Contents of the GEBCO Digital Atlas (GDA)
2.3 EXTRACTING DATA FROM THE GDA
Most of the data are stored on the CD-ROM in two forms:
a) as a set of binary formatted files and indices designed for direct input to the GDA Software Interface and optimised for speed of processing and display. These formats need not concern the user.
b) as a set of simple flat ASCII files directly accessible to the user. These include:
18 files of the GEBCO Fifth Edition bathymetric contours and coastlines with one GEBCO sheet per file -the geographie coverage of each sheet is as shown in Figure 2 (page 13).
18 files of the GEBCO Fifth Edition trackline control (including the outlines of survey boxes) with one GEBCO sheet per file - these complement the above files of bathymetric contours.
10 files of the mCM First Edition bathymetric contours and coastlines with one IBCM sheet per file - the geographic coverage of each sheet is as shown in Figure 5 (page 23).
8 files of GEODAS (March 1993) bathymetric tracklines showing the coverage of data held at the IRO Data Centre for Digital Bathymetry with one file per geographic octant.
Each of the directly accessible files is buHt up as aseries of vector streams relating either to bathymetric contours or coastlines or to tracklines or the outlines of survey boxes. Each file is formatted in the same flat ASCII format as described in Table 2 (page 39).
In copying files directly from the CD-ROM, the user is constrained to accepting the geographic coverage of the file and all the contour levels contained therein. A more flexible approach is provided through the GDA Software Interface, which allows the user to define precisely his geographic area of interest, irrespective of sheet boundaries. It also allows the user to specify which contour levels are to be selected and also to merge trackline vectors in the same file as the bathymetric contours and coastlines.
When vector streams are exported through the GDA Software Interface, the user is offered a choice of formats. The vectors can either be exported in the simple flat ASCII format described in Table 2 or in the DXF format. The DXF originated as a proprietary format in the commercially available 'AUTOCAD' package, but is now a standard input format for many other commercially available software packages. With this option the user is able to avail hirnself of the presentational and manipulative capabilities of such packages for displaying the vector information in the GDA. In DXF the vector streams are expressed as polylines in geographic co-ordinates with each contour depth assigned to aseparate layer and aseparate logical colour. Each of the layers is labelIed according to the same feature code system as given in Table 2.
The GDA Software Interface provides the only mechanism whereby the user can copy the World Vector Shoreline (WVS), or parts thereof, into his own files. The user can specify his geographic area of interest and the scale version of WVS to be exported. When output, the WVS can be merged in the same file as the bathymetric contours and tracklines as required. If WVS is selected prior to contour export, then it will replace the existing coastline in the GEBCO or mCM charts.
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TABLE 2. A simple Oat ASen format for GDA vector streams
Within this format, files are built up as a series of labelIed vector streams relating to bathymetric contours, coastlines, tracklines or the outlines of survey boxes. A new vector stream is started for each contour segment, coastline segment, trackline or survey box.
Bach vector stream is preceded by a header record containing a feature code 'ICODE' for the vector and a count 'ICOUNT' of the number of succeeding co-ordinate pairs making up the vector. Each co-ordinate pair is stored in a record with a geographie latitude 'ALA T' and longitude 'ALONG', each expressed in decimal degrees.
By default, latitude is stored in the range -90° to +90° with north positive and south negative; with longitude stored in the range -180° to +180° with east positive and west negative. However, it is possible for the user to specify that longitude should be stored in the range 0° to 360°. A further 9ption enables the longitude to be expressed in a 360°
range within the band 0° to +540° although this option will only be used in exceptional cases for maintaining a contiguous geographie area.
