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Diving Deeper: Episode 13 (July 15, 2009) —
What is Hydrography?

HOST: Welcome to Diving Deeper where we interview National Ocean Service scientists on the ocean topics and information that are important to you! I’m your host Kate Nielsen.

Today’s question is….What is hydrography?

Hydrography, also known as hydrographic surveying, deals with the measurement and description of the physical features on the bottom of the sea floor for the primary purpose of safety of navigation.

To help us dive a little deeper into this question, we will talk with Jerry Mills on hydrography – what it is and why it is important to us. Jerry is a Technical Advisor with the Office of Coast Survey. Hi Jerry, welcome to our show.

JERRY MILLS: Thanks Kate, it’s great to be here and talk about hydrography and how it impacts us every day.

HOST: Jerry, can you explain hydrography to us a little bit more?

JERRY MILLS: Sure thing. The word hydrography broadly means water mapping and because of that there are three different definitions. Oceanographers use the term to describe and map the physical characteristics of water such as temperature, salinity, and chemical content. Geographers and geologists use hydrography to define the water surface in the U.S. and the direction and volume of water flow between water bodies. Hydrography, as used by my office, is really focused on identifying any hazards to safe navigation which includes shallow depths, shipwrecks, rocks, or any other dangerous objects. 

HOST: What does hydrographic data tell us?

JERRY MILLS: Kate, the most important thing we can learn from hydrographic data is the depth of the water in a particular area. In order for large commercial ships, military watercraft, and pleasure boats to safely navigate on America’s oceans and coasts, mariners need to know where it is safe – or not safe – to steer their vessels.

Hydrographic data collected by my office is used to update NOAA’s nautical charts. Nautical charts are like road maps for mariners. Charts are dotted with little numbers, or depth measurements, and symbols which tell a ship captain if an area is too shallow or too dangerous to operate their vessels.

Detailed depth information is also useful in determining fisheries habitat, understanding marine geologic processes, and assisting coastal engineers.

HOST: Jerry, many of our listeners today do not live right along the coast. Why is hydrography important to them?

JERRY MILLS: Hydrography is important to everyone in the U.S. whether you live along the coast or in the middle of the country. Roughly 98 percent of all goods and products that are brought to our country are brought here by ship. This includes petroleum, automobiles, sports equipment, cameras, and many others. In order for ships to safely dock into port, they need to safely travel along our coasts, Great Lakes, canals, bays, and rivers.  You can’t navigate safely if you don’t have a NOAA nautical chart. If ships can’t safely dock in port, then people in the middle of the country can’t get their products.

HOST: Jerry, what type of equipment is needed to map the bottom of the sea floor?

JERRY MILLS: To better understand the equipment we currently use it might be a good idea to talk about how measurements were made in the past. The first evidence that man was measuring the depth of water is found in tomb paintings of ancient Egypt dating from 1800 B.C. One showed a man using a long slender pole on the bow of a large vessel under oar and sail. Another showed a man measuring depths by means of a weight attached to a line, called a lead line, which had distinct markings. Believe it or not, lead lines were the main way to determine water depths until approximately 1940 and over half the depths on today’s nautical charts of the U.S. were determined from lead lines.

HOST: That’s pretty amazing and seems like it could be a somewhat inaccurate way to collect water depth measurements. What technology replaced lead lines for measuring depth today?

JERRY MILLS: In the 1920s and 1930s my office developed instruments that used SONAR, which stands for SOund NAvigation and Ranging, to determine water depth. These new echo sounders would send out a sound signal and measure the time for it to travel to the sea floor and back to the ship. It would be like dribbling a basketball and measuring the time between when the ball left your hand, bounced off the floor, and returned to your hand. By knowing the speed of sound in water the time could be converted to depth. These devices, also called depth finders or single beam sonars, gained popularity in the 1940s and are still in use today.

HOST:  Has any new sonar technology been developed that improves our ability to measure water depth and find objects underwater?

JERRY MILLS: Actually Kate, two new types of sonar have been developed over the past 45 years and they are called multibeam sonar and side scan sonar. The name multibeam really describes what this device does. Essentially it is like having hundreds of single-beam echo sounders pointing in different directions down from the ship in a fan shape. This creates a line of depths or a swath across the sea floor so as the ship moves forward through the water it covers an area on the sea floor, sort of like mowing your lawn. This is a big improvement over the old single-beam sonars because objects could exist between single-beam lines but not be detected.

