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Bathymetric Maps

Bathymetric Maps

ALEXANDR IOFFE

Bathymetric mapping refers to construction of ocean and sea mapsbathymetric maps (BM). Bathymetric maps represent the ocean (sea) depth depending on geographical coordinates, just as topographic maps represent the altitude of Earth's surface at different geographic points. Bathymetric maps are critical to submarine navigation, submarine evasion tactics, and in predicting the location of ocean signal channels.

The most popular kind of bathymetric maps is one on which lines of equal depths (isobaths) are represented. Like geographical maps of the surface of Earth, bathymetric maps are constructed in definite cartography projection. Mercator projection is used perhaps more often in constructing bathymetric maps, and has been used for a long time in constructing sea charts that are used for sailing in all latitudes except Polar ones.

The creation of a bathymetric map of a given region depends above all on the amount of depth measurement data for that region. Before the invention of the echosounder in the 1920s, ocean (sea) depth could be measured only by lead. Such measurements were quite rare; these measurements were made only in isolated points, and creation of bathymetric mapping was practically impossible. Thus, the structure of the ocean floor was virtually 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 investigated only after World War II. Another important factor for creating bathymetric mapping is determining geographical coordinates of the point where the depth measurement is made. It is evident that when these determinations are more precise, then the maps are better. As of 2003, the GPS (Global Positioning System) is used for determining the coordinates of the measurement points.

When constructing topographic maps of land, one can always measure the altitude of any point of the surface precisely. However, when constructing a bathymetric map, it is practically impossible to determine the exact depth of any point of the bottom of the sea. Obviously, bathymetric maps are more precise when more data of depth measurement per surface area unit in the given region are available. Currently, the most precise and detailed bathymetric maps result from using data from multibeam echosounding. The multibeam echosounder is a special kind of echosounder, which is located on board of the vessel and measures the depth simultaneously in several points of the bottom. These points are located on the straight line perpendicular to the vessel track. These points themselves are determined by the reflection of several acoustical pulses (beams) directed from one point at different angles to the vertical. The determination of depth in this method is performed regularly within periods of several seconds during the vessel motion. The measurement data are stored in a computer, and using them the map of an isobath of narrow bottom stripe can be represented periodically, or these data can be represented on a monitor.

It should be noted that in addition to the multibeam echosounder, other devices that measure depths simultaneously in several points of the ocean bottom have been developed, but all of them are based on the reflection of sound signals from the bottom.

If there are a lot of measurement data (more precisely this means that the average amount of measurement data per surface area unit is relatively big, and the measurement points themselves are located uniformly on the surface investigated), then computer methods of isobath construction are used. In this case, two stages of the work are executed: first using the measurement data obtained in arbitrary points of the surface, the values of the depth in knots of a regular grid are calculated (sometimes this stage is known as digital surface model construction), and then using these grid values, coordinates of different isobaths are determined (grid values are used also for other forms of bathometric mapping representations, 3-D views, for example). There are many algorithms of digital model creation, such as the least mean square method, and the so-called Kriging method, as well as algorithms of constructing an isobath of its own using depth grid values. To construct a precise map of the region it is necessary to perform echosounding surveying on it in such a manner that map stripes, obtained in different vessel tracks, would be as close to each other as possible, or even overlap. After performing such surveying, all data are joined together, and the map of the entire region is constructed.

It should be noted that currently, only small part of Earth's ocean bottom (several percent) is covered by such precise measurements. In some places, little data is available in a study area, obtained by one beam echosounder, or there is no data at all. In these cases, scientists try to use results of other geophysical measurements, first of all gravimetric measurements, to determine ocean depth. For example, methods of determination of ocean bottom topography using satellite altimetry or marine gravimetry data are useful. Even with using otherwise accurate satellite technology, indirect geophysical methods for determining the ocean bottom depth can always contain a mistake. The Earth's surface is a very complex formation, so the precise value of the ocean depth at a given point should be determined if necessary only by direct measurement.

In the case where depth measurement data are small in numbers for a given region, indirect methods are used in constructing bathymetric mapping, such as geomorphology analysis, for example. Scientists also take into account geological considerations and even human intuition, which can at times be useful.

Several international organizations are currently working on bathymetric mapping. The unclassified General Bathymetric Chart of the Oceans (GEBCO, in the scale 1:5000000), which may be considered a reference map, is one example. In this map, data of many regional bathymetric maps are collected, taking into account the different methods of their construction. There is also a digital version of this map (on CD), where files are represented in different formats, and in ASCII codes in particular, and where isobaths are represented in the so-called vector format.

Bathymetric mapping is finding increasing scientific and commercial use. For example, bathymetric maps are important in forging different underwater communications.

FURTHER READING:

BOOKS:

Barnes, J. Basic Geological Mapping, 3rd ed. New York: John Wiley and Sons, 1995.

PERIODICALS:

Perez, P. "SPOT Satellite Data Analysis for Bathymetric Mapping." Image Processing, vol. 3 (2000):464467.

Opderbecke, J. "Depth Image Matching for Underwater Vehicle Navigation." Image Processing, vol. 2(1999):624629.

SEE ALSO

Mapping Technology

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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 Oceanthe Middle-Atlantic ridgewas discovered and began to be studied only after World War II (19391945). 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.

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