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QGIS User Guide, Release 1.8.0
• Load the gtopo30 elevation raster as above, then start the GRASS toolbox and click on the
GRASS shell.
In the shell window type the command r.shaded.relief map=gtopo30
shade=gtopo30_shade2 azimuth=315 zmult=3 and press [Enter].
• After the process finishes shift to the Browse tab and double click on the new gtopo30_shade2 raster to
display in QGIS.
• As explained above, shift the shaded relief raster below the gtopo30 raster in the Table of Contents, then
check transparency of the colored gtopo30 layer. You should see that the 3D effect stands out more strongly
compared to the first shaded relief map.
Figure 15.14: Displaying shaded relief created with the GRASS module r.shaded.relief
Raster statistics in a vector map
The next example shows how a GRASS module can aggregate raster data and add columns of statistics for each
polygon in a vector map.
• Again using the Alaska data, refer to Importing data into a GRASS LOCATION to import the trees shapefile
from the shapefiles directory into GRASS.
• Now an intermediary step is required: centroids must be added to the imported trees map to make it a
complete GRASS area vector (including both boundaries and centroids).
• From the toolbox choose Vector → Manage features, and open the module v.centroids.
• Enter as the output vector map ‘forest_areas’ and run the module.
• Now load the forest_areas vector and display the types of forests - deciduous, evergreen, mixed in different colors: In the layer Properties window, Symbology tab, choose from Legend type
‘Unique value’ and set the Classification field to ‘VEGDESC’. (Refer to the explanation of the symbology
tab :ref:sec_symbology in the vector section).
• Next reopen the GRASS toolbox and open Vector → Vector update by other maps.
• Click on the v.rast.stats module. Enter gtopo30, and forest_areas.
• Only one additional parameter is needed: Enter column prefix elev, and click [run]. This is a computationally heavy operation which will run for a long time (probably up to two hours).
15.9. The GRASS toolbox
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