HOST: So is it correct to say that multibeam sonar gives us a more complete picture of what is on the sea floor?

JERRY MILLS: That’s right and with modern computers, this data can be displayed so it looks like the water has been removed and we are looking directly at the sea floor.

HOST: Wow, that’s really amazing. You mentioned side scan sonar too a little bit earlier. What is the difference between this and the multibeam sonar?

JERRY MILLS: Side scan sonars were developed before multibeam sonars and can actually find smaller objects on the sea floor than multibeam. A side scan sonar is typically towed behind the ship in what is called a towfish. This allows the instrument to get closer to the sea floor, but the operator has to be careful not to let it run into a rock or shipwreck or the sea bottom. It operates in a slightly different manner than multibeam in that it measures the strength of the returning sound wave and converts that into a black and white image. It almost looks like a black and white photograph. However, it does not provide accurate depth information so that is why multibeam and side scan are often used together on a survey.

HOST: Does any of this sonar technology harm or disturb marine life, like maybe whales for example?

JERRY MILLS: Kate, that’s a common misconception about the sonar technologies that are used for hydrographic purposes. The sonars that appear to be harmful to marine mammals such as whales are very powerful, low frequency devices that transmit sound over hundreds of miles. Our sonars operate at a much lower power and higher frequency so the sound waves travel much shorter distances, such as a few hundred meters, and do not harm marine life. We’ve even seen porpoises swimming alongside our side scan towfish and rubbing against it, presumably trying to get it to play.

HOST: Jerry, you touched on this a little bit earlier with some of the different uses of hydrographic data, but what are the economic benefits associated with hydrographic surveying? Do you have an example maybe that you can share with us?

JERRY MILLS: Well, just like our trucks on our highways, captains of commercial ships require hydrographic data on NOAA’s nautical charts to safely transit marine highways. More than two billion tons of domestic and international freight travel over our marine highways annually including 3.3 billion barrels of oil to meet U.S. energy demands. Waterborne commerce is a major part of the U.S. economy contributing more than $1 trillion to our gross domestic product.

Also, U.S. ports receive more than 22,000 visits a year from ocean-going container ships. That’s a lot of products coming to our store shelves from these ships. Keep in mind, if you live in the middle of the country, these products need to be transported by trucks, trains, or river-bound tugboats and barges from the ocean ports. Operating this marine transportation system requires more than 13 million jobs, another great number for our economy.

HOST: Thanks Jerry. Those are some really amazing statistics and I think so far today you’ve highlighted for us a few of the roles of your office in hydrographic surveying, but can you expand a little bit more on the role of the National Ocean Service in this effort?

JERRY MILLS: Certainly Kate. My office, the Office of Coast Survey is part of the National Ocean Service and is responsible for producing and maintaining a suite of more than 1,000 nautical charts that cover the coastal waters of the U.S. To do this, we conduct hydrographic surveying with the support of other offices within the National Ocean Service. The Remote Sensing Division of the National Geodetic Survey provides shoreline information for our surveys and the Center for Operational Oceanographic Products and Services provides critical tide and water level data.

HOST: Jerry, I think that most of us are familiar with some of the environmental issues that NOAA tackles or things like weather, fisheries, and the oceans. How is hydrography and nautical charting connected to the other parts of NOAA?

JERRY MILLS: This is a great question Kate. Everyone is aware, particularly after the Exxon Valdez accident in Alaska in 1989 that the most damaging substance to marine life is oil and other petroleum products. All ships and boats carry some amount, usually in the form of diesel fuel, but large tankers may be loaded with as much as 3 million barrels of oil. The results of any vessel hitting an object on the sea floor that is hard enough to penetrate the hull is likely to spill oil, in some cases catastrophically.

