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Hydrographic Surveying

30 August 2011, 6:33 pm

Port of Seattle

Hydrographic Surveying

  • Why survey our coasts? Did you know that most of the things you buy in the store probably traveled, at some point, on a ship? Our nation's marine highways carry more than three-quarters of all U.S. goods and supplies. Each year, ships move two billion tons of freight in and out of our nation's ports. Keeping our marine transportation system functioning in a way that is safe, efficient, and environmentally sound requires information about water depth, the shape of the sea floor and coastline, the location of possible obstructions, and other physical features of water bodies. Hydrography is the science behind this information, and surveying is a primary method of obtaining hydrographic data.

  • A little history. In 1807, President Thomas Jefferson signed a mandate ordering a survey of our young nation’s coast. And so began the history of the Survey of the Coast, an organization that would go through changes to its name and structure, but not its fundamental mission: providing nautical charts to the maritime community for safe passage into American ports and along 95,000 miles of U.S. coastline. Today, as it did in Jefferson's day, the Office of Coast Survey conducts the hydrographic surveys that are the fundamental component of nautical charts. Depending on the charting requirement, NOAA uses two kinds of sonar to survey the sea floor: multibeam and side scan.

  • Using sonar to survey. Multibeam sonar measures the depth of the sea floor by analyzing the time it takes for sound waves to travel from a boat to the sea floor and back. It provides amazing detail of the sea floor, especially in rocky and rough terrain, where it gives a complete picture of the bottom. Multibeam sonar is very useful in areas such as the Northeast U.S. and Alaska, where the sea floor is complex and often strewn with thousands of rocks. However, in relatively shallow, flat areas, like the mid-Atlantic coast, the multibeam is not very efficient. So, NOAA uses another tool called side scan sonar. This kind of sonar creates an image of the sea floor, but does not determine depths. If surveyors find a wreck or obstruction using side scan sonar, NOAA will investigate the item and determine the depth with a multibeam sonar or with divers.

  • Determining Positions. The other essential ingredients to charting are accurate positioning and tides or water levels. Until the 1970s, hydrographers determined positions of water depths by using a sextant. Today’s modern Global Positioning System provides accuracy and efficiencies never dreamed of only a few years ago. Measuring and predicting the rise and fall of tides, and accurate positioning, are critical to mariners as they guide large ships in and out of our ports. In many cases, ships are less than three feet off the bottom and within inches of clearance below a highway bridge. Today’s technology gives pilots and mariners the knowledge to maneuver along America’s coastline and into our ports and harbors.

  • Data Processing. Once hydrographic data has been collected using sonar, it is put into context, or referenced to the location at which it was collected. The raw soundings are adjusted to a standard or absolute water level (datum) using predictions from tidal observations collected at water-level stations. Horizontal positions (latitude and longitude) are recorded using Differential Global Positioning System equipment and need no further adjustment. Information about the coastline is compared extensively to recent coastal survey maps to identify new features and modifications. Eventually, the sounding data are re-processed to produce the final soundings.

  • Nautical Charts. The Office of Coast Survey uses collected hydrographic data primarily to update NOAA’s suite of over 1,000 nautical charts. A nautical chart is kind of like a map for navigating a waterway and is one of the most fundamental tools available to the mariner. The numbers on a nautical chart are depth measurements – obtained via hydrographic surveys. Charts also include other information critical to safe navigation and ecological protection. Symbols tell ship captains or recreational boaters if an area is too shallow or too dangerous to operate their vessels or if they are approaching sensitive marine sites.

  • Survey Parties. Hydrographers, from centuries ago until today, have developed nautical charts to tell mariners where waters are safe. Back in the 16th century, charts showed the location of ghoulish "monsters," ready to devour complete vessels. Today, NOAA hydrographers search for modern monsters—undiscovered underwater geologic formations and other dangers to navigation. There are nearly 3,500,000 square nautical miles of our nation’s waters to monitor, understand, and chart. The Office of Coast Survey evaluates the 500,000 square nautical miles that are navigationally significant. Then, each spring, hydrographic field parties set out to survey sections of the 43,000 square nautical miles deemed to be critical by the Coast Survey's evaluation. Sometimes teams break loose from normal surveying to respond to disasters such as hurricanes.

  • Disaster Response. Following hurricanes or other types of coastal disasters, navigation response teams, part of the Office of Coast Survey, conduct hydrographic surveys of the ocean floor, looking for changes in depth or debris from storms below the surface of the water that could pose great danger to vessel traffic above. Six navigation teams are strategically located around the country, ready to collect data needed to inform officials of navigational hazards and to help the U.S. Coast Guard find alternative routes for commercial and military ships.

  • Other Applications. In addition to generating the data needed to update nautical charts, hydrographic surveys support a variety of activities such as port and harbor maintenance (dredging), coastal engineering (beach erosion and replenishment studies), coastal zone management, and offshore resource development. Detailed depth information is also useful in determining fisheries habitat and understanding marine geologic processes.

Original article: Hydrographic Surveying