Bathymetric Maps

Bathymetric Maps

A bathymetric map represents ocean depths depending upon geographical coordinates, in much the same way a topographic map represents the altitude of the Earth's surface in given different geographic points. Bathymetric maps have provided useful forensic evidence in court when certain types of crimes involving the sea are committed, or disputes arise about fishing boundaries or national boundaries at sea. Bathymetric maps have also been used by treasure-seekers when investigating the sea floor to identify the most likely areas to seek sunken ships, and aided in the search for the H.M.S. Titanic in the 1980s.

The most common type of bathymetric map displays lines called isobaths that indicate ocean depths. Like geographical maps of the Earth's surface, bathymetric maps are usually constructed in Mercator projection. Mercator projection is a mathematical method for displaying the surface of the Earth on a flat sheet of paper or computer screen. Mercator projection maps have been used for centuries in constructing sea charts that are used for sailing in all latitudes except the polar regions. Mercator projections are not used at extreme northern and southern latitudes because of the increasing degree of map distortion (the difference between map depiction and geographical reality) as one nears the poles.

The creation of a bathymetric map for a given region depends on the amount of depth measurement data for that region. Since before the invention of the echo sounder (an instrument that uses sound waves to gauge the depth of a body of water or of objects below the surface) in the 1920s, ocean (sea) depth measurements were quite rare; these measurements were made only in isolated points, and the creation of a bathymetric map was practically impossible. Thus, the structure of the ocean floor was unknown. It should be noted, for example, that the most important structure in the Atlantic Ocean—the Middle-Atlantic ridge—was discovered and began to be studied only after World War II (1939–1945). Another important factor for creating bathymetric maps lies in the determination of the geographical coordinates of the point where the depth measurement is made. In order to produce precise maps, precise geographical determinations are needed. GPS (Global Positioning System) technology is usually used for determining the coordinates of measurement points in bathymetric mapping.

Bathymetric maps of a country's continental shelf (the gradually sloping seabed around a continental margin) are important due to the special legal status of sea areas. These maps are important not only for defining territorial waters; they are also important because the shelf is home for intensive mineral deposits and mineral output, such as oil from beneath the sea floor off the coasts of the United Kingdom, Norway, and Mexico.

The United Nations Convention on the Law of the Sea (1982) states that, "The fixed points comprising the line of the outer limits of the continental shelf on the seabed. . .either shall not exceed 350 nautical miles from the baselines from which the breadth of the territorial sea is measured or shall not exceed 100 nautical miles from the 2,500 meter isobath, which is a line connecting the depth of 2,500 meters." It is clear that this statement implies that bathymetric maps are essential to draw precise boundaries of continental shelves. The Law of the Sea also determines that the foot of a continental slope will be set as the point where the slope's gradient change at its base is the greatest. A gradient is the maximum angle of the surface of a slope at a given point, and on the sea floor, a gradient can only be determined with bathymetric mapping.

When constructing topographic land maps, one can always measure the altitude of any point of the surface precisely. However, when constructing bathymetric maps, it is practically impossible to determine the depth of any one point on the ocean bottom. Obviously, bathymetric maps are more precise when more depth measurements per surface area unit in the given region are available. The most precise and detailed bathymetric maps are constructed using data provided by multi-beam echo sounding. The multi-beam echo sounder is a special kind of sonar located on board the research vessel that measures the depth simultaneously in several points of the ocean bottom, creating a swath of data. Depth determination by this method is performed regularly every few seconds while the vessel is in motion.

Bathymetric maps are finding more and more use both for practical forensic and scientific purposes. They have documented evidence that has resulted in laws to protect the environment of a given area (for example, locating areas of the sea and estuaries stressed by pollution off South Florida in 1999). In 1997, also in South Florida, bathymetric maps served as evidence of environmental compliance violations when they illustrated detrimental changes in submerged wetlands after sea grass was removed by illegal dredging.

Bathymetric mapping is also important for projects conducted in port territories; in these cases, usually a very detailed bathymetric map is constructed. Besides their uses in international courts, bathymetric maps are important for scientists who study the development of the Earth, the formation of seas and oceans, and the changing sea floor.

see also Accident investigations at sea; Remote sensing.