• bathymetry
    Bathymetric maps often use false colors to illustrate different aquatic depths. The shallow waters of Lake Michigan, above, for instance, are not actually red.

    Map courtesy NOAA/NGDC, NOAA National Ocean Service, Canadian Hydrographic Service, and Army Corps of Engineers

    Surf's Down!
    Surfing is much more than just "riding the waves"it starts with what lies beneath. The seafloor transforms ordinary waves into good waves . . . and good waves into great surfing. Bathymetry, or measuring the depth and rise of the seafloor, is important to good surfers.

    If there is a steep ascent of the ocean floor near the beach, it will cause waves to rise more quickly, and become bigger. If, however, the ocean floor has a slow and gradual ascent, the waves will come in more slowly, and not break as big.

    The famous El Porto surf area off the coast of Los Angeles, California, is a good example of how big waves develop. An underwater canyon focuses the energy of underwater currents, and the canyon's steep walls cause waves to rise quickly, producing huge, powerful waves.

    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 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.


    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.

  • Term Part of Speech Definition Encyclopedic Entry
    accuracy Noun

    condition of being exact or correct.

    achieve Verb

    to accomplish or attain.

    ancient Adjective

    very old.

    angular resolution Noun

    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 Noun

    large group.

    ascent Noun

    climb or movement upward.

    bathymetric map Noun

    representation of spatial information displaying depth underwater.

    bathymetry Noun

    measurement of depths of bodies of water.

    Encyclopedic Entry: bathymetry
    canyon Noun

    deep, narrow valley with steep sides.

    Encyclopedic Entry: canyon
    cargo Noun

    goods carried by a ship, plane, or other vehicle.

    centimeter Noun

    metric unit of measurement, equal to about .34 inch.

    characteristic Noun

    physical, cultural, or psychological feature of an organism, place, or object.

    crew Noun

    workers or employees on a boat or ship.

    current Noun

    steady, predictable flow of fluid within a larger body of that fluid.

    Encyclopedic Entry: current
    data bank Noun

    source or organization that supplies a large amount of information, usually about a specific topic.

    digital Adjective

    having to do with numbers (or digits), often in a format used by computers.

    dinosaur Noun

    very large, extinct reptile chiefly from the Mesozoic Era, 251 million to 65 million years ago.

    echo sounder Noun

    device that measures the depth of water using sound pulses. Also called a sonic depth finder.

    economic Adjective

    having to do with money.

    elevation Noun

    height above or below sea level.

    Encyclopedic Entry: elevation
    engineer Noun

    person who plans the building of things, such as structures (construction engineer) or substances (chemical engineer).

    environment Noun

    conditions that surround and influence an organism or community.

    estimate Verb

    to guess based on knowledge of the situation or object.

    fossil Noun

    remnant, impression, or trace of an ancient organism.

    Encyclopedic Entry: fossil
    gradual Adjective

    rising or falling by a small amount.

    Hawaii-Emperor Seamount Chain Noun

    underwater mountain range in the north Pacific Ocean, stretching from the U.S. state of Hawaii to southeast Japan.

    hurricane Noun

    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.

    hydrography Noun

    measurement and study of the surface waters of the Earth.

    inaccurate Adjective

    untrue.

    integral Adjective

    very important.

    International Hydrographic Organization (IHO) Noun

    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 Noun

    body of water surrounded by land.

    marine Adjective

    having to do with the ocean.

    mountain Noun

    landmass that forms as tectonic plates interact with each other.

    multibeam echo sounder Noun

    device that measures the depth of water using a variety of sound pulses.

    National Geophysical Data Center (NGDC) Noun

    U.S. government organization that provides scientific leadership, products and services for geophysical data from the Sun to the Earth and Earth's seafloor and solid earth environment.

    nautical chart Noun

    representation of spatial information displaying data on bodies of water and coastal areas.

    navigation Noun

    art and science of determining an object's position, course, and distance traveled.

    Encyclopedic Entry: navigation
    navigator Noun

    person who charts a course or path.

    ocean Noun

    large body of salt water that covers most of the Earth.

    Encyclopedic Entry: ocean
    ocean trench Noun

    a long, deep depression in the ocean floor.

    Encyclopedic Entry: ocean trench
    organism Noun

    living or once-living thing.

    pressure Noun

    force pressed on an object by another object or condition, such as gravity.

    reef Noun

    a ridge of rocks, coral, or sand rising from the ocean floor all the way to or near the ocean's surface.

    reliable Adjective

    dependable or consistent.

    river Noun

    large stream of flowing fresh water.

    Encyclopedic Entry: river
    salinity Noun

    saltiness.

    sandbar Noun

    underwater or low-lying mound of sand formed by tides, waves, or currents.

    seafloor Noun

    surface layer of the bottom of the ocean.

    seamount Noun

    underwater mountain.

    simulate Verb

    to create an image, representation, or model of something.

    steep Adjective

    extreme incline or decline.

    surfing Noun

    the sport of riding down a breaking wave on a board.

    Encyclopedic Entry: surfing
    temperature Noun

    degree of hotness or coldness measured by a thermometer with a numerical scale.

    Encyclopedic Entry: temperature
    topographic map Noun

    map showing natural and human-made features of the land, and marked by contour lines showing elevation.

    topography Noun

    study of the shape of the surface features of an area.

    tsunami Noun

    ocean waves triggered by an earthquake, volcano, or other movement of the ocean floor.

    uniform Adjective

    exactly the same in some way.

    valley Noun

    depression in the Earth between hills.

    vary Verb

    to change.

    volcano Noun

    an opening in the Earth's crust, through which lava, ash, and gases erupt, and also the cone built by eruptions.

    Encyclopedic Entry: volcano
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