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.
GEBCO 08 Global Relief Grid: Local Processing of Products
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:
Preliminary Reading (in
OceanTeacher, unless otherwise indicated):
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The first CD of the old 2-CD collection has been replaced by a DVD that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
|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 >
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.
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
|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
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
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
Palettes for Standard Product Rasters there are special methods
needed to show the exact land border visually.
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.