Encyclopedic Entry

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.

Vocabulary

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.

Credits

Media Credits

The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit. The Rights Holder for media is the person or group credited.

Writers

Kim Rutledge
Melissa McDaniel
Diane Boudreau
Tara Ramroop
Santani Teng
Erin Sprout
Hilary Costa
Hilary Hall
Jeff Hunt

Illustrators

Tim Gunther
Mary Crooks, National Geographic Society

Editors

Kara West
Jeannie Evers

Educator Reviewer

Nancy Wynne

Producer

Caryl-Sue, National Geographic Society

Sources

Dunn, Margery G. (Editor). (1989, 1993). "Exploring Your World: The Adventure of Geography." Washington, D.C.: National Geographic Society.

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