Merged IBCAO/ ETOPO5 Topography
(AOMIP)

• Background
• Motivation
• Meshing Details
• Remapping IBCAO
• FORTRAN Code
• Download Data

Motivation

• The recent beta release of the International Bathymetric Chart of the Arctic Ocean (IBCAO) has provided an updated, high-resolution (i.e., 2.5 km) digital topographic data base ideal for use in numerical models of the Arctic Ocean. Previously, the best available data source was the Earth-Topography-Five-Minute Gridded Elevation Data Set (ETOPO5) available at a lower resolution (i.e., 10 km).

• One difficulty with the two data sets mentioned above is that the IBCAO data is on a polar stereographic projection grid and the ETOPO5 is on a spherical coordinate grid. For use in the Arctic Ocean Model Intercomparison Project (AOMIP) the two data sets are here meshed together to form a single data set in spherical coordinates.

Meshing Details

• A FORTRAN code fragment can be used to read a data file which holds a single, direct access record containing the merged IBCAO and ETOPO5 topographic data sets.

• The merged data product resides on the same grid as the original ETOPO5 data. Since the IBCAO is at a higher resolution (i.e., ~ 2.5 km) than the ETOPO5 (i.e., ~10 km) this results in some loss of IBCAO data resolution. Nonetheless, the merged data set presented is still an improvement over the original ETOPO5 data because of the incorporation of many of the new topographic measurements that lie within the IBCAO data set.

• The merged data product has an Arctic 'focus' with no specific attention paid to the Antarctic topography. However, the Antarctic Bedmap data base will be included when publicly released.

• The merged data set is created by (i) remapping the polar stereographic projection of the original IBCAO data onto a spherical coordiante grid and (ii) merging the new spherical coordinate IBCAO grid with the ETOPO5 grid using a 'linear-ramp' weighting factor.

• The 'linear-ramp' weighting is defined such that over the southernmost 5 degree band of the IBCAO data the weights varies linearly from 1.0 to 0.0 (i.e., a 1.0 weight implies fully replacing the ETOPO5 data by the IBCAO data; a 0.0 weight implies simply taking the orginal ETOPO5 data).

• The available gzip compressed data file (TOPO_IBCAO_ETOPO5.gd.gz ) is 23 MB. When gzip uncompressed the data file (TOPO_IBCAO_ETOPO5.gd) is 37 MB.

• The direct access record length of the data file is 37350724 bytes (i.e., 4 bytes per topographic value x 4321 longitudes x 2161 latitudes).

• The direct access record is read into the data array TOPO_IBCAO_ETOPO5 which has starting array index (1,1) corresponding to (longitude 0.0, latitude -90.0) and terminal array index (4321,2161) corresponding to (longitude 360.0, latitude + 90.0).

• The data array holding the topography is declared with 4 byte precision (i.e., single precision) and NOT 8 byte (i.e., double precision) because the data file is in GrADS/IEEE read/write format.

• The longitudinal and latitudinal grid spacing are exactly 1/12 of a degree everywhere on the globe (i.e. five-minutes of arc resolution).

• If you download the merged data set please let me know. If you have any difficulties using the data set or have suggestions for improving it please contact me.
  

Remapping IBCAO

• The objective is to exactly remap the original IBCAO data from its polar stereographic projection onto the spherical coordinate system of the earth.

• The original IBCAO data is on a right-handed Cartesian grid with origin at the North Pole and x-axis aligned 90 degrees counterclockwise to the direction of the prime meridian. This coordinate system has measure in units of m.

• The IBCAO polar stereographic projection grid cuts the spherical earth along the plane intersecting the earth at latitude .

• The earth's coordinate system is labeled as which correspond to longitude and latitude respectively. This coordinate system has measure in units of degrees.

• The radius of the earth is taken as .

• The discretization of the original IBCAO stereographic grid is 2.5 km in both x and y. The remapping creates a spherical coordinate grid having spacing of 0.10 degrees in longitude and 0.05 degrees in latitude. This new grid extends from a latitude of to the pole.

• The desired topographic height at any point on the spherical grid is derivable from knowledge of the IBCAO topographic height via the coordinate transformation

• This mapping tells us that at point on the original IBCAO stereographic grid we will find the desired topographic height corresponding to the point on the spherical grid. The quantity in square brackets is referred to as the map scale factor. Since this point will in general not coincide with any of the original IBCAO grids points we determine the value of by using interpolation over the original IBCAO grid.

• This procedure is repeated for every point on the spherical grid thus completing the remapping.

• Incidentally, performing the above coordinate transformation while ignoring the map scale factor can lead to a signifincant distortion whereby true geographical positions can be erroneously displaced by approximately 100 km. This is evident by looking at the position of the coastline (defined by the color transition from blue to green) from the merged IBCAO/ETOPO5 data as compared to the coastline (shown as white squiggly line) as taken from an accurate coastline data base. By comparison, when the maps scale factor is used there is almost an exact match between the merged IBCAO/ETOPO5 data coastline and the accurate coastline data base.

FORTRAN Code

      !declare single precision local array to hold topography
      REAL, DIMENSION (4321,2161) :: TOPO_IBCAO_ETOPO5

      !open i/o channel to data file
      OPEN  (UNIT  = 10,     
     &           FILE  ='TOPO_IBCAO_ETOPO5.gd',
     &           FORM  ='UNFORMATTED',
     &           ACCESS='DIRECT',
     &           ACTION='READ',
     &           RECL  = 4 * 4321 * 2161)

 

      !retrieve direct access record from data file into local array
      READ (10, REC = 1) TOPO_IBCAO_ETOPO5 (:,:)

Download Data

• Descriptor File ( in GrADS / ASCII control file format)
• Data File (gzip compressed, in GrADS / IEEE binary format)

. © David Holland. All Rights Reserved.

If you would like further information concerning any of the above topics please send email .