![]() ![]() Hydrography is the study of the depth and characteristics of water. 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. The NGDC also operates a worldwide digital data bank of bathymetric measurements on behalf of the member countries of the International Hydrographic Organization. The presence of undersea trenches or mountains can directly affect the strength and path of a tsunami or hurricane. The NGDC, for example, creates digital elevation models that are used to simulate tsunamis. Their bathymetric measurements support safe navigation and protect marine environments around the globe. National Geophysical Data Center (NGDC) and the International Hydrographic Organization (IHO) measure and archive bathymetric data. These samples proved that the seamounts stood above the water in the time of the dinosaurs. These seamounts contained fossils of reef-building organisms that lived in shallow waters during the Cretaceous period. Using bathymetric tools, samples of rocks from the tops of these seamounts confirmed the theory. Scientists thought they were ancient volcanoes, but they could not be sure. These seamounts, called the Hawaii-Emperor Seamount Chain, rise 1,000 or more meters (3,280 feet) above the seafloor. For example, thousands of seamounts were discovered in the central Pacific Ocean, near the U.S. Many interesting discoveries have been made by bathymetric technology. If the material is hard, the signal from the echo sounder will come back stronger. For instance, they can indicate whether the feature is made of hard or soft sediments. Multibeam echo sounders can also provide information about the physical characteristics of a seafloor feature. They also allow scientists to map more seafloor in less time than a single-beam echo sounder. Multibeam echo sounders correct for the movements of the boat at sea, further increasing the measurements’ accuracy. 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. Angular resolution is the ability to measure different angles, or points of view, of a single object. This array of pulses provides very high angular resolution. Multibeam echo sounders feature hundreds of very narrow beams that send out sound pulses. To address these problems, engineers developed multibeam echo sounders. The ocean’s constant movement makes bathymetry difficult. The ocean has different currents, with different temperatures and salinities. In general, sound travels faster as temperature, salinity, and pressure increase. The speed of sound in water also varies, depending on the temperature, salinity (saltiness), and pressure of the water. Reflections from undersea organisms, such as whales, can disrupt the sound wave’s path. The ship from which the measurements are taken is moving, changing the depth to the seafloor by centimeters or even feet. However, the accuracy of these measurements is still limited. An echo sounder is able to measure a small area of the seafloor. The longer it takes, the deeper the water. The time it takes for the pulse to leave and return to the ship determines the topography of the seafloor. ![]() An echo sounder sends out a sound pulse from a ship’s hull, or bottom, to the ocean floor. Echo Sounders Today, echo sounders are used to make bathymetric measurements. This could lead to danger for a ship’s crew and economic losses if the ship hit the sandbar and lost its cargo. Other times, an ocean trench or sandbar would surprise navigators. Sometimes, the seafloor’s hills and valleys were easy to predict. More often, scientists and navigators estimated the topography of the seafloor. To get a clear picture of the seafloor, scientists would have had to take thousands of rope measurements. The rope could also only measure depth one point at a time. The rope often did not travel straight to the seafloor, but was shifted by currents. These measurements, however, were inaccurate and incomplete. 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. It can also indicate a seamount, or underwater mountain. A circular shape with increasingly smaller circles inside of it can indicate an ocean trench. On bathymetric maps, they connect points of equal depth. On topographic maps, the lines connect points of equal elevation. Bathymetric maps look a lot like topographic maps, which use lines to show the shape and elevation of land features. Bathymetry is the measurement of the depth of water in oceans, rivers, or lakes. ![]()
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