20. Preparazione dei dati dell’esercizio

Nota

Questo processo è inteso per organizzatori di corsi o per utenti QGIS con maggiore esperienza, i quali desiderano creare campio di dati georeferenziati per i loro corsi. I dataset predefiniti sono forniti con il Manuale di Formazione, ma puoi seguire queste istruzioni se desideri sostituire i dataset predefiniti.

The sample data provided with the Training Manual refers to the town of Swellendam and its surroundings. Swellendam is located about 2 hours” east of Cape Town in the Western Cape of South Africa. The dataset contains feature names in both English and Afrikaans.

Chiunque può usare questo dataset senza difficoltà, ma potresti preferire usare dati del tuo paese o della tua città. Se tu scegli di fare così, i tuoi dati georeferenziati verranno usati in tutte le lezioni dal Modulo 3 al Modulo 7.2. In seguito o moduli usano sorgenti di dati più complessi che potrebbero essere o non essere disponibili per la tua regione.

Nota

Queste istruzioni presuppongono che tu abbia una buona conoscenza di QGIS e non sono finalizzate ad essere usate come materiale di insegnamento.

20.1. hard Try Yourself Create OSM based vector Files

Se desideri sostituire i dati predefiniti con i dati georeferenziati del tuo corso, questo può essere facilmente fatto con gli strumenti costruiti in QGIS. La regione che tu scegli di usare dovrebbe avere un buon insieme di aree rurali ed urbane, contenere strade di livello diverso, aree con confini (quali riserve naturali o fattorie) e superfici con acqua, quali corsi d’acqua e fiumi.

  1. Apri un nuovo progetto QGIS

  2. Select Layer ▶ Data Source Manager to open the Data Source Manager dialog

  3. In the Browser tab, expand the XYZ Tiles drop-down menu and double-click the OpenStreetMap item.

    ../../../_images/browser_xyztiles.png

    A map of the world is now visible on the map canvas.

  4. Close the Data Source Manager dialog

  5. Move to the area you’d like to use as study area

    ../../../_images/swellendam_neighborhood.png

Now that we have the area we’ll extract the data from, let’s enable the extraction tools.

  1. Go to Plugins ▶ Manage/Install Plugins…

  2. In the All tab, type QuickOSM in the search box

  3. Select the QuickOSM plugin, press Install Plugin and then Close the dialog.

    ../../../_images/quickosm_plugin_download.png
  4. Execute the new plugin from Vector ▶ QuickOSM ▶ QuickOSM… menu

  5. In the Quick query tab, select building in the Key drop-down menu

  6. Leave the Value field empty, meaning that you are querying all buildings.

  7. Select Canvas Extent in the next drop-down menu

  8. Expand the Advanced group below and uncheck all geometry types on the right except Multipolygons.

  9. Press Run query

    ../../../_images/building_query_builder.png

    A new building layer is added to the Layers panel, showing buildings in the selected extent.

  10. Proceed as above to extract other data:

    1. Key = landuse and Multipolygons geometry type.

    2. Key = boundary, Value = protected_area and Multipolygons geometry type.

    3. Key = natural, Value = water and Multipolygons geometry type.

    4. Key = highway and check Lines and Multilines geometry types.

    5. Key = waterway, Value = river and check Lines and Multilines geometry types.

    6. Key = place and Points geometry type.

This process adds the layers as temporary files (indicated by the indicatorMemory icon next to their name).

../../../_images/osm_data_loaded.png

You can sample the data your region contains in order to see what kind of results your region will yield.

We now need to save the resulting data to use during your course. We’ll be using ESRI Shapefile, GeoPackage and SpatiaLite formats depending on the data.

To convert the place temporary layer to another format:

  1. Click the indicatorMemory icon next to the place layer to open the Save Scratch Layer dialog.

    Nota

    If you need to change any of the temporary layer’s properties (CRS, extent, fields…), use the Export ▶ Save Features as… contextual menu instead, and ensure the Add saved file to map option is checked. This adds a new layer.

