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1 Your web browser (Safari 7) is out of date. For more security, comfort and the best experience on this site: Update your browser Ignore BATHYMETRY For the complete encyclopedic entry with media resources, visit: Bathymetry is the measurement of the depth of water in oceans, rivers, or lakes. Bathymetric maps look a lot like topographic maps, which use lines to show the shape and elevation of land features. On topographic maps, the lines connect points of equal elevation. On bathymetric maps, they connect points of equal depth. A circular shape with increasingly smaller circles inside of it can indicate an ocean trench. It can also indicate a seamount, or underwater mountain. In ancient times, scientists would conduct bathymetric measurements by throwing a heavy rope over the side of a ship and recording the length of rope it took to reach the seafloor. These measurements, however, were inaccurate and incomplete. The rope often did not travel straight to the seafloor, but was shifted by currents. The rope could also only measure depth one point at a time. To get a clear picture of the seafloor, scientists would have had to take thousands of rope measurements. More often, scientists and navigators estimated the topography of the seafloor. Sometimes, the seafloor s hills and valleys were easy to predict. Other times, an ocean trench or sandbar would surprise navigators. This could lead to danger for a ship s crew and economic losses if the ship hit the sandbar and lost its cargo. Echo Sounders Today, echo sounders are used to make bathymetric measurements. An echo 1 of 7

2 sounder sends out a sound pulse from a ship s hull, or bottom, to the ocean floor. The sound wave bounces back to the ship. The time it takes for the pulse to leave and return to the ship determines the topography of the seafloor. The longer it takes, the deeper the water. An echo sounder is able to measure a small area of the seafloor. However, the accuracy of these measurements is still limited. The ship from which the measurements are taken is moving, changing the depth to the seafloor by centimeters or even feet. Reflections from undersea organisms, such as whales, can disrupt the sound wave s path. The speed of sound in water also varies, depending on the temperature, salinity (saltiness), and pressure of the water. In general, sound travels faster as temperature, salinity, and pressure increase. The ocean has different currents, with different temperatures and salinities. The ocean s constant movement makes bathymetry difficult. To address these problems, engineers developed multibeam echo sounders. Multibeam echo sounders feature hundreds of very narrow beams that send out sound pulses. This array of pulses provides very high angular resolution. Angular resolution is the ability to measure different angles, or points of view, of a single object. Having high angular resolution means a single feature of the seafloor like the top of an undersea mountain would be measured from a variety of angles, from the sides as well as the top. Multibeam echo sounders correct for the movements of the boat at sea, further increasing the measurements accuracy. They also allow scientists to map more seafloor in less time than a single-beam echo sounder. Multibeam echo sounders can also provide information about the physical characteristics of a seafloor feature. For instance, they can indicate whether the feature is made of hard or soft sediments. If the material is hard, the signal from the echo sounder will come back stronger. 2 of 7

3 Many interesting discoveries have been made by bathymetric technology. For example, thousands of seamounts were discovered in the central Pacific Ocean, near the U.S. state of Hawaii. These seamounts, called the Hawaii-Emperor Seamount Chain, rise 1,000 or more meters (3,280 feet) above the seafloor. Scientists thought they were ancient volcanoes, but they could not be sure. Using bathymetric tools, samples of rocks from the tops of these seamounts confirmed the theory. These seamounts contained fossils of reef-building organisms that lived in shallow waters during the Cretaceous period. These samples proved that the seamounts stood above the water in the time of the dinosaurs. Bathymetric Data The U.S. National Geophysical Data Center (NGDC) and the International Hydrographic Organization (IHO) measure and archive bathymetric data. Their bathymetric measurements support safe navigation and protect marine environments around the globe. The NGDC, for example, creates digital elevation models that are used to simulate tsunamis. The presence of undersea trenches or mountains can directly affect the strength and path of a tsunami or hurricane. The NGDC also operates a worldwide digital data bank of bathymetric measurements on behalf of the member countries of the International Hydrographic Organization. The IHO, based in Monaco, works to achieve uniformity in nautical charts, adopt reliable methods of carrying out ocean surveys, and develop the sciences in the field of hydrography. Hydrography is the study of the depth and characteristics of water. Bathymetry is a part of hydrography. It is an integral part in this science of surveying and charting bodies of water. Vocabulary 3 of 7

