Le Gall, J. Y.

English

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AN OCEANOGRAPHIC GRID SYSTEM FOR COASTAL AND CONTINENTAL MARGIN AREAS <*>b y  J . Y .  Lf. G a l l  Centre Océanologique de Bretagne, BrestIn the Marsden Square Grid System, or its derivations —  and this is the system that most national and international oceanographic data acquisition centres employ —  the information is located geographically for statistical purposes by means of a grid consisting of 10°, 5° or 1° “ squares The system, an early one dating from 1850, has three major disadvantages, and this has meant that its numbering principle cannot be used for a grid whose smallest lattice is less than 1“ of latitude by 1° of longitude.The first drawback is the numbering system itself. Beyond latitudes 10° N and 10“ S it becomes impossible to know as to what sector of the globe a given number refers without recourse to a chart or to a table giving conversions into geographical coordinates.The second drawback is linked to the first, and concerns electronic data processing techniques. The use of a Marsden Square number for indicating geographic coordinates on a flatbed plotter necessitates either a numerical sub-program or a program stored in memory.The third drawback of the Marsden system is that four figures are necessary in order to determine a quadrilateral of 5 “ X  5° and five figures for one of 1° X  1°- The Marsden system was not designed to go beyond this limit.In order to mitigate some of these drawbacks B a u d i n - L a u r e n c i n  (1966) devised an improved Marsden Square numbering system using the same number of figures. Its great improvement lies in the fact that it permits conversion of code to position from information given by a three or a five figure number. Once again, the system’s limits were purposely set at 1° X  1° quadrilaterals.However, a surface of 1° latitude by 1“ longitude is an area too vast for use in connection with the collection and processing of data required for hydrological, sedimentological, bionomie and fisheries studies in coastal regions. For more precision we must be able to go as far as minutes of latitude and longitude.(*) Contribution No. 187 o f the “ Département Scientifique du Centre Océanologique de Bretagne” .I. —  PURPOSE, AND ADVANTAGESThe problem of locating data geographically is a particularly difficult one when the data are submitted by the many different workers in the marine field (for instance, fishermen, or other workers —  to employ the word in its widest sense).These difficulties are of two kinds. Firstly, the fisherman or the collector in general who has to give a position may not wish to do so with too much precision (he may wish to protect his personal fishery grounds, or to ensure that industrially exploitable mineral deposits are protected.) Secondly, he may be beset by practical difficulties of accurate geographic positioning.It was, therefore, necessary to devise a numbering system with the following characteristics :—  Provide a degree of precision of as much as a minute of latitude and longitude;—  Indicate increasing degrees of precision by a system of grading according to the importance of the information (in the same way as the words in a Thesaurus are graded) ;—  Permit the independent reader to deduce the geographic position in longitude and latitude uniquely from the figures of the numbering code, each figure being taken successively and individually.—  Be suitable for data processing with the same techniques as auto­mated cartography (computer-linked plotters), whatever the degree of precision employed to define the location of the information.The proposed system fulfills these four requirements and necessitates only a seven-figure number for designating the desired precision (minutes of arc), providing that a preliminary positioning to within a 10° X  10° area has been carried out.II. —  THE NUMBERING PRINCIPLEConsidering the degree of precision required, it would be logical to presume that the finder, the collector, the analyst or the user of the inform­ation already knows the geographic position of the spot to within 10° —  in other words, we assume he is capable of deducing that the position of his data falls within a quadrilateral of 10“ of latitude by 10° of longitude. At this stage the present Marsden numbering should be used. As an example, the French coasts fall almost entirely between 40° and 50° latitude North. Similarly, if we study a region of even the size of Brittany we know we shall be between 47° - 48°  North and 1" -5 °  West.50'40'30'20'10'47° N5°W 50' 40' 30' 20' 10 4°WThe first code figure in the proposed system signifies the latitude and longitude units. The reader will thus retain in mind the 10° units of latitude and longitude defining the work area in a general way. The data processer will incorporate these two