13.1. Lesson: Configuration de GRASS

Using GRASS in QGIS requires you to think of the interface in a slightly different way. Remember that you’re not working in QGIS directly, but working in GRASS via QGIS. Hence, make sure you have installed QGIS Desktop with Grass support.

Le but de cette leçon : commencer un projet GRASS dans QGIS.

13.1.1. Follow Along: Start a New GRASS Session

To launch GRASS from within QGIS, you need to activate it as with any other plugin

1. First, open a new QGIS project.

2. Dans le Gestionnaire d’extension, activez GRASS dans la liste :

   
   
   

   The GRASS toolbar and the GRASS panel will appear:

   

   GRASS toolbar

   

   GRASS Panel

The GRASS panel is not active because, before you can use GRASS, you need to create a Mapset. GRASS always works in a database environment, which means that you need to import all the data you want to use into a GRASS database.

The GRASS database has a straightforward structure, even if at a first look it seems very complicated. The most important thing you should know is that the upper level of the database is the Location. Each Location can contain different Mapset: in every Mapset you will find the PERMANENT Mapset because it is created by default by GRASS. Each Mapset contains the data (raster, vector, etc) in a particular structure, but don’t worry, GRASS will take care of this for you.

Just remember: Location contains Mapset that contains the data. For more information visit the GRASS website.

GRASS database structure (from GRASS docs)

13.1.2. Follow Along: Démarrer un nouveau projet GRASS

1. Click on the Plugins –> GRASS –> New Mapset menu:

   
   
   

   You’ll be asked to choose the location of the GRASS database.

2. Définissez le dossier qui sera utilisé par GRASS pour y placer sa base de donnée :

   
   
   

3. Cliquez sur Suivant

GRASS needs to create a Location, which describes the maximum extent of the geographic area you’ll be working in, also known as Grass Region.

Note

the Region is extremely important for GRASS because it describes the area in which all layers will be taken into account for GRASS. Everything that is outside will not be considered. Don’t worry, you can always change the extent of the GRASS Region after the Location has been created

1. Call the new location SouthAfrica:

   
   
   

2. Cliquez sur Suivant

3. We’ll be working with WGS 84, so search for and select this CRS:

   
   
   

4. Cliquez sur Suivant

5. Maintenant, sélectionnez la région Afrique du Sud depuis la liste déroulante et cliquez sur Définir :

   
   
   

6. Cliquez sur Suivant

7. Créez un jeu de donnée, qui est le fichier sur lequel vous allez travailler.

   
   
   

   Once you’re done, you’ll see a dialog asking with a summary of all the information entered.

   
   
   

8. Cliquez sur Terminer.

9. Cliquez sur OK dans la fenêtre de succès.

You will see that the GRASS Panel will become active and you can start to use all GRASS tools.

13.1.3. Follow Along: Chargement des données vecteur dans GRASS

You have now a blank map and before you can start to use all the GRASS tools you have to load data into the GRASS database, specifically into the Mapset. You cannot use GRASS tools with layer that are not loaded into a GRASS Mapset.

There are many different ways to load data in the GRASS database. Let’s start with the first one.

13.1.3.1. Follow Along: Load data using the QGIS Browser

In section Le panneau de recherche we saw that the easiest and quickest way to load the data in QGIS is the Browser Panel.

GRASS data are recognized from the QGIS Browser as real GRASS data and you can notice it because you will see the GRASS icon next to the GRASS Mapset. Moreover you will see the icon next to the Mapset that is opened.

Note

You will see a replication of the GRASS Location as normal folder: GRASS Mapset data are those within the folder

You can easily drag and drop layers from a folder to the GRASS Mapset.

Let’s try to import the roads layer into the grass_mapset Mapset of the SouthAfrica Location.

Go to the Browser, and simply drag the roads layer from the training_data.gpkg GeoPackage file into the grass_mapset Mapset.

That’s it! If you expand the Mapset you will see the imported roads layer. You can now load in QGIS the imported layer like all the other layers.

Astuce

You can also load layers from the Layer Legend Panel to Mapset in the Browser Panel. This will speed up incredibly your workflow!

13.1.3.2. Follow Along: Load data using the GRASS Panel

We will use now the long method to load the rivers.shp layer into the same Mapset.

1. Load data into QGIS as usual. Use the rivers.shp dataset (found in the exercise_data/shapefile/ folder)

2. As soon as it is loaded, click on the Filter box of the GRASS Panel and find the vector import tool by entering the term v.in.ogr.qgis:

   Avertissement

   There are 2 similar tools: v.in.ogr.qgis and v.in.ogr.qgis.loc. We are looking for the first one.

