Objective: Create a 3D visualization of an elevation map created with elevation values from ground control points, excluding water bodies. Learn how to import layers into R, perform basic manipulations in GRASS within QGIS and GRASS standalone.

Step 1: If you haven't done so already, save the 'region_hydrique' in the UTM 18N (NAD83) CRS. Start a New project and set the CRS of the current canvas to UTM 18N (NAD83). Add the region_hydrique file (in UTM) and the 'Elevation_points.shp' file in QGIS (this file is already in UTM).

Step 2: Make sure that the GRASS plugin is activated and that the GRASS toolbar is selected under View­­ > Toolbar. Click on the 'New Mapset' icon and define a new GRASS database folder, create new location (give it a name such as 'Workshop2', choose the UTM 18N (NAD83) CRS, set the region to the current QGIS extent, and choose a name for the mapset (e.g. 31h05).

Step 3: You now have to import the files into GRASS. Click on the 'Open GRASS tools icon', click on 'module tree', and find File management > import into GRASS > import vector into GRASS > v.in.ogr.qgis - Import loaded vector. Select the vector layer region hydrique (in UTM) and find name for the output vector map. Repeat these steps for the Elevation_points vector file. Load the newly created GRASS files into the canvas by clicking on the 'Add GRASS vector layer' icon.

Step 4: Click on the 'Display current GRASS region' icon and then on the 'Edit Current GRASS region' icon, and select a GRASS region that just encompasses all the elevation points. Set the Cell width and height to 30 m.

Step 5: Open GRASS tools and find the v.to.rast.constant module to convert the water bodies file to raster. Keep the raster value at 1.

Step 6: Close QGIS. We could decide to continue working within QGIS, but instead, we will move to GRASS outside of QGIS.

Step 7: Open GRASS and select the Database, Location and Mapset that you have created within QGIS.

Step 8: Load the Raster map you have created in Step 5, and the elevation points you have imported in Step 3 with the 'Add raster map layer' and 'Add vector layer' icons.

Step 9: Use the Raster ­­­> Mask module (r.mask) to create a mask layer for the next step. Select the 'Create inverse mask' and put 1 in 'Category Value to use for Mask'. Click the 'Run' button to run the module.

Step 10: Use the v.surf.rst module (Raster > interpolate surfaces) to create an interpolated elevation map using the Elevation_points layer. Specify a name for the output elevation map in the output tab and specify the ELEVATION column as the 'Name of the attribute column to be used for approximation'.

Step 11: Generate a slope map from this new elevation layer using the r.slope.aspect module (Raster > Terrain Analysis). Specify an output name for the slope layer.

Step 12: View the resulting map in three dimensions in the NVIZ module (File > NVIZ).

• DATABASE (Géodatabase): Répertoire principal de GRASS.
• LOCATION (Secteur): Répertoire de travail. SCR unique.
• MAPSET (Jeu de données): Sous-répertoires sous le secteur.
• REGION (Région): Définie par une étendue et une résolution spécifiques.

La liste complète des commandes GRASS est ici.

On first use (within R):

install.packages('spgrass6', dependencies=TRUE)

For Mac users, run instead:
install.packages('spgrass6', type='source', dependencies=TRUE)

spgrass 6 commands:

execGRASS               execute GRASS commands (when launched from R)
system                  execute GRASS commands (when launched from GRASS)
readRAST6               read GRASS raster files
writeRAST6              write GRASS raster files
readVECT6               read GRASS vector object files
writeVECT6              write GRASS vector object files
gmeta6                  read GRASS metadata from the current LOCATION
getLocationProj         return a PROJ.4 string of projection information
gmeta2grd               create a GridTopology object from the GRASS region
vInfo                   return vector geometry information
vColumns                return vector database columns information
vDataCount              return count of vector database rows
vect2neigh              return area neighbours with shared boundary length