constants in an optional program.Progressive degrees o f precision.Within an area defined by 10° latitude by 10" longitude the first step in positioning data will be the use of basic quadrilaterals of 1 ° X  1° identified by a two-figure number. This 1° X  1° area will then be sub­divided, as is usually the case on charts, into 36 sub-divisions of 10' X  10' and referenced with a four-figure number. The following step is within these 10' X  10' areas where the subdivision is into twenty-five 2 ' X  2' quadrilaterals. This last division was selected on account of the two limiting factors already mentioned —  the fact that some local fishermen55 54 53 52 51 50r45 44/42// 4 ' 40357 *' 3368482808zsrjSSBEB02000Î 312'3025/2 3 /j 6 ‘ 6’ 4  2' 22 4t i \ 20i5/ 14 13 12 II 1005 04i03i02 01 0 0do not wish to disclose positions with great accuracy and the concern that everyone may have the possibility of preserving his own professional secrets. Those who do not need to safeguard such interests and who are equipped with the necessary navigational means will wish to use the next and fourth degree of precision, that is to say the 1' X  1' division.Procedure for graded numbering.The example given is for coding a quadrilateral of 1' X  1'-cp =  47-27' —  47-28' N G =  05u20' —  05-21' Wïn the system proposed here, this code will read :75 22 60 1The code is explained below.K now n locality.Atlantic coast of Francebetween latitudes cp =  40“ —  50° N between longitudes G =  0" —  10° WFirst Sub-division ( 1 " X  1 ")•Code 757 =  Number indicating the unit of a degree in latitude i.e. c p = 7 +  [constant =  40] =  47° —  48° N5 =  Number indicating the unit of a degree in longitudei.e. G =  5 +  [constant =  0] =  5° —  6 ° WSecond Sub-division (10' X  10').Code 75 222 =  Number indicating tens of minutes in latitude47-20' —  47“30' N2 =  Number indicating tens of minutes in longitude05-20' —  05”30' WThird Sub-division (2' X  2').Code 75 22 606 =  Number indicating minute units (even numbers only) withinthe ten-minute unit of latitude defined above.47°26' —  4 7 -2 8 'N0 =  Number indicating minute units (even numbers only) within the ten-minute unit of longitude defined above.05°20' —  05°22' WFourth Sub-division (1' X  10-Code 75 22 60 1=  Code for the inter-cardinal compass pointsNE =  1 SE =  2 NW  =  3 SW  =  4__+  1' in latitude+  0 in longitude75 22 60 1 q> =  47”27' —  47°28'NG  :=  05-20' —  05-21' WCONCLUSIONThe numbering system proposed necessitates a seven-figure number to define a quadrilateral of 1' of latitude and longitude. Note that we started with the position already defined as falling within two parallels of latitude and two meridians of longitude 10° (or multiples of 10°) apart.In this way, processing of information —  whatever the degree of precision of its location —  is made possible in terms of a degree to a minute of arc, and with this system we can pass direct from the code to the geographic position without need of tables or charts. Furthermore it is compatible with computer techniques in automated cartography.Thus it is not applicable to the problems usually encountered in cases of a precision of the degree or larger, for which it was not intended, and where the Marsden Square system —  or even better, the improved B a u d i n - L a u r e n c i n  system —  will generally be utilised.The applications of the system of numbering described here are essentially to a wide variety of work on coastal strips from the coastline up to the continental margin. Its precision and its excellent definition were initially devised for collecting information about the grounds of local fishermen, but the system could be used in such fields as the improve­ment of offshore concessions and the monitoring of the effect of urban sewage on benthic life in studies of the movement of coastal sediments.REFERENCESJ o h n s o n , C.R. (1965) : On the Marsden Square System : A Progress Report Unpublished manuscript). U.S. National Oceanographic Data Center.B a u d i n - L a u r e n c i n , F. (1966) : Sur une amélioration concernant la numé­rotation des carrés statistiques Marsden. Communication Symposium sur l’Océanographie et les ressources halieutiques de l’Atlantique tro­pical (1er), Abidjan, pp. 425-426.National O c e a n o g r a p h i c  Data C e n t e r  (Washington) (1973) : User’s Guide to NODC’s Data Services. Key to Oceanographic Records, Documenta­tion No. 1, February 1973, 72 p.(Manuscript submitted in French).IMPRIMERIE LOUIS-JEANP ub lications sc ien tifiques  e t litté ra ires  TYPO - OFFSET 05002 GAP -  TéMphone 51-35-23 * D * p ô t  lé  g e l  3 7  • 1974

An Oceanographic Grid System for Coastal and Continental Margin Areas

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An Oceanographic Grid System for Coastal and Continental Margin Areas

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