GEBCO 08 Global Relief Grid: Online Processing of Products
Alongside the well-known and widely used vector contour products, the
General Bathymetric Chart of the Oceans (GEBCO) project has published a
new set of global gridded relief data. [These replace an earlier
not-well-received product.] The global 30-second grid, named GEBCO 08, synthesizes several
high-quality grids and survey tracks. [A companion 1-minute version
is also available, used in this exercise for faster results.] If you want to do the data
processing locally (i.e. download the entire dataset and subset it by
geographic coordinates), then go to the companion exercise
GEBCO 08 Global Relief Grid: Local Processing
Preliminary Reading (in
OceanTeacher, unless otherwise indicated):
General Bathymetric Chart of the Oceans (GEBCO) Digital Atlas
- "The GEBCO One Minute Grid, a global one minute
interval bathymetric grid, based primarily on the bathymetric contours
of the October 2002 version of the GDA. It also incorporates land
elevations derived from the Global Land One-km Base Elevation (GLOBE)
project data set. " [From the website]
|1. Open the GEBCO website
above, and read about the program and its products.
|2. Scroll down the page to
find and click the DOWNLOAD DATA tab.
|3. Read about the gridded
dataset. This version ("GEBCO 08") replaces a grid published in the
1990's that was essentially a gridding of all the XYZ coordinates of the
vector contour lines. This new version is a compilation of all the
best available grids, and has been recommended for use by the World Data
Center for Geodesy and Geophysics (NGDC in Boulder, Colorado, USA).
|4. Scroll down the page, and
you will find this announcement.
- If you are already registered, then LOG IN
- If you are not already registered, then click REGISTER
|"To download the
data you are required to be a registered BODC web user. If you have already
register to become a new user."
|5. Enter all the
registration information, and then click NEXT.
|6. Check the I HAVE READ
item, and then click on ACCESS THE DATA.
|7. Click on CONTINUE.
|8. This data selection screen
appears with a global map.
|9. Enter the coordinates of
your area of interest. Be extremely careful with the N, E, S and W
|10. Click on the zoom icon.
|11. Your area will now appear
on the map. If it doesn't look good, then go back to Panel 9.
|12. When the map looks good,
then select the dataset you want. We don't use Mr. SID grids, so you
want the 30-second grid or the 1-minute grid.
For our use here, select the
1-minute grid. You can return here later to work with the 30-second
grid on your own time.
|13. Click on DOWNLOAD
DOCUMENTATION and examine the document provided.
|14. Now you can click on
15. Navigate to DATA > BASEMAP > RELIEF > GEBCO and save the file as
|16. Run Saga and open MODULES
> IMPORT/EXPORT GDAL-OGR > GDAL: IMPORT RASTER
|17. Make these selections:
- FILE - select the new NC file you just created. [Ignore the CDF
extension you see in the image]
- MULTIPLE BANDS - Unchecked
- TRANSFORMATION - Check this to use the correct geographic coordinates
(instead of a default page coordinate grid with 0,0 at the lower left
- INTERPOLATION - The author suggests BICUBIC SPLINE INTERPOLATION, but
you can experiment with other choices; this may not even be relevant
for this type of grid
Then click on OK.
|18. The grid now appears in
your list of objects. Visualize it, as you see here, with ADD TO MAP.
Check the LEGEND to make sure you have the correct grid values, as you see
FILE > SAVE AS to save the new Saga format grid in the folder PRODUCTS > SAGA >
GRIDS with the filename
will need it later for other exercises.
If you need a NetCDF format grid,
then follow these steps:
- Select IMPORT/EXPORT> GDAL/OGR > EXPORT RASTER and use FORMAT =
NETWORK COMMON DATA FORMAT
- All the other default settings in this tool should be OK.
- Navigate to LIBERIA > PRODUCTS > SAGA > GRIDS and use
relief_liberia_gebco08_online_saga_1min.nc for the filename.
- Check the new raster in
ncBrowse to make sure
it looks fine.
You will also need his file later for other exercises.
|20. Just for your
information, here's the same grid with a 100-color rainbow palette.
Look closely to see that the land includes some small mountains. Also the World Borders country map has been added. Notice that the
yellow-brown interface is NOT the coast.
|21. The GEBCO 08 global
relief datasets 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 find ETOPO1 in an
exercise, then replace it with GEBCO 08 (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.