12.1. Lesson: Impostazione GRASS

Usare GRASS in QGIS richiede di pensare all’interfaccia in un modo leggermente diverso. Ricordati che non stai lavorando in QGIS direttamente, ma lavori in GRASS via QGIS. Quindi, assicurati di aver installato QGIS Desktop con il supporto di Grass.

To open a QGIS session with GRASS available on Windows you have to click on the QGIS Desktop with GRASS icon.

L’obiettivo di questa lezione: Iniziare un progetto GRASS in QGIS.

12.1.1. Follow Along: |Iniziare una nuova sessione GRASS

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

1. First, open a new QGIS project.

2. In the Plugin Manager, enable GRASS in the list:

   

   The GRASS toolbar and the GRASS panel will appear:

   

   Fig. 12.28 GRASS toolbar

   

   Fig. 12.29 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.

Fig. 12.30 GRASS database structure (from GRASS docs)

12.1.2. Follow Along: Start a New GRASS Project

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

   

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

2. Set it as the directory that will be used by GRASS to set up its database:

   

3. Click Next.

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.

Nota

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. Click Next.

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

   

4. Click Next.

5. Now select the region South Africa from the dropdown and click Set:

   

6. Click Next.

7. Create a mapset, which is the map file that you’ll be working with.

   

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

   

8. Click Finish.

9. Click OK on the success dialog.

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

12.1.3. Follow Along: Loading Vector Data into 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.

Follow Along: Load data using the QGIS Browser

In section Il Pannello di Navigazione 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.

Nota

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.

Suggerimento

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

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:

   Avvertimento

   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:

   

   Nota

   Note the extra import options provided under Advanced Options. These include the ability to add a WHERE clause for the SQL query used for importing the data.

4. Click Run to begin the import.

5. When it’s done, click the View output button to see the newly imported GRASS layer in the map.

6. Close first the import tool (click the Close button to the immediate right of View output), then close the GRASS Tools window.

7. Remove the original rivers layer.

Now you are left with only the imported GRASS layer as displayed in your QGIS map.

12.1.4. Follow Along: Loading Raster Data into 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.

Nota

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. Open the GRASS Tools dialog again.

3. Click on the Modules List tab.

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. Click Run.

7. When the process is done, click View output.

8. Close the current tab, and then Close the whole dialog.

   

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

12.1.5. Try Yourself Aggiungere layer a 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

12.1.6. 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.

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.

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!

Suggerimento

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

12.1.7. In Conclusion

The GRASS workflow for ingesting data is somewhat different from the QGIS method because GRASS loads its data into a spatial database structure. However, by using QGIS as a frontend, you can make the setup of a GRASS mapset easier by using existing layers in QGIS as data sources for GRASS.

12.1.8. What’s Next?

Ora che i dati sono stati importati in GRASS, possiamo vedere le operazioni di analisi avanzate che GRASS offre.