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.
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.
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.
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.
Term Part of Speech Definition Encyclopedic Entry accuracy Noun
condition of being exact or correct.
to accomplish or attain.
angular resolution Noun
ability to measure different angles, or points of view, of a single object. Also called spatial resolution.
to keep records or documents.
climb or movement upward.
bathymetric map Noun
representation of spatial information displaying depth underwater.
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.
metric unit of measurement, equal to about .34 inch.
physical, cultural, or psychological feature of an organism, place, or object.
workers or employees on a boat or ship.
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.
having to do with numbers (or digits), often in a format used by computers.
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.
having to do with money.
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).
conditions that surround and influence an organism or community.
to guess based on knowledge of the situation or object.
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.
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.
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.
body of water surrounded by land.
Encyclopedic Entry: lake marine Adjective
having to do with the ocean.
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.
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.
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.
force pressed on an object by another object or condition, such as gravity.
a ridge of rocks, coral, or sand rising from the ocean floor all the way to or near the ocean's surface.
Encyclopedic Entry: reef reliable Adjective
dependable or consistent.
large stream of flowing fresh water.
Encyclopedic Entry: river salinity Noun
underwater or low-lying mound of sand formed by tides, waves, or currents.
surface layer of the bottom of the ocean.
to create an image, representation, or model of something.
extreme incline or decline.
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.
study of the shape of the surface features of an area.
ocean waves triggered by an earthquake, volcano, or other movement of the ocean floor.
exactly the same in some way.
depression in the Earth between hills.
an opening in the Earth's crust, through which lava, ash, and gases erupt, and also the cone built by eruptions.
Encyclopedic Entry: Plate Tectonics and Volcanic Activity