Chapter 8. Automating QGIS Analysis

This book has introduced you to using Python from the command line, in a Jupyter Notebook, and in an IDE to perform geospatial tasks. While these three tools will allow you to accomplish your tasks, there are many times when work needs to be done using desktop GIS software.

QGIS, a popular open source GIS application, provides desktop GIS functionality with the ability to work in a Python console and the ability to write toolboxes and plugins using Python. In this chapter, you will learn how to manipulate desktop GIS data using Python and how to automate these tasks using toolboxes and plugins.

In this chapter, you will learn how to:

• Load and save layers
• Create layers from API data sources
• Add, edit, and delete features
• Select specific features
• Call geoprocessing functions
• Write geoprocessing toolboxes
• Write plugins

Working in the Python console

The QGIS Python console is a Python console. You can perform all of your normal Python tasks with the added benefit of having the QGIS libraries added. From the console, you can manipulate GIS data and display it on the screen, or not.

The Python console is located under the Plugins menu on the QGIS toolbar. You can also access it by pressing Ctrl + Alt + P on the keyboard. The console will usually open in the bottom of the main window. You can undock it by clicking on the title bar (where it says Python Console), holding down the mouse button, and dragging the window to another location on the screen or by clicking the window button at the top-right of the console:

A screenshot of the Python console

The console has buttons for clearing the window, importing GIS and QGIS specific libraries, running the current command (you can press Enter instead of clicking this button), showing the editor, modifying options, and viewing theHelpfiles. The editor launches a simplified...

Writing custom toolboxes

Writing toolboxes will allow you to automate several tasks and make that code available to users as a GUI, or to other developers as an algorithm that can be executed using processing. In this section, you will learn how to create a toolbox and call it from processing.

In this chapter, you have learned how to load data from a file and from a PostGIS database. In this example, you will learn how to bring data in to QGIS from the SeeClickFix Application Program Interface (API).

Note

SeeClickFix is a 311 reporting system that is used by many cities in the United States. It contains geospatial data and has a very well documented, and user-friendly API.

To create a new script, open the processing toolbox in QGIS. This will open an editor window. You will write your code in this window and save it using the save icon. The file name will become a toolbox under Tools|User scripts|File name. Save the file and name it SeeClickFix.

Now that you have an empty toolbox, we can start...

Summary

In this chapter, you have learned how to use Python in QGIS. You started by learning the basics of loading a layer and displaying it on the map, and then progressed to adding, editing, and deleting features. You learned how to select features, highlight the selection, and how to use expressions. Then, we took advantage of pre-built geoprocessing tools and you learned how to call toolbox algorithms using processing. Lastly, you learned how to write your own toolbox.

In the next chapter, you will learn how to use Python with Esri tools. You will learn how to use Jupyter Notebooks in a browser to interact with cloud-based datasets and how to use the Esri API for Python to perform basic geospatial analysis and to create ArcGIS Online web maps.