Marine Data Literacy 2.0

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Home > 7. Bathy/Topo Data > 7.1 GEBCO Local

7.1 GEBCO 08 Global Relief Grid: Local Processing of Products

VERY IMPORTANT NOTE:  7.1 GEBCO 08 Global Relief Grid: Local Processing of Products, this exercise, has been deprecated, in favor of 7.4 GEBCO 08 Global Relief Grid:  Online Processing of Products which is infinitely easier and refers to better data; 8-26-2015.  7.1 will be deleted very soon.
  • Exercise Title:  GEBCO 08 Global Relief Grid: Local Processing of Products

  • Abstract:  Alongside the well-known and widely used vector contour product, the General Bathymetric Chart of the Oceans (GEBCO) project has published a new set of global gridded relief data.   A global 30-second grid, named GEBCO 08, and a 1-minute version (named gridone) are available.  [Gridone is used here for faster results.]  Users who want follow this method must obtain one of the global datafiles online, but the extraction of local data from it can be accomplished with Saga.  Special software for this extraction, provided by GEBCO, is not necessary.  If you want to do the data processing online (i.e. without downloading the global file), then go to the companion exercise GEBCO 08 Global Relief Grid: Online Processing.

  • Preliminary Reading (in OceanTeacher, unless otherwise indicated):

  • Required Software:

  • Other Resources: 

    • General Bathymetric Chart of the Oceans (GEBCO) Digital Atlas - The first CD of the old 2-CD collection has been replaced by a DVD that includes the GEBCO 08 grids (30 arc-second or 1 arc-minute resolution).  Saga can be used to read, display or subset the data, so the GridViewer software offered at the GEBCO website is not necessary for use in MDL exercises.  Use GEBCOLite if you can't find the CD.

    • GEBCO Gridded Bathymetry Data - If you don't have the digital atlas (the latest version with the DVD), then download the data from here.  Registration required. 

    • frame_grid_liberia_0.01deg.sgrd - Dummy grid made in the exercise Creating Grid Templates in Saga

  • Author:  Murray Brown

  • Version:  10-1-2013

1.  Open the Digital Atlas webpage (link above).  Spend a few minutes reading over the materials, if you aren't already familiar with GEBCO.
2.  Open the gridded data page (link above) and read about these relatively new products.  [These replace an earlier not-well-received product.] 
3.  Find this statement, and use either LOG IN (if you already have registered) or REGISTER. To download the data you are required to be a registered BODC web user. If you have already registered, please log in, alternatively please register to become a new user.
4.  If you are registering, then you can use something like these entries in the form.  Then click on LOGIN TO PROCEED.
5.  Make cure to check the box, then click ACCESS.
6.  You'll see these choices:
  • 30-sec Grid - Very large, highest resolution relief set
  • 30-sec SID Grid - Classified global grid, where each cell value is a code to identify the source of that part of the GEBCO 08 grid
  • 1-min Grid - Smaller, easier to store and use

For purposes of this exercise select the third choice, the 1-minute grid.  You can come back later to get the 30-sec version, if you wish.

7.  Just below, select DOWNLOAD DOCUMENTATION and save it in the folder DATA > BASEMAP > RELIEF > GEBCO. 

After you download the documentation, you will be allowed to check the DOWNLOAD DATA button.

8.  Download the data file to the same folder with the documentation with the new name relief_global_gebco08_1min_cdf.zip.  Then unzip it and rename the content file relief_global_gebco08_1min.cdf.

NOTE:  The filename extension used by GEBCO, NC, is not correct.  These grid files are in CDF format.

9.  Run Saga.
10.  Select MODULES > IMPORT/EXPORT-GDAL/OGR > GDAL: Import Raster
11.  This is a very simply import operation, so just make these selections and click OK.
12.  After the load operation (which takes a few minutes), you'll see this new object in Saga.  Notice that the grid system has a cellsize of 0.016667, which is what we expected for 1/60 of a degree, or 1 minute.

Right-click on it and select ADD TO MAP.

13.  Here's the global grid (shown in shades of gray), showing the extremely fine detail that is available.
14.  We need to resample these data down to the Liberia area.  To do this we need the Liberia dummy grids.  The specific one we need is the 0.01-degree version, which is just slightly smaller than the 0.016667 cellsize of the GEBCO grid.

Use FILE > LOAD > GRID to load the 0.01-degree dummy grid.

15.  Select MODULES > GRID TOOLS > RESAMPLING
16.  Make these selections:
  • GRID SYSTEM:  The system of the GEBCO data grid
  • GRID:  The global GEBCO data grid
  • PRESERVE DATA TYPE:  Check
  • TARGET GRID:  Grid

Then click OK.

17.  Now you are asked to specify the GRID for resampling:
  • GRID SYSTEM:  0.01-degree system
  • GRID:  0.01-degree dummy grid

Then click OK

18.  For INTERPOLATION METHOD, select NEAREST NEIGHBOR for the easiest/fastest processing.  Then click OK.

Experiment with the other method on your own later, so see if there are significant differences.

19.  This new object (#2) appears under the 0.01-degree system.
20.  And here, with ADD TO MAP, you can see the new Liberia relief grid..

 

21.  Please note that the blue-brown interface is not the coast.  The actual coast is about about 1 degree north of it!    As you can learn in the exercise Creating Color Palettes for Standard Product Rasters there are special methods needed to show the exact land border visually.
22.  Use FILE > SAVE AS to save the new Saga grid in the folder PRODUCTS > SAGA > GRIDS with the filename relief_liberia_gebco08_saga_1min.  You will need it later for many exercises.
23.  The GEBCO 08 global relief dataset will be merged into existing MDL exercises, replacing the old ETOPO1 relief grid.  This will be a gradual process, and should not change any exercises radically.  If you come to any mention of ETOPO1, then replace it with gridone (1 min).  If you have the 30-sec version of GEBCO 08, then ask your instructor for special steps to deal with the change in resolutions.