gaps between trackline data by providing relative "ups and downs."
It was not until the late 1940s, that researchers confirmed the existence of the huge mid-ocean ridge system that spans the globe like the seams on a baseball. Furthermore, the geological significance of the ridges' medial rift valley wasn't appreciated for another decade. Without global data on the shape of the Earth's surface, we still might not understand the processes that continue to alter the globe.
The scientific term used to describe the study of the ocean's depth (below sea level) is bathymetry (also note that you have bathymetry in lakes, which may or may not be above or below sea level, depending on the amounts of evaporation and precipitation that particular lake region experiences). Topography (i.e., geography), on the other hand, is a term used to describe the land above sea level. Lastly, global relief is used as a catch-all term to describe the Earth's entire surface, whether it be above or below sea level. Essentially, all of the Earth's surface is a continuum sharing similar features, it is just that some of them are above sea level, while others are submerged.
NGDC's Bathymetry, Topography, & Relief Data
NGDC archives and distributes public domain bathymetric data from several sources, including side-scan sonar and satellite data (i.e., ETOPO2 data and Sea floor Topography derived from Satellite Altimetry). On the other hand, NGDC collects public domain topography (i.e., digital elevation models) through such projects as TerrainBase (a research quality collection of digital terrain models), and the Global Land One-kilometer Base Elevation (GLOBE) Program (an international project to develop a global digital elevation model on a 30-second lat/lon grid—approximately 1-kilometer at low latitudes).
NGDC's bathymetry and topographic data and information are available to everyone including the general public, large engineering firms, scientific researchers, agriculture and government policy makers. To accommodate this large user group, NGDC's Marine Geology & Geophysics Division offers a wide range of both standard and custom data products. For each standard data product, NOAA provides a online "flier" or data announcement giving more information about the data set and instructions on how to order it. Many of the more popular standard data sets can be ordered directly from NGDC's online store. Online store links, where available, are provided on the data announcement Web page for the product. Many digital databases are offered on CD-ROM with full access software and regular updates. Also check NGDC's Anonymous FTP site for data, which can be directly downloaded from the Internet.
Products also include data in non-digital form (such as microfilm, paper copies, charts and maps, etc.) and a printed report series. Slide sets and printed posters are available depicting beautiful images of MGG data of various kinds, including color shaded relief of global bathymetry/topography. For pricing and order information, please contact Robin Warnken, NOAA/NGDC Mail Code E/GC3, 325 Broadway, Boulder, CO USA 80305-3328. Phone (303) 497-6338, Fax (303) 497-6513. NGDC can also produce custom images (on request) for many of the databases. Please contact Dr. Peter W. Sloss, NOAA/NGDC Mail Code E/GC3, 325 Broadway, Boulder, CO USA 80305, Phone (303) 497-6119, Fax (303) 497-6513, for details. Perhaps the easiest way to retrieve data is online, where users can conduct their own search for data using one (or more) of the following criteria. Note that the following are all global relief data sets, but differ with regard to data source, geographic area and level of detail.
Note that searches may also be conducted online by NGDC Product type, form and geographic area. All products can be purchased from the NGDC online store.
One of the most popular NGDC product sites is the NGDC image page, which provides links to all NGDC images (i.e., .GIF .JPG, .MOV, or .MPEG images produced in the National Geophysical Data Center's Marine Geology & Geophysics Division, or from MGG data in cooperation with scientists from other organizations). These images are in the public domain, with the exception of images in outside publications to which links are provided. If you use them, please credit NOAA/NGDC for the image and the underlying data sources/funding institutions of data from which the images were derived.
The ocean bottom is divided into three major areas: the continental shelf, the continental slope, and the deep ocean basin. The continental shelf extends underwater from each of the major land masses and can best be characterized as the submerged portion of the continents. The shelf has features similar to those we see on land, including hills, ridges, and canyons. On the map below, the continental shelf is the light-blue area that surrounds the continents. The shelf varies in size—it may be virtually non-existent in some areas, while elsewhere it may extend out several hundred miles from the shore. The shelf's average distance is about 64 kilometers (40 miles). It is beyond the continental shelf that the "deep sea" begins. The shelf ends at a depth of about 200 meters (660 feet), giving way to the steeper continental slope, which descends about 3,700 meters (12,000 feet) to the deep ocean basin. Here, the ocean floor deepens sharply and its features again resemble those on land (i.e., great plains and mountains) only on a much larger scale. In fact, the Earth's longest mountain range lies under the sea. More than 56,000 kilometers (35,000 mi) long, this mountain range, called the Mid-Ocean Ridge system, snakes its way around the globe. The Mid-Ocean Ridge marks the areas where the Earth's crustal plates are moving apart and is one of the most geologically active areas on Earth. It is here that new sea floor is created, giving rise to
hydrothermal vents and volcanoes.