Each record is made up of 20 bytes as folio ws:
Header record: ICODE, ICOUNT in format (2I6,6X),CR,LF Co-ordinate pair record: ALAT, ALONG in format (F8A,FlOA),CR,LF
The feature code of each vector contains:
'0' for coastlines
'depth in corrected metres' for bathymetric contours '22222' for tracklines
'22223' for outlines of survey boxes, special compilations or areas of dense sounding coverage
Within each file, the vector streams are stored in ascending order of their feature code value 'ICODE' - thus all coastline vectors will appear before the bathymetric contour vectors which, in turn, will be arranged in ascending order of their depths. If present, the tracklines will follow the bathymetric contours and then be followed by the outlines of survey boxes.
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2.4 GEBCO FIFfH EDITION BATHYMETRIC CONTOURS AND COASTLINES This data set was digitized from stable base transparencies of the masters of the eighteen printed sheets of the GEBCO Fifth Edition and all data are stored in geographie co-ordinates. The series comprises 16 sheets on Mercator projection at a scal~ of 1: 10 million (at the equator) in the region
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and two polar sheets on polar stereographic projection at a scale of 1:6 million at 75° latitude. These sheets were based on the intemational1924 (Hayford) ellipsoid. The polar sheets were only digitized ton°
latitude Le. to the limits of the Mercator sheets. The sheet areas digitized are shown in Figure 2 (page 13).Where sheets overlapped, the overlap area was only digitized from the most recently published sheet. It is estimated that, in general, the digitized bathymetric contours lie weIl within 0.5mm of their positions on the printed sheets. At the completion of digitization, a detailed edgematching of contours was carried out across sheet boundaries so as to provide a seamless global bathymetry - thus digitized contours at sheet boundaries may not always accord with those on the printed sheets although any adjustments made were carried out taking due account of the underlying trackline control information.
All bathymetric contours present on the published sheets were digitized, including the basie GEBCO contours of 200m, 500m, and 500m intervals thereafter. The actual contours depieted vary considerably from sheet to sheet and intermediate contours are often included to better define the bathymetry in certain regions. However, all sheets include, as aminimum, the basic GEBCO contours and these were digitized as closed contours without open ends. All contour levels are expressed in corrected metres Le.
based on echo-soundings corrected for the speed of sound according to the Echo-Sounding Correction Tables (see section 2.11).
The bathymetric contours were digitized from the published GEBCO Fifth Edition sheets except in two regions:
a) 500S to 7°N; 700W to 200E where they were digitized from the approved and edited pre-publication compilation sheets for revised GEBCO sheet 5.12. In general, these sheets were compiled and digitized on Mercator projection at a scale of 1:5',737,447 at the equator, except in the area OoS to 3°S where they were prepared at a scale of 1:2,500,000. The area 0° to 7°N, 200W to 200E was taken from the digitized version of GEBCO sheet 5.08.
b) OON to 46°40'N; l000E to 1800E, where the contours were provided in digital form by the Japan Oceanographic Data Center from their standard 1:1 million bathymetric charts of the region. These were subsequently reduced in volume by 89% using the Douglas-Peucker algorithm with a lateral tolerance of 0.08mm.
The digital coastline included in this data set is essentially that portrayed on the GEBCO Fifth Edition sheets Le. based on the Carte Generale du Monde of the Institut Geographique National in Paris, France, and, for the Antarctic continent, on maps
provided by the Scott Polar Research Institute in Cambridge, England. However, there are two exceptions:
a) for the region 50DS to 7DN; 700W to 200E of revised GEBCO sheet 5.12, the digital coastline has been taken from the high resolution W orld Vector Shoreline digital data set reduced in volume by 84% using the Douglas-Peucker algorithm with a lateral tolerance of O.04mm at a scale of 1:5,737,447.
b) for the region OON to 46°40'N; lOOOE to 180oE, the coastline was provided in digital form by the Japan Oceanographic Data Center based on their standard 1:1 million bathymetric charts of the region. The volume of this data set was reduced by 89%
using the Douglas-Peucker algorithm with a lateral tolerance of O.08mm.
This data set appears in two forms on the CD-ROM; one tailored for access and display through the GDA Software Interface, while the other is directly accessible to the user with one fIle per GEBCO sheet, formatted according to the specification given in Tahle 2 (page 39).