1.2. Préparation des données d’exercice

Note

Ce processus est prévu pour les formateurs ou les utilisateurs plus expérimentés de QGIS qui souhaitent créer des jeux de données locaux pour leur formation. Les jeux de données par défaut sont fournis avec le manuel d’exercice mais vous pouvez suivre les instructions qui suivent si vous désirez les remplacer.

Les données exemples fournies avec le manuel d’apprentissage concernent la ville de Swellendam et ses abords. Swellendam est située à environ 2 heures à l’est de Cape Town dans l’ouest de l’Afrique du Sud. Les noms des entités contenues dans le jeu de données sont en Anglais et en Afrikaans.

Tout le monde peut utiliser ce jeu de données sans aucune difficulté mais vous préférerez sans doute utiliser les données de votre propre pays ou votre propre ville. Si vous choisissez cette voie, vos données propres seront utilisées dans toutes les leçons du module 3 au module 7.2. D’autres modules utilisent des sources de données plus complexes qui ne seront peut-être pas disponibles pour votre région.

Note

Ces instructions supposent que vous avez une bonne connaissance de QGIS et n’ont pas vocation à être utilisées comme ressource pédagogique.

1.2.1. Try Yourself Create OSM based vector Files

Si vous voulez remplacer le jeu de données par défaut avec le votre pour la formation, vous pouvez le faire facilement avec les outils inclus dans QGIS. La région que vous choisissez devrait avoir un mélange correct entre les zones urbaines et rurales, contenir des routes de différentes classes, des limites administratives (comme les réserves naturelles ou les fermes) et des surfaces en eau telles que les ruisseaux et les rivières.

1. Ouvrez un nouveau projet 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.

   

   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

   

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.

   

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

   

   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 icon next to their name).

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

   Note

   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.

   

4. Press 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 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.

   

5. Press 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 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.

   

4. Press 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):

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

1.2.2. Try Yourself Créer des fichiers Tiff du MEN SRTM

For modules Module: Créer des données vectorielles and Module: Rasters, you’ll also need raster images (SRTM DEM) which cover the region you have selected for your course.

Le CGIAR-CGI (http://srtm.csi.cgiar.org/) fournit des MNE SRTM que vous pouvez télécharger depuis http://srtm.csi.cgiar.org/SELECTION/inputCoord.asp.

You’ll need images which cover the entire region you have chosen to use. To find the extent coordinates, in QGIS , zoom to the extent of the largest layer and pick the values in the 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).

Une fois que vous avez téléchargé le/les fichier(s) requis, vous devriez les sauvegarder dans le répertoire exercise_data sous raster/SRTM.

1.2.3. Try Yourself Créer des fichiers Image Tiff

In Module Module: Créer des données vectorielles, Follow Along: Sources de Données 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:

1.2.4. Try Yourself Remplacement des variables

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.

Les variables qui doivent être remplacées sont les suivantes:

• 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.