Marine Data Literacy 2.0

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Home > 2. Marine GIS > 2.6 GEBCO Contours

2.6 Adding GEBCO Depth Contours to a Project Map in Saga

  • Exercise Title:  Adding GEBCO Depth Contours to a Project Map

  • Abstract:  Shapefile depth contours are extracted from the General Bathymetric Chart of the Oceans, Digital Edition 2003 (GEBCO) for the project area.  The contours include the coastline (the World Vector Shoreline) and depths at increments of 500 m; the coastline only is also extracted as a separate product (resolution 1:250000).

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

  • Required Software:

  • Other Resources: 

    • N/A

  • Author:  Murray Brown

  • Version:  6-1-2014

1.  Run the GEBCO software (GebcoCE.exe on the CD), and you'll see 2 windows appears (both shown here).  The top window is a small control console; the bottom window, the Chart Definition window which only appears when you need to specify a new map, includes 8 tabs for different types of display options.
2.  You can open the Chart Definition window at any time by clicking on SELECT > CHART DEFINITION.
3.  On the DATA SOURCE page, make these selections:
  • IBCM is a Mediterranean dataset, which we don't need here
  • WVS is the World Vector Shoreline, borrowed by GEBCO from the US Navy.
  • We don't use the GEBCO Grid Files in this exercise.
4.  On the AREA page, make these selections:
  • The Chart Name section refers to existing named chart areas in the GEBCO system, which we don't need.
  • The Select From the Map section would allow you to pick a project area by zooming on a map, but we already know our project area limits
  • The "Equidistant Cylindrical" projection is actually just the simple geographic system, which is not really a projection at all.  In this system all the grid cells are perfect squares.
  • The Latitudes and Longitudes are entered for the "Exact Area," with N, S, E and W to indicate hemispheres; don't try to use signed numbers
5.  Do not select the PALETTE tab.  It takes a long time to load, and we don't need it because we won't be making any colored depth grid maps.
6.  On the CONTOURS page, select only the 1000's depths, manually

Not all of these contours are available for all world areas.  You should spend time later with GEBCO to see exactly what is available for your own area.

7.  On the CHART VIEW page, make these selections:
  • Bathymetric Contours - display
  • Legend - display
  • Contour Line Style - solid

Look below at Panel 17 to see the effects of turning ON some of these features that are not checked.  You can return later to GEBCO to see what the other options provide, visually.  But GEBCO is not a GIS system, so we need to move on in that direction. 

8.  On the GRATICULE page, make these selection:
  • Decimal Degrees - the only way that scientists today store and use geographic coordinates
  • 180...0...180 Degree Range - Method for Atlantic-centered maps. [Use 0...360 only for maps that cover the center of the Pacific Ocean.]
  • Grid/Graticule > Display.  If your map is very large, you could use 2 or 5 degrees to avoid crowding
9.  When all these settings have been chosen, then you can click OK.  Be patient, because GEBCO works slowly.
10.  A map like this should appear eventually.  It contains only selected bathymetric contours, and no coastline.
12.  Make these selections (Shapefile, Ordered, Contiguous), then click on BROWSE.  Navigate to DATA > BASEMAP > RELIEF, and enter the filename bathymetry_cons_liberia_gebco2003_1000s Then click on OK to export the data.
13.  Check in the destination folder right now to see if you have 3 files with this filename, and the extensions SHP, SHX, and DBF.  If you don't, then go back to panel 11 and perform the steps more carefully.
14.  Now select DISPLAY > BATHYMETRIC CONTOURS > DISPLAY OFF to remove the contours from the map.

Then check DISPLAY > COASTLINE to add the WVS coastline to the map.

15.  This map will be drawn, showing only the coastline.
16.  Use the same steps above to save the coastline in the folder DATA > BASEMAP > BORDERS, with the filename coastline_liberia_wvs_gebco2003
17.  This panel is provided only to show you how nice the GEBCO graphics can be when some of the features turned off in the CHART VIEW page are turned on.  You can explore GEBCO later on your own to make these kinds of maps.  Note that they are very easy to save as high-quality graphic images.

GEBCO is not a GIS system, and you cannot add any data to it.  Only the features in the above panels are available.

You can close GEBCO.

18.  Run Saga.  And select FILE > PROJECT > LOAD PROJECT and either select or navigate to the project_map_liberia SPRJ file to re-open the project collection.
19.  Select FILE > SHAPES > LOAD SHAPE and navigate to the contours shape to load it.  Then load the coastline shape in the same way.
20.  Both of these shapes now appear in the workspace, denoted LINE shapes.

Right-click on the contours shape, and select ADD TO MAP.

NOTE:  The older command "Show Shapes" has been dropped.

21.  When this MAP SELECTION window opens, select the existing map number 1 (already containing the world borders), instead of NEW.  Then click OK.
22.  This places the contours on the existing map, as you see here.

We can't tell what they mean, since they are all the same color.  So we need to look at the properties to differentiate them.

23.  In the properties window, select DISPLAY:COLOR CLASSIFICATION > TYPE > GRADUATED COLOR

Notice several rows below there is an item labeled ATTRIBUTE.  This is where you can find a drop-down menu, where you could pick the attribute to connect to color.  But with the GEBCO contours, there is only one attribute, DEPTH, and it is already selected.


24.  The depth contours are now color coded.  [They don't show very well in this reduced image, unfortunately.]
25.  You can use the COLORS menu (as in a previous exercise) to set the number of colors to a convenient small number that matches with the number of contours here.  In this case:
  • COUNT = 6
  • PRESETS = Rainbow

looks good, as you see here.  This legend is displayed by clicking the LEGEND tab at the bottom of the properties window.

In most cases, with "classified" type data, where there are only a finite number of data values, you should use a LOOKUP TABLE and not let Saga choose the colors automatically.  We will deal with lookup tables in a different exercise.  The simple legend/palette combination you see here is merely a coincidence, because most classified datasets don't match up automatically this well.


26.  OPTION:  Rename the contours object to GEBCO depth contours, then click SETTINGS > APPLY.  Then rename the coastline to GEBCO/WVS coastline, and click SETTINGS > APPLY.
27.  Select FILE > PROJECT > SAVE PROJECT to save the new data into the existing project.
28.  On your own:  Make a new map with only the world borders shape and the coastline shape from GEBCO, to compare them.  Here you see just a small part of the Liberia coastline.  Which one is more detailed?  Why?