  2. Select the ESRI Shapefile format

  3. Use the button to browse to the exercise_data/shapefile/ folder and save the file as places.shp.

    ../../../_images/save_osm_place.png
  4. Premi OK

    In the Layers panel, the temporary place layer is replaced with the saved places shapefile layer and the temporary icon next to it removed.

  5. Double-click the layer to open its Layer Properties ▶ Source tab and update the Layer name property to match the file name.

  6. Repeat the process for other layers, renaming them as follows:

    • natural_water into water

    • waterway_river into rivers

    • boundary_protected_area into protected_areas

    Each resulting data set should be saved in the exercise_data/shapefile/ directory.

The next step is to create a GeoPackage file from the building layer to use during the course:

  1. Click the indicatorMemory icon next to the building layer

  2. Select the GeoPackage format

  3. Save the file as training_data.gpkg under the exercise_data/ folder

  4. By default, the Layer name is filled as the file name. Replace it with buildings.

    ../../../_images/save_osm_building.png
  5. Premi OK

  6. Rename the layer in its properties dialog

  7. Repeat the process with the highway layer, saving it as roads in the same GeoPackage database.

The last step is to save the remaining temporary file as a SpatiaLite file.

  1. Click the indicatorMemory icon next to the landuse layer

  2. Select the SpatiaLite format

  3. Save the file as landuse.sqlite under the exercise_data/ folder. By default, the Layer name is filled as the file name. Do not change it.

    ../../../_images/save_osm_landuse.png
  4. Premi OK

You should now have a map which looks something like this (the symbology will certainly be very different, because QGIS randomly assigns colors when layers are added to the map):

../../../_images/post_osm_import.png

The important thing is that you have 7 vector layers matching those shown above and that all those layers have some data.

20.2. hard Try Yourself Crea un DEM SRTM tiff

For modules Module: Creare dati vettoriali and Module: Raster, you’ll also need raster images (SRTM DEM) which cover the region you have selected for your course.

The CGIAR-CGI provides some SRTM DEM you can download from http://srtm.csi.cgiar.org/srtmdata/.

You’ll need images which cover the entire region you have chosen to use. To find the extent coordinates, in QGIS , zoomToLayer zoom to the extent of the largest layer and pick the values in the extents Extents box of the status bar. Keep the GeoTiff format. Once the form is filled, click on the Click here to Begin Search >> button and download the file(s).

Una volta che hai scaricato il file (i files) richiesto, dovranno essere salvati nella cartella exercise_data, all’interno delle sottocartelle raster/SRTM.

20.3. hard Try Yourself Crea un Files tiff di immagine

In Module Module: Creare dati vettoriali, Follow Along: Data Sources lesson shows close-up images of three school sports fields which students are asked to digitize. You’ll therefore need to reproduce these images using your new SRTM DEM tiff file(s). There is no obligation to use school sports fields: any three school land-use types can be used (e.g. different school buildings, playgrounds or car parks).

For reference, the image in the example data is:

../../../_images/field_outlines.png

20.4. basic Try Yourself Sostituisci gli Emblemi

Having created your localised dataset, the final step is to replace the tokens in the substitutions.txt file so that the appropriate names will appear in your localised version of the Training Manual.

Gli emblemi che hai bisogno di sostituire sono i seguenti:

  • majorUrbanName: this defaults to «Swellendam». Replace with the name of the major town in your region.

  • schoolAreaType1: this defaults to «athletics field». Replace with the name of the largest school area type in your region.

  • largeLandUseArea: this defaults to «Bontebok National Park». Replace with the name of a large landuse polygon in your region.

  • srtmFileName: this defaults to srtm_41_19.tif. Replace this with the filename of your SRTM DEM file.

  • localCRS: this defaults to WGS 84 / UTM 34S. You should replace this with the correct CRS for your region.