Data to the Project Map Extent with a "Dummy" Grid in Saga
is the method to resize or change the resolution of a data grid in GIS
systems. By this means, every aspect of the original grid (columns,
rows, cellsize, exact corner locations) can be altered. This can be
done manually, by specifying each of the new parameters, but it is
infinitely easier to accomplish with a set (library) of pre-made dummy
grids. A major new project, such as the creation of the Liberia data
collection, requires you to make this library, and always use it for
resampling, to insure that all project grids are identical.
Preliminary Reading (in
OceanTeacher, unless otherwise indicated):
|1. Before we can
begin gridding, we need to load the "dummy grids" that are needed by most gridding
algorithms in Saga as blueprints for the grids to create.
|2. In Saga, use
FILE > GRID > LOAD GRID to load all the dummy grids you have for Liberia.
You should have at least the 1-degree version and the 0.1-degree version.
|3. Also in Saga,
use FILE > GRID > LOAD GRID and navigate to the DATA > BASEMAP > GEOLOGY
folder to load the global sediment thickness grid, sediments_global_ngdc,
you worked with in the Marine GIS exercises. If you did not do those
exercises, then download the zipfile sediments_global_ngdc_sdat.zip
to the same folder location and unzip it.
|4. This is approximately what
you should see in Saga, if you have opened a map showing the global sediment
|5. If your map doesn't look
quite like the above, it's because we have adjusted the color palette to
emphasis "low end" numbers (i.e. we have applied a logarithmic color scale
that gives more weight to numbers at the lower end of the range). The
full range of sediment thickness values is about 0 to 18000 m.
You can do
this in the properties window, by setting the DISPLAY MODE to LOGARITHMIC
(UP). Also a STRETCH FACTOR must be set, and values from 10-1000 often
do a good job.
[Of course, LOGARITHMIC (DOWN) is available for value distributions that
need more emphasis at the high end of the range.]
|6. Now to begin the
resampling, select MODULES > GRID TOOLS > RESAMPLING
|7. For GRID SYSTEM, drop down
the menu and select the system that contains the grid you want to resample.
That is the huge 0.0833333-degree system.
|8. Now make these additional
- For GRID, choose the only one available, the global sediments grid
- For PRESERVE DATA TYPE, check the box
- For TARGET GRID, choose GRID (which means we will name a grid later)
Click OK and this window closes.
|9. This window opens.
For GRID SYSTEM, select the dummy grid system that is closest in resolution
to the 0.083333-degree grid to be resampled.
NOTE: Data "purists"
claim that resampling should never be done with a grid that is even slightly
smaller in cell-size than the original. In this case, we are certainly
|10. For GRID, make sure it
says CREATE (or it might over-write an existing other file).
This window closes.
|11. The selection of the method depends on the nature of the resampling:
- If there aren't any no-data areas, then use BICUBIC SPLINE
- If there are no-data areas (as we have here), then;
- If you are resampling to a grid cell size that is very close to the
original size, then choose NEAREST NEIGHBOR to avoid the creation of false
cell values in no-data areas.
- If you are resampling to a radically different cell size, then use an
interpolation method like BICUBIC SPLINE INTERPOLATION, and then (in a
second operation) use an appropriate land- or depth-mask to eliminate
false cell values that will be created in no-data areas. Masking is
covered in the exercise
Masking Methods for
Depth- or Height-Limited Grids in Saga
You can see the results of both methods in Panel 13 below, to illustrate
Then click OK.
|12. A new grid object appears
in the 0.1-degree system. It has a misleading name ("global"), but that
is just to help you recognize the original source.
Right-click on the
object and select SHOW GRID.
|13. Here is your new grid (on
the bottom). It matches up very well with the World Vector Shoreline
The improper grid on the top was created with BICUBIC SPLINE
INTERPOLATION, and you can easily see that a band of false cells extends
onto the land by at least 2 cell-widths.
|14. So that's all
there is to it. With the pre-made dummy grids, you can easily and
quickly make identical grids from all sorts of data. You'll find this
necessary many time in the process of making a data collection, because grid
operations usually require grids to have specific geometric "systems."
Also, and perhaps even as important, resampled grids are often much smaller
than the parent grid, saving storage space, loading time, and calculation
|15. Now to save the grids,
right-click on it in the objects menu and select SAVE GRID AS. Then
navigate back to the PRODUCTS > SAGA > GRIDS folder and save the grid as