4 Term accuracy Term condition of being exact or correct. achieve verb to accomplish or attain. ancient angular resolution adjectivevery old. ability to measure different angles, or points of view, of a single object. Also called spatial resolution. archive verb to keep records or documents. array large group. ascent climb or movement upward. bathymetric map representation of spatial information displaying depth underwater. bathymetry measurement of depths of bodies of water. canyon deep, narrow valley with steep sides. cargo goods carried by a ship, plane, or other vehicle. centimeter metric unit of measurement, equal to about.34 inch. characteristic physical, cultural, or psychological feature of an organism, place, or object. crew workers or employees on a boat or ship. current data bank digital dinosaur echo sounder economic steady, predictable flow of fluid within a larger body of that fluid. source or organization that supplies a large amount of information, usually about a specific topic. having to do with numbers (or digits), often in a format adjective used by computers. very large, extinct reptile chiefly from the Mesozoic Era, 251 million to 65 million years ago. device that measures the depth of water using sound pulses. Also called a sonic depth finder. adjectivehaving to do with money. elevation height above or below sea level. 4 of 7

5 engineer person who plans the building of things, such as structures (construction engineer) or substances (chemical engineer). environment conditions that surround and influence an organism or community. estimate verb to guess based on knowledge of the situation or object. fossil remnant, impression, or trace of an ancient organism. gradual Hawaii- Emperor Seamount Chain hurricane hydrography inaccurate integral International Hydrographic Organization (IHO) adjectiverising or falling by a small amount. underwater mountain range in the north Pacific Ocean, stretching from the U.S. state of Hawaii to southeast Japan. tropical storm with wind speeds of at least 119 kilometers (74 miles) per hour. Hurricanes are the same thing as typhoons, but usually located in the Atlantic Ocean region. measurement and study of the surface waters of the Earth. adjectiveuntrue. adjectivevery important. group that works to provide marine data, products, and service to advance maritime safety and efficiency, and support the protection and sustainable use of the marine environment. lake body of water surrounded by land. marine Term mountain adjectivehaving to do with the ocean. landmass that forms as tectonic plates interact with each other. multibeam echo sounder device that measures the depth of water using a variety of sound pulses. 5 of 7

6 Term National U.S. government organization that provides scientific Geophysical leadership, products and services for geophysical data Data Center from the Sun to the Earth and Earth's seafloor and solid (NGDC) earth environment. nautical chart representation of spatial information displaying data on bodies of water and coastal areas. navigation art and science of determining an object's position, course, and distance traveled. navigator person who charts a course or path. ocean large body of salt water that covers most of the Earth. ocean trench a long, deep depression in the ocean floor. organism living or once-living thing. pressure force pressed on an object by another object or condition, such as gravity. reef a ridge of rocks, coral, or sand rising from the ocean floor all the way to or near the ocean's surface. reliable adjectivedependable or consistent. river large stream of flowing fresh water. salinity saltiness. sandbar underwater or low-lying mound of sand formed by tides, waves, or currents. seafloor surface layer of the bottom of the ocean. seamount underwater mountain. simulate verb to create an image, representation, or model of something. steep adjectiveextreme incline or decline. surfing the sport of riding down a breaking wave on a board. degree of hotness 6 of 7or coldness measured by a

7 temperature degree of hotness or coldness measured by a thermometer with a numerical scale. Term topographic map showing natural and human-made features of the map land, and marked by contour lines showing elevation. topography study of the shape of the surface features of an area. tsunami ocean waves triggered by an earthquake, volcano, or other movement of the ocean floor. uniform adjectiveexactly the same in some way. valley depression in the Earth between hills. vary verb to change. volcano an opening in the Earth's crust, through which lava, ash, and gases erupt, and also the cone built by eruptions. Articles & Profiles National Geographic News: Laser-Armed Planes Map Uncharted Seas at Top Speed National Geographic Adventure: The Big, Record-Breaking, Green, Disease- Busting, Deep-Sea, Crime-Fighting IDEAS for 2009 Maps NOAA: National Geophysical Data Center US Bathymetric and Fishing Maps National Geographic Society. All rights reserved. 7 of 7