13.1. Lesson: Configuración 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.

El objetivo de esta lección: Comenzar un proyecto GRASS en QGIS.

13.1.1. basic 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. En el Adiministrador de Complementos, activa GRASS en la lista:

    ../../../_images/enable_grass.png

    The GRASS toolbar and the GRASS panel will appear:

    ../../../_images/grass_toolbar.png

    GRASS toolbar

    ../../../_images/grass_panel.png

    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.

../../../_images/grass_database.png

GRASS database structure (from GRASS docs)

13.1.2. basic Follow Along: Comienza un Nuevo Proyecto GRASS

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

    ../../../_images/grass_menu.png

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

  2. Definelo como el directorio que será usado por GRASS para crear su base de datos:

    ../../../_images/grass_folder.png
  3. Haz clic en Siguiente.

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:

    ../../../_images/new_location.png
  2. Haz clic en Siguiente.

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

    ../../../_images/wgs_84_selected.png
  4. Haz clic en Siguiente.

  5. Ahora, selecciona la región South Africa del menú desplegable y haz clic en Establecer:

    ../../../_images/set_south_africa.png
  6. Haz clic en Siguiente.

  7. Crea un directorio de mapa, que el archivo de mapa con el que estarás trabajando.

    ../../../_images/grass_mapset.png

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

    ../../../_images/grass_final.png
  8. Clic Terminar.

  9. Haz clic en Aceptar en el diálogo de éxito.

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

13.1.3. basic Follow Along: Cargando datos vector en 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. basic Follow Along: Load data using the QGIS Browser

In section El panel de exploración 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 grassMapsetOpen icon next to the Mapset that is opened.

../../../_images/grass_browser.png

Nota

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

../../../_images/grass_browser_import.png

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.

Truco

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. basic 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:

    Advertencia

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

    ../../../_images/grass_panel_import.png

    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:

    ../../../_images/grass_tool_selected.png

    Nota

    hard Nota las opciones de importación extra dentro de Advanced Options. Éstas incluyen la habilidad de añadir expresiones WHERE para una consulta SQL utilizada al importar los datos.

  4. Haz clic en Ejecutar para comenzar la importación.

  5. Cuando termine, haz clic en el botón Ver salida para ver la capa recien importada a GRASS en el mapa.

  6. Cerrar primero la herramienta de importación (haga clic en el botón Cerrar inmediatamente a la derecha de Vista de salida), a continuación cierre la ventana Herramientas GRASS.

  7. Remove the original rivers layer.

Ahora tienes solamente la capa importada de GRASS visible en tu mapa de QGIS.

13.1.4. basic Follow Along: Cargando Datos Ráster en 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. Abre el diálogo Herramientas GRASS de nuevo.

  3. Haz clic en la pestaña Lista de módulos.

  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.

    ../../../_images/g_dem_settings.png
  6. Haz clic en Ejecutar.

  7. Cuando el proceso termine, haz clic en Ver salida.

  8. Haz clic en Cerrar para cerrar la pestaña, y entonces haz clic en Cerrar para cerrar el cuadro de diálogo completo.

    ../../../_images/g_dem_result.png
  9. You may now remove the original srtm_41_19_4326.tif layer.

13.1.5. basic 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. basic 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. basic 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:

    ../../../_images/grass_open_mapset1.png
  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. basic 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 GRASS icon next to it). You will see some options.

  4. Click on Open mapset:

    ../../../_images/grass_open_mapset_browser.png

The Mapset is now open and ready to use!

Truco

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 cadena de trabajo en GRASS para procesar datos es diferente del método que QGIS utiliza porque GRASS carga sus datos en una estructura de base de datos espacial. Sin embargo, utilizando QGIS como interfaz, puedes hacer los ajustes de un directorio de mapas de GRASS más fácilmente utilizando capas existentes en QGIS como fuente de datos para GRASS.

13.1.8. What’s Next?

Ahora que los datos están importado a GRASS, podemos ver las operaciones de análisis avanzado que GRASS ofrece.