Grades 9-12: See That Sound (Hydrographic Surveying) Hydrography is the science of measuring and describing the physical features of bodies of water and adjacent land areas that are periodically underwater (such as areas that are covered and uncovered by rising and falling tides). The most common use of hydrographic data is as an aid to safe navigation. This educational activity explains the concept and major components of sonar systems used to collect hydrographic data. Thunder Bay National Marine Sanctuary: Shipwreck Alley "Shipwreck Alley" is the final resting place for scores of ships that have fallen victim to Lake Huron’s murky fog banks, sudden gales, and rocky shoals. The cold, fresh waters of Lake Huron slow down natural processes that corrode iron and degrade wood, so even the oldest shipwrecks are often in excellent condition. The shipwrecks of Thunder Bay tell us a great deal about life on the Great Lakes over the past 200 years, and offer opportunities to study the structure, rigging, and other details of sailing ships that are hard to find anywhere else. Nautical Charts and Marine Navigation: Plot your Course One of the most fundamental tools used by marine navigators is the navigational chart. The task of charting U.S. coastal waters began in 1807 when President Thomas Jefferson ordered the first survey of the nation’s coast. Coastal Currents and Navigation: Ready, Set, Drift! Anyone who operates a boat in coastal waters needs to understand the movement of currents and how to handle their effects. Shipmasters, in particular, need accurate real-time information about coastal water movements to avoid dangerous and expensive groundings and collisions. NOAA Education NOS Education