Hydrography

Hydrography is a scientific field that studies the physical features of bodies of water and the adjacent areas. Hydrographic surveys measure the depth and bottom structure of water bodies. The primary purpose of this information is to update nautical chards and develop hydrographic models, although the data are increasingly being used for additional purposes as well.

Brief History

Hydrography has been important since the first time humans set sail on the ocean. Without knowledge of water depths and compositions, sailors ran extreme risk of running aground and sinking. In the United States, the Office of Coast Survey was tasked with surveying the country’s coastlines to allow for the safe passage of ships. It was formed by Thomas Jefferson in 1807. Since then, the Office has grown enormously in its size and responsibilities, eventually forming the National Oceanic and Atmospheric Administration.

Hydrographic Tools

Hydrographic survey methods through the years. Credit: NOAA OCS [4]

The tools used in hydrographic surveying have changed over the years as technology has changed and improved. Early hydrographers used lead lines to measure water depths. Weighted wire dragging was added to the hydrographer’s toolbox. Finally came the development of sonar.

• Lead lines were long ropes weighted with lead and marked with depth measurements. These lines were periodically dropped into the ocean (in an orderly pattern) and the depth markers on the line would reveal the depth measurement (known as a depth sounding) in that location. The position of the ship would be charted using sextants and multiple mapped reference points. The combination of known location and depth would be used to make nautical charts for other seafaring vessels.
• Weighted wire dragging began in the early 1900’s. These types of surveys used a weighted wire dragged between two vessels to find obstructions beneath the surface. The wire was set down to a certain depth using weights and then dragged between the two vessels. If the wire snagged on something, the location and depth of the hazard was marked.
• The development of sonar (which uses sound to measure distances) in the 1930’s allowed many more data points to be collected, which allowed for more accurate maps. The early type of sonar is known as a single beam echo sounder or single beam sonar. However, despite the increased accuracy over lead line surveys, the single beam surveys still had some gaps in the data.

As technology has progressed, so have the types of sonar and their abilities to accurately collect data. Modern hydrographic tools allow for much greater accuracy and knowledge about the seafloor. The primary modern hydrographic tools are side scan sonar, multibeam sonar, and Lidar.

• Side scan sonar is usually used specifically to detect objects on the seafloor. This sonar system is comprised of three different components: a topside processing unit, a towfish (a unit towed behind the boat), and a transmission cable. The towfish sends out a transmission signal and the return echo of the signal is recorded, creating a picture of the objects on the seafloor. The drawback of side scan sonar is that it cannot determine depth, which is why hydrographers also use multibeam echo sounders.
• Multibeam echo sounders (MBES) transmits sound waves from a unit directly beneath the survey vessel’s hull and records the amount of time it takes for the signal to return. This allows hydrographers to determine the depth the seafloor or objects protruding from the seafloor. Multibeam sonar is an improvement over single beam sonar because it allows for a wider coverage area, which reduces or negates the gaps produced by earlier methods.
• LIDAR is an acronym that stands for Light Detection And Ranging; it determines distance and depth by analyzing reflections from laser pulses. These systems are usually attached to aircraft and flown above the area scientists are mapping. Lidar uses different frequencies of light: a lower frequency (with longer wavelengths) reflects off the surface of the water and helps determines shoreline elevations while the lower frequency (shorter wavelength) penetrates the water and reflects off the bottom to determine water depth. Depending upon the clarity of the water, Lidar systems are usually useful up to depths of 50 meters. It’s usually used in around shallow, rocky coastlines where it is too difficult for a surface vessel to travel.

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  Side Scan Sonar diagram. Credit: NOAA OCS [1]

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  Multibeam Sonar diagram. Credit: NOCAA OCS [2]

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  This Lidar image shows the coastline terrain (green and red) and the ocean depths (blue). Credit: NOAA OCS [3]

Uses of Hydrography

A sunken ship discovered by sonar. Credit: NOAA OCS [5]

The primary use of hydrography is the creation of nautical charts. Nautical charts are maps of the ocean. They depict water depths, obstructions, and any other information a ship might need to safely navigate. Without nautical charts, sailing would be much more dangerous and expensive. This is important because ships are the backbone of overseas trade. Nearly 80% of the United States overseas trade is conducted through marine transportation. However, hydrography is used for much more than just the creation of nautical charts. Among other uses, hydrography has been used to chart and manage oyster beds, identify wrecks, help economies recover after hurricanes, assist in the preservation of coral reefs, locate damaged pipelines underwater, and monitor the foundation of ocean wind turbines.

Related Links

• Hydrography: More than Nautical Charts
• Hydrographic Surveying