PrintFor the final part of this section, let’s switch from the Python console in QGIS to writing a standalone script that uses qgis. You can use your editor of choice to write the script and then execute the .py file from the OSGeo4W shell (see again Section 4.4.1) with all environment variables set correctly for a qgis and QT5 based program.
We are going to repeat the task from Section 3.9.1 of creating buffers around the centroids of the countries within a rectangular (in terms of WGS 84 coordinates) area around southern Africa. We will produce two new vector GeoPackage files: a point based one with the centroids and a polygon based one for the buffers. Both data sets will only contain the country name as their only attribute.
We start by importing the modules we will need and creating a QApplication() (handled by qgis.core.QgsApplication) for our program that qgis can run in (even though the program does not involve any GUI).
Important note: When you later write you own qgis programs (e.g. in the L4 homework assignment), make sure that you always "import qgis" first before using any other qgis related import statements such as "import qgis.core". We are not sure why this is needed, but the other imports will most likely fail tend to fail without "import qgis" coming first.
import os, sys import qgis import qgis.core
To use qgis in our software, we have to initialize it and we need to tell it where the actual QGIS installation is located. To do this, we use the function getenv(…) of the os module to get the value of the environmental variable “QGIS_PREFIX_PATH” which will be correctly defined when we run the program from the OSGeo4W shell. Then we create an instance of the QgsApplication class and call its initQgis() method.
qgis_prefix = os.getenv("QGIS_PREFIX_PATH")
qgis.core.QgsApplication.setPrefixPath(qgis_prefix, True)
qgs = qgis.core.QgsApplication([], False)
qgs.initQgis()
Now we can implement the main functionality of our program. First, we load the world borders shapefile into a layer (you may have to adapt the path!).
layer = qgis.core.QgsVectorLayer(r'C:\489\TM_WORLD_BORDERS-0.3.shp')
Then we create the two new layers for the centroids and buffers. These layers will be created as new in-memory layers and later written to GeoPackage files. We provide three parameters to QgsVectorLayer(…): (1) a string that specifies the geometry type, coordinate system, and fields for the new layer; (2) a name for the layer; and (3) the string “memory” which tells the function that it should create a new layer in memory from scratch (rather than reading a data set from somewhere else as we did earlier).
centroidLayer = qgis.core.QgsVectorLayer("Point?crs=" + layer.crs().authid() + "&field=NAME:string(255)", "temporary_points", "memory")
bufferLayer = qgis.core.QgsVectorLayer("Polygon?crs=" + layer.crs().authid() + "&field=NAME:string(255)", "temporary_buffers", "memory")
The strings produced for the first parameters will look like this: “Point?crs=EPSG:4326&field=NAME:string(255)” and “Polygon?crs=EPSG:4326&field=NAME:string(255)”. Note how we are getting the EPSG string from the world border layer so that the new layers use the same coordinate system, and how an attribute field is described using the syntax “field=<name of the field>:<type of the field>". When you want your layer to have more fields, these have to be separated by additional & symbols like in a URL.
Next, we set up variables for the data providers of both layers that we will need to create new features for them. The new features will be collected in two lists, centroidFeatures and bufferFeatures.
centroidProvider = centroidLayer.dataProvider() bufferProvider = bufferLayer.dataProvider() centroidFeatures = [] bufferFeatures = []
Then, we create the polygon geometry for our selection area from a WKT string as in Section 3.9.1:
areaPolygon = qgis.core.QgsGeometry.fromWkt('POLYGON ( (6.3 -14, 52 -14, 52 -40, 6.3 -40, 6.3 -14) )')
In the main loop of our program, we go through all the features in the world borders layer, use the geometry method intersects(…) to test whether the country polygon intersects with the area polygon, and, if yes, create the centroid and buffer features for the two layers from the input feature.
for feature in layer.getFeatures():
if feature.geometry().intersects(areaPolygon):
centroid = qgis.core.QgsFeature()
centroid.setAttributes([feature['NAME']])
centroid.setGeometry(feature.geometry().centroid())
centroidFeatures.append(centroid)
buffer = qgis.core.QgsFeature()
buffer.setAttributes([feature['NAME']])
buffer.setGeometry(feature.geometry().centroid().buffer(2.0,100))
bufferFeatures.append(buffer)
Note how in both cases (centroids and buffers), we first create a new QgsFeature object, then use setAttributes(…) to set the NAME attribute to the name of the country, and then use setGeometry(…) to set the geometry of the new feature either to the centroid derived by calling the centroid() method or to the buffered centroid created by calling the buffer(…) method of the centroid point. As a last step, the new features are added to the respective lists. Finally, all features in the two lists are added to the layers after the for-loop has been completed. This happens with the following two commands:
centroidProvider.addFeatures(centroidFeatures) bufferProvider.addFeatures(bufferFeatures)
Lastly, we write the content of the two in-memory layers to GeoPackage files on disk. This works in the same way as in previous examples. Again, you might want to adapt the output paths.
qgis.core.QgsVectorFileWriter.writeAsVectorFormat(centroidLayer, r'C:\489\centroids.gpkg', "utf-8", layer.crs(), "GPKG") qgis.core.QgsVectorFileWriter.writeAsVectorFormat(bufferLayer, r'C:\489\buffers.gpkg', "utf-8", layer.crs(), "GPKG")
Since we are now done with using QGIS functionalities (and actually the entire program), we clean up by calling the exitQgis() method of the QgsApplication, freeing up resources that we don’t need anymore.
qgs.exitQgis()
If you run the program from the OSGeo4W shell and then open the two produced output files in QGIS, the result should look as shown in the image below.
