Adding GEBCO Depth
Contours to a Project Map
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):
|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
- 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
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
- 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
|11. Select FILE > EXPORT
DATA > VECTOR DATA
|12. Make these selections (Shapefile,
Ordered, Contiguous), then click on BROWSE. Navigate to DATA > BASEMAP
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
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.
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
|23. In the properties window,
select DISPLAY:COLOR CLASSIFICATION > TYPE > GRADUATED COLOR
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.
Now click SETTINGS > APPLY.
|24. The depth contours are
now color coded. [They don't show very well in this reduced image,
|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
- 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.
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
|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.
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?