   
   
   

   The v stands for vector, in means a function to import data into the GRASS database, ogr is the software library used to read vector data, and qgis means that the tool will look for a vector from among the vectors already loaded into QGIS.

3. Once you’ve found this tool, click on it to bring up the tool itself. Choose the rivers layer in the Loaded Layer box and type and name it g_rivers to prevent confusion:

   

   Note

   Notez les options supplémentaires d’import fournies dans Options avancées. Cela inclut la possibilité d’ajouter une clause WHERE pour la requête SQL utilisée lors de l’import de donnée.

4. Cliquez sur Exécuter pour commencer l’import.

5. Une fois fait, cliquez sur le bouton Voir le résultat pour voir la nouvelle couche GRASS importée dans la carte.

6. Fermez d’abord l’outil d’import (cliquez sur le bouton Fermer juste à droite de Voir le résultat), puis fermez la fenêtre Outils GRASS.

7. Remove the original rivers layer.

Vous ne disposez désormais plus que de la couche GRASS affichée dans la carte QGIS.

13.1.4. Follow Along: Chargement des données Raster dans GRASS

You can import a raster layer in the same ways we imported vector layers.

We are going to import in the GRASS Mapset the layer srtm_41_19_4326.tif.

Note

the raster layer is already in the correct CRS, WGS 84. If you have layers in different CRS you must reproject them in the same CRS of the GRASS Mapset

1. Load the srtm_41_19_4326.tif layer in QGIS

2. Ouvrez à nouveau les Outils GRASS.

3. Cliquez sur l’onglet Liste des modules.

4. Search for r.in.gdal.qgis and double click the tool to open the tool’s dialog.

5. Set it up so that the input layer is srtm_41_19_4326.tif and the output is g_dem.

   
   
   

6. Cliquez sur Exécuter.

7. Une fois l’opération réussie, cliquez sur Voir le résultat.

8. Vous pouvez Fermer l’onglet actuel puis cliquez dans la boîte de dialogue suivante sur Fermer.

   
   
   

9. You may now remove the original srtm_41_19_4326.tif layer.

13.1.5. Try Yourself Add Layers to Mapset

Try to import in the GRASS Mapset the vector layers water.shp and places.shp from the exercise_data/shapefile/ folder. As we did for rivers rename the imported layer as g_water and g_places to avoid confusion

Check your results

13.1.6. Open an existing GRASS Mapset

If you have an existing GRASS Mapset you can easily reopen it in another session of QGIS.

You have several method to open a GRASS Mapset, let’s explore some of them.

Let’s close the Mapset by clicking on the Close Mapset button of the GRASS Tools window.

13.1.6.1. Follow Along: Using the GRASS plugin

1. Click on the Plugins –> GRASS –> Open Mapset menu next to the Plugins –> GRASS –> New Mapset menu that we saw in the previous section.

2. Browse to the GRASS database folder: be careful! You must choose the parent folder, not the GRASS Mapset one. Indeed GRASS will read all the Locations of the database and all the Mapsets of each Location:

   
   
   

3. Choose the Location SouthAfrica and the Mapset grass_mapset that we have created before.

That’s it! The GRASS Panel will become active meaning that the Mapset has been correctly opened.

13.1.6.2. Follow Along: Using the QGIS Browser

Even faster and easier is opening a Mapset using the QGIS Browser:

1. Close the Mapset (if it is open) by clicking on the Close Mapset button of the GRASS Tools window.

2. In the QGIS Browser, browse to the folder of the GRASS database.

3. Right click on the Mapset (remember, the Mapset has the GRASS icon next to it). You will see some options.

4. Click on Open mapset:

   
   
   

The Mapset is now open and ready to use!

Astuce

Right click on a GRASS Mapset offers you a lot of different settings. Try to explore them and see all the useful options.

13.1.7. In Conclusion

La méthode de travail de GRASS pour la gestion des données est quelque peu différente de celle de QGIS car GRASS les charge dans une base de donnée à structure spatiale. Toutefois, en utilisant QGIS comme interface, vous pouvez rendre plus facile la création d’un jeu de donnée GRASS grâce aux couches QGIS comme sources de données pour GRASS.

13.1.8. What’s Next?

Maintenant que les données sont importées dans GRASS, nous pouvons nous intéresser aux opérations d’analyses plus évoluées que nous permet GRASS.