<i>Importance of sea floor nautical charts in delimiting maritime zones and boundaries</i>
The sea floor chart is a valuable tool in order to display the limits of maritime zones under national jurisdiction as per UNCLOS 1982. However it must be borne in mind that the nautical chart was specifically designed for the safe passage of vessels over the oceans and seas and these days it shows the coastline and the seaward area over which the delimitation is to be made. The chart is recognized by all the coastal states and it projects in good detail, the configuration of the coast and the morphology of the coastal zone including the sea floor. At present, in almost all countries, the nautical chart is the only type of chart or map which is close to meeting the needs of legal establishments and cartographers responsible for carrying out the task of boundary delimitation. It is however very well known that due to the mathematical nature of its projection, the charts so prepared contain distortions/inaccuracy which may affect its use in delimitation. The technical elements considered when choosing the chart are -chart projection, chart scale, horizontal datum and vertical datum. These factors have fundamental importance influence on the practical definition of maritime boundaries. The charts used for the purposes of delimitation between the countries should reflect the present situation as accurately as possible and they should be based on the most recent surveys carried out by the hydrographers of the coastal countries. Some projections are "Conformal" in that angles are preserved and the shape of areas is retained even if the scale must vary from one point to another. Lambert conformal projections are suitable for most delimitation tasks from 4 deg to 72 deg latitude North and South. The range of suitable scales will normally be from 1:1,000.000 to 1:1,000,000 for delimitation of the EEZ and CS while the scale for territorial sea boundary determinations should be of the order of 1:50,000 to 1:100,000, as more accuracy is required. Nautical charts used by ships/vessels contain only a part of the hydrographic data available, and represent a limited number of "contour lines" depending on scale, water depths, distance from the coast, intended use of the chart and availability of bathymetric data. If we want to have an accurate representation of the morphology of the sea floor in order for instance to determine the relationship with the terrestrial morphology of the coastal state, it might be useful to integrate the charts with data supplied by organizations qualified to conduct bathymetric and oceanographic surveys. The need for detailed data becomes particularly important when the determination of lines and points having special interest is required and in such cases, a more exhaustive and accurate documentation of " water depths" seismic profiles must also be available. The latter shall be in detail for at least the first 1,000 meters of the sub-bottom and be supplied jointly with the documents on the geology of the coastal zone and the interpretation of the presence and thickness of the sedimentary rocks. Through these documents it will be possible to obtain morphological profiles of the sea floor to determine the points, or zones, which divide the various parts of the sea floor (shelf, slope, rise, abyssal plain) and the external margin of the continental shelf. Adjacent States must use a Transverse Mercator projection chart to define a maritime boundary to its advantage as the narrow strip of sea and land is mapped with a minimum distortion. If the area of sea and land between States is not much extended in latitude, then also the Mercator projection may be used advantageously. The Transverse Mercator has the main advantage of allowing manual geometrical construction and the measurement of distances, angles and surfaces with errors which are acceptable when the graphic method is accepted as valid by the parties involved. It is recommended that a common geodetic datum be adopted and the adjustments necessary to pass from on geodetic system to the other one be quantified to avoid any possibility of cartographic incompatibility when positioning maritime boundary delimitations. If this transformation is not possible for lack of data, or even if feasible it is not convenient, the best solution is to agree on the use of common charts during the negotiations. The adoption of a common Geodetic Datum could avoid problems mainly in the use of the equidistance method. In order to determine maritime boundary delimitations, it may be necessary to perform surveys both on land and at sea through Geodetic and topographic surveys, Tidal and oceanographic surveys, Bathymetric surveys and Geo-scientific surveys. Most importantly determination of the low-water line which defines the normal baseline and base points for straight line segments of a baseline system, positioning of the geodetic coordinates of bench marks and other re-adjustment of ancient and or distorted surveys must be carried out beforehand. Thorough reconnaissance is to be done to identify and select those points to be as the baselines along the coast. Such points may be those used as turning points or terminal points of a straight baseline system or the low-water line itself. Off-lying rocks, islands and low-tide elevations are particularly important in this respect. Efforts should be to locate all geodetic control points in the area which may be used to reference the baseline points. When the coastline is deeply indented and cut into, or fringed with islands with numerous embayments, headlands and off-lying rocks and islands and it is planned to use a system of straight baselines, care must be taken to carry out a detailed reconnaissance in order that the points along the baselines may be well chosen . An accurate chart of large scale is most desirable for this purpose. The location of the low-water line will require careful tide measurements in areas where the coast is gently sloping as any error in vertical measurement may result in considerable horizontal displacement. It is extremely important to obtain the exact height above low water datum of all off lying rocks, sandbars and other features. At the same time the tidal range must be precisely determined in order that it will be known if these features are above or below high water. Whether or not a feature is a low-tide elevation or a feature permanently above high water may assume considerable importance. It is therefore very important to pay total attention to the trend of tide as well as to the possible influence on the sea level of meteorological factors. Determining the coastline become more difficult in zones that have large differences in tide. Photo-grammetric surveys may be used in addition to the geodetic methods to define precisely the entire extent of the low-water line providing details between the surveyed points. Low lying coastlines, subject to intensive erosive processes, with large tidal ranges, and bordered by extensive soft mud banks may make classical survey methods difficult. In such cases, aerial photograph and satellite images may be utilized at low-water, and bathymetric surveys at high-water, in order to establish the low-water It is therefore important for Bangladesh to ensure that we have correct and appropriate charts prepared and we know how to use the charts to our advantage. As the preparation of charts would take months and years, it is advisable that enough attention to its minutest details are given now before even we try to sit with our neighbours for talks on delimitation of various maritime zones and boundaries. We have to consider the difference between the various chart datums and mean sea levels which have been established throughout the world and have significant in terms of boundary delimitation. Chart datums are considered as the charted low-water line and are the line from which maritime boundaries and limits of jurisdictional zones are measured. The fact that there are different levels of Chart Datum means that adjacent or opposite States may use different levels at which to establish their baselines. Consequently, differences in the development of equidistance lines can result. We must remember that considerable differences may also occur in areas where rocks, islands or reefs, exposed at low tide, are used as baseline points. The choice of the level of chart datum may decide whether they are charted as features permanently below water, and thus eliminated from boundary delimitation calculations or as low-tide elevations, and thereby possibly included in such calculations. Make no mistake in the point that the precise definition of chart datum is crucial in boundary delimitation along with the accuracy of the data used in the actual calculations. The accuracy of the vertical datum depends on the length of the tidal records and the remoteness of the area to be delimited from the secondary station. There may be a requirement to produce a special report, including the working documents and calculations, to substantiate the results of the survey measurements and consequent delineation of the coastline. It is therefore urgent for the Foreign Ministry to make sure that we have all types of coastal and deep sea floor charts to substantiate our claims with our neighbours when demarcating our maritime boundaries or with the UN bodies when we are going to claim our CS on or before 2011. Special cell or body must start looking seriously into our preparations so that bad home works again and again do not deprive us from getting our due share of the Bay of Bengal and its resources and in spite of repeated warnings through this column someone someday could be held responsible for playing with the future of this country's resources. The author is a freelancer
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