Marine Data Literacy 2.0

Providing instruction for managing, converting, analyzing and displaying oceanographic station data, marine meteorological data, GIS-compatible marine and coastal data or model simulations, and mapped remote sensing imagery

 

 

 

 

Home > 7. Bathy/Topo Data > 7.4 GEBCO Online

7.4 GEBCO 08 Global Relief Grid: Online Processing of Products

  • Exercise Title:  GEBCO 08 Global Relief Grid: Online Processing of Products

  • Abstract:  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):

  • Required Software:

  • Other Resources: 

    • 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]

  • Author:  Murray Brown

  • Version:  10-3-2014

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 registered, please log in, alternatively please 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 specifications.
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 DOWNLOAD DATA.
15.  Navigate to DATA > BASEMAP > RELIEF > GEBCO and save the file as gebco_1min_-23_-6_-3_9.nc

 

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 corner)
  • 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 here.
19.  Use FILE > SAVE AS to save the new Saga format grid in the folder PRODUCTS > SAGA > GRIDS with the filename relief_liberia_gebco08_online_saga_1min.sgrd.  You 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.