How Deep Is the Ocean?
The deepest known point on Earth is at the bottom of the Mariana Trench, a depression in the floor of the western Pacific Ocean, just east of the Mariana Islands. This trench is 1,554 miles long and 44 miles wide. Near its southwestern extremity (210 miles southwest of Guam) lies the deepest point on Earth. This point—known as the Challenger Deep—is where the ocean bottom lies at a depth of nearly 7 miles (variously reported to be at least 36,198 to 38,518 feet deep). In 1960, the Trieste, a manned submersible owned by the U.S. Navy, descended to the bottom of the Mariana Trench. The depth reported is still only an estimate, based on a mathematical conversion from measured pressure (more than 8 tons per square inch), interpreted in terms of the integral of water density from the surface to the bottom. For an animated video of a simulated cruise through the trench and more information, try the following link: http://www.ngdc.noaa.gov/mgg/image/marianas.html.
Other Ocean Facts
Relevant Web Sites
NGDC Data: Frequently Asked Questions
Search NGDC products by form and geographic area
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April 12, 2002 — Beneath the world's oceans lie rugged mountains, active volcanoes, vast plateaus and almost bottomless trenches. The deepest ocean trenches could easily swallow up the tallest mountains on land. Around most continents are shallow seas that cover gently sloping areas called continental shelves, which reach depths of about 650 feet (200 meters). The continental shelves end at the steeper continental slopes, which lead down to the deepest parts of the ocean. (Click on the image to the left for a link to NOAA's Global Data Images.)
In the days of wooden sailing ships, little was known about the depth of the open oceans other than that the water was deep enough to allow safe passage of a ship's keel. Even less was known about the incredible structures and processes that shaped the oceans and continents. The only direct measurements of the water's actual depth were obtained by lowering a heavy weight on the end of a long line (rope or cable) marked in fathoms and trying to tell when it hit bottom. Although the length of the line from the ship to the weight could theoretically derive accurate water depth measurements, inaccuracies due to the effects of currents, ship motion, and stretching on the line were inevitable. Furthermore, this technique allowed only single point measurements that may (or may not) be representative of the entire region being sampled. Therefore, a complete and accurate map of the submerged ocean floor would have required millions of such observations. Without modern technology, sailors would still be at sea dropping weighted long lines overboard, while the truth about the world beneath the waves would still be buried under theory, conjecture and fantasy.
Global bathymetry and topography are two of the most extensively used types of scientific data for a wide variety of scientific, practical and artistic applications. Global relief forms a framework for visualizing other types of data and information, and provides boundary conditions for environmental modeling. NOAA's National Geophysical Data Center has a strong program in bathymetric data compilation in both coastal and open ocean areas. NGDC operates the International Hydrographic Organization Data Center for Digital Bathymetry (IHO DCDB) and is an active participant in international ocean mapping projects sponsored by the IHO and the Intergovernmental Oceanographic Commission.
A Global View of Our Earth's Geography
IMAGE SOURCE: This image is a color shaded relief image from the new ETOPO2 2-minute world elevation data set. (Click on the NOAA image to the left for a larger view. Please credit "NOAA" if using this image.) This image was generated from digital data bases of sea floor and land elevations on a 2-minute latitude/longitude grid (1 minute of latitude = 1 nautical mile, or 1.852 km). Assumed illumination is from the west; shading is computed as a function of the east-west slope of the surface with a nonlinear exaggeration favoring low-relief areas. A Mercator projection was used for the world image, which spans 390 degrees of longitude from 270 West eastward to 120 East; latitude coverage is from 80 degrees North to 80 degrees South. The resolution of the gridded data varies from true 2-minute for the Atlantic, Pacific, and Indian Ocean floors and all land masses to 5 minutes for a few locations on the Antarctic Ocean floor.
The National Geophysical Data Center
Patricia Viets, NOAA Satellite Service, (301) 457-5005