As an example, back in November 2004, the oil tanker Athos I struck an object on the sea floor of the Delaware River puncturing its hull and resulting in an oil spill of 473,000 gallons of heavy crude oil. The damage to the environment was rather extensive, not only in the direct loss of life to fish and marine birds, but the general damage of the area for many years after the accident. Modern hydrographic surveys would almost certainly have found the objects preventing the resulting ecological disasters and saving millions of dollars in damages. This is really the connection between hydrographic surveys and the environment.

HOST: Thanks Jerry for making that connection for us. That really helps out a lot. How far off of the coast, does your office collect this hydrographic data?

JERRY MILLS: The Office of Coast Survey is responsible for surveying the entire U.S. Exclusive Economic Zone. This area is essentially all waters adjacent to the U.S. and its territories out to a distance of 200 nautical miles offshore for a total of 3.4 million square nautical miles, and this is more than 20 percent larger than the land area of the U.S. However, most of our hydrographic surveys are conducted within a few miles of shore.

HOST: Thanks Jerry. That is quite an extensive area to map and keep updated on nautical charts. How often do we need to resurvey an area? 

JERRY MILLS: Well, Kate, as with many questions, the answer is it depends. Some areas, particularly those close to the mouths of rivers, are constantly having mud and sand deposited. At the mouth of the Mississippi River for example, hydrographic surveys need to be done every day because of the constantly changing conditions and water depths. Other areas, like much of New England, do not change much at all over time and do not normally need to be resurveyed very frequently, perhaps every 50 years or so, unless there is a large advance in technology.

Earlier I mentioned that over half of the depths on nautical charts are from lead line measurements and most of the others are from single-beam echo sounders. That means that most areas have not been surveyed with modern side scan sonars and multibeam so there may be undiscovered wrecks or rocks on the sea floor. Those areas need to be resurveyed to ensure they are safe for navigation. In addition, with global climate change, many glaciers in Alaska have been receding, some as much as ten miles in ten years. Since previous charts showed no water depths, only ice, they need to be resurveyed, especially since cruise ships are traveling in those uncharted waters.

HOST: Do we need to survey more frequently after major natural events such as hurricanes?

JERRY MILLS: Yes, we actually do. When there’s a major event of nature such as a hurricane or an earthquake, we need to conduct surveys rapidly to determine whether there’s been any changes in water depth or new obstructions. Hurricane Katrina created a lot of destruction not only on land, but also in navigation channels where vessels, land vehicles, and building debris were found by our hydrographic surveys. The Office of Coast Survey has six navigation response teams that are first on the ground to start recording this information after a hazardous natural event.

HOST: These navigation response teams sound like they are critical to collecting the information right after a hazardous event. Do these teams support response and data collection for only natural hazardous events?

JERRY MILLS: With hurricanes, our navigation response teams and our fleet of NOAA ships have some lead time to prepare and plan for emergency hydrographic surveying. However, sometimes, we have no notice when a man-made disaster, such as an oil spill or airline accident, happens. 

A recent example is in the U.S. Airways emergency plane landing on New York’s Hudson River. Within minutes, our New England-based Navigation Response Team became aware of the situation, and began work immediately. Unfortunately, at the time, their boat was unavailable, but the team used their close connections with the New York and New Jersey police and fire boats, and also the FBI and Army Corps of Engineers, to provide surveying expertise in finding debris from the emergency landing including the missing engines.

Other notable incidents that your listeners may not know about NOAA is that our hydrographic survey vessels discovered the site of John F. Kennedy, Jr.’s, plane crash off Martha’s Vineyard in 1999 as well as the wreckage of TWA flight 800 off New York’s Long Island in 1996.
HOST: Thanks Jerry. These are definitely great examples and a lot of things I wasn’t aware that NOAA and your office was involved with. Do you have any final closing words for our listeners today?

JERRY MILLS: The one thought I would like to leave with your listeners is that we know more about the surface of the Moon and even Venus, which is constantly shrouded by dense clouds, than we do about the shape of the ocean floor. It seems rather remarkable but with modern technology we are slowly making progress. Hydrographic surveys are the key to better understanding what lies beneath the ocean’s surface.

HOST: Thank you Jerry for joining us on today’s episode of Diving Deeper and talking about hydrography, how we collect the data, and why this is so important to us. To learn more about hydrography, please visit

That’s all for this week’s show. Please tune in on July 29th for our next episode on land cover.