Polygonised General Bathymetric Chart of the Oceans (GEBCO) bathymetric data

Brief description

A polygonised bathymetric dataset covering the Southern Ocean. The dataset was compiled from data sourced from the General Bathymetric Chart of the Oceans (GEBCO) Digital Atlas, 2003 edition, and the Antarctic Digital Database, version 4.

Lineage

Progress Code: completed

Statement: The data processing was undertaken using ESRI ArcGIS ArcInfo 8.3 software. Bathymetric contours with depth a multiple of 500 m were extracted from the GEBCO 2003 data as a shapefile. The data were projected to a Polar Stereographic Projection (Latitude of True Scale 71 degrees South). The data were clipped to the required mapping extent, converted to a double precision coverage, and cleaned with a dangle length of 150 m to resolve the minor dangles associated with the arcs. The data were then checked for dangles. Dangle errors were either arc undershoots or overshoots. Overshoots were deleted, whilst connectors were added to join and complete arcs, which were undershoots. Once the dangle errors were resolved the data were cleaned with a dangle length of 5m and checked to ensure all dangles, except those round the Antarctic Coastline, had been resolved.

The coastline from the Antarctic Digital Database, version 4 scale of capture data, was then integrated with the bathymetry. The integrated coverage was again cleaned, this time with a small tolerance of 0.25 m. The dangles associated with the integration of the coastline were then resolved. As before overshoots were deleted, whilst connectors were added to join and complete arcs, which were undershoots. A depth of 0m was assigned to the Antarctic coastline arcs. The integrated dataset was built for arcs and polygons.

Minimum and maximum depth attribute values were then assigned to all polygons. The difference in these values was usually either 0m or 500m. (The difference is 0m if a polygon is comprised of arcs of the same depth, in other words a closed loop). If the difference differed from either 0m or 500m the polygons were tagged and checked. There are a number of cases where the entire required GEBCO bathymetric contour does not exist. For example a 2000m contour line may be missing, and hence a polygon has a depth range of 1000m (from 1500m to 2500m). These were deemed to be valid values.

Once the final checks were completed the coverage was attributed to comply with the Scientific Committee on Antarctic Research (SCAR) Feature Catalogue.

There are some 'large' polygons (area greater than 1000 sq km) that have the majority, but not all, of higher depth bounding lines the same value and/or the majority, but not all, of lower depth bounding lines the same value. For example, a 'large' polygon between 1000m and 1500m depth that contains a 'small' polygon that is bounded by a single 2000 m contour. That is, no 1500m contour surrounding the 'small' polygon and enclosed by the 'large' polygon. For this 'large' polygon the 1000_2000 value in the Interval field would not be an appropriate indication of the depth range for the majority of the polygon for map display purposes. Consequently, the values in the Interval field for polygons like the one mentioned above were changed so they are according to the majority higher and lower depth values, giving an indication of the depth range for the majority of the polygon for map display purposes. That is 1000_1500 rather than 1000_2000 in this example.

Finally, polygons representing depressions were identified and attributed as such in the 'Relief' field. The procedure to do this started with 'flat' polygons - polygons that have the same minimum and maximum height. Polygons with 'Interval' value 0_0 were excluded from this procedure as these represent areas of land. For each 'flat' polygon, the polygon surrounding it was looked at - if the minimum depth of the polygon surrounding it was less than the minimum depth of the 'flat' polygon then the 'flat' polygon was classed as a depression. The LPOLY# and RPOLY# fields in the arc attribute table were used to determine polygon neighbours and therefore the polygon surrounding a 'flat' polygon. Microsoft Access was used to link the arc and polygon attribute tables and compare the depths and then create the table that satisfied the criteria.