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  • QGIS Python Programming Cookbook, Second Edition
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QGIS Python Programming Cookbook, Second Edition

QGIS Python Programming Cookbook, Second Edition

By : Joel Lawhead
1.5 (2)
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QGIS Python Programming Cookbook, Second Edition

QGIS Python Programming Cookbook, Second Edition

1.5 (2)
By: Joel Lawhead

Overview of this book

QGIS is a desktop geographic information system that facilitates data viewing, editing, and analysis. Paired with the most efficient scripting language—Python, we can write effective scripts that extend the core functionality of QGIS. Based on version QGIS 2.18, this book will teach you how to write Python code that works with spatial data to automate geoprocessing tasks in QGIS. It will cover topics such as querying and editing vector data and using raster data. You will also learn to create, edit, and optimize a vector layer for faster queries, reproject a vector layer, reduce the number of vertices in a vector layer without losing critical data, and convert a raster to a vector. Following this, you will work through recipes that will help you compose static maps, create heavily customized maps, and add specialized labels and annotations. As well as this, we’ll also share a few tips and tricks based on different aspects of QGIS.
Table of Contents (10 chapters)
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Preface
Preface
What this book covers
What you need for this book
Who this book is for
Sections
Conventions
Reader feedback
Customer support
Free Chapter
1
1. Automating QGIS
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1. Automating QGIS
Introduction
Installing QGIS 2.18 for development
Using the QGIS Python console for interactive control
Automatically starting the Python console
Using the Python's Script Runner plugin
Setting up your QGIS IDE
Debugging QGIS Python scripts
Navigating the PyQGIS API
Creating a traditional QGIS plugin
Creating a Processing Toolbox plugin
Distributing a plugin
Building a standalone application
Storing and reading global preferences
Storing and reading project preferences
Accessing the script path from within your script
2
2. Querying Vector Data
2. Querying Vector Data
Introduction
Loading a vector layer from a file sample
Loading a vector layer from a geodatabase
Examining vector layer features
Examining vector layer attributes
Filtering a layer by geometry
Filtering a layer by attributes
Buffering a feature
Measuring the distance between two points
Measuring distance along a line
Calculating the area of a polygon
Creating a spatial index
Calculating the bearing of a line
Loading data from a spreadsheet
Accessing layer metadata
3
3. Editing Vector Data
3. Editing Vector Data
Introduction
Creating a vector layer in memory
Adding a point feature to a vector layer
Adding a line feature to a vector layer
Adding a polygon feature to a vector layer
Adding a set of attributes to a vector layer
Adding a field to a vector layer
Joining a shapefile attribute table to a CSV file
Changing vector layer geometry
Changing a vector layer feature's attribute
Removing data from a vector layer
Deleting a vector layer feature's attribute
Reprojecting a vector layer
Converting a shapefile to KML or GeoJSON
Merging shapefiles
Splitting a shapefile
Generalizing a vector layer
Dissolving vector features
Performing a union on vector shapes
Rasterizing a vector layer
Exporting a layer to the GeoPackage format
4
4. Using Raster Data
4. Using Raster Data
Introduction
Loading a raster layer
Loading a NetCDF file
Getting the cell size of a raster layer
Obtaining the width and height of a raster
Counting raster bands
Swapping raster bands
Querying the value of a raster at a specified point
Reprojecting a raster
Creating an elevation hillshade
Creating vector contours from elevation data
Sampling a raster dataset using a regular grid
Adding elevation data to a line vertices using a digital elevation model
Creating a common extent for rasters
Resampling raster resolution
Counting the unique values in a raster
Mosaicing rasters
Converting a TIFF image to a JPEG image
Creating pyramids for a raster
Converting a pixel location to a map coordinate
Converting a map coordinate to a pixel location
Creating a KML image overlay for a raster
Classifying a raster
Converting a raster to a vector
Georeferencing a raster from control points
Clipping a raster using a shapefile
5
5. Creating Dynamic Maps
5. Creating Dynamic Maps
Introduction
Accessing the map canvas
Changing the map units
Iterating over layers
Symbolizing a vector layer
Setting a transparent layer fill
Using a filled marker symbol
Rendering a single band raster using a color ramp algorithm
Setting a feature's color using a column in a CSV file
Creating a complex vector layer symbol
Using icons as vector layer symbols
Using an outline for font markers
Using arrow symbols
Creating a graduated vector layer symbol renderer
Creating a categorized vector layer symbol
Using live layer effects
Creating inverted polygon shapeburst fills
Creating a map bookmark
Navigating to a map bookmark
Setting scale-based visibility for a layer
Using SVG for layer symbols
Using pie charts for symbols
Using the 2.5D renderer
Using XYZ tiled map services
Using Esri ArcGIS map services
Labeling a feature
Using null symbols
Change map layer transparency
Adding standard map tools to the canvas
Using a map tool to draw points on the canvas
Using a map tool to draw polygons or lines on the canvas
Building a custom selection tool
Creating a mouse coordinate tracking tool
6
6. Composing Static Maps
6. Composing Static Maps
Introduction
Creating the simplest map renderer
Using the Map Composer
Adding labels to a map for printing
Adding a scale bar to a map
Adding a North arrow to the map
Adding a logo to the map
Adding a vertical legend to the map
Adding a horizontal legend
Adding a custom shape to the map
Adding a grid to the map
Adding a table to the map
Adding a world file to a map image
Saving a map to a project
Loading a map from a project
7
7. Interacting with the User
7. Interacting with the User
Introduction
Using log files
Creating a simple message dialog
Creating a warning dialog
Creating an error dialog
Displaying a progress bar
Creating a simple text input dialog
Creating a file input dialog
Creating a combobox
Creating radio buttons
Creating checkboxes
Creating a dock widget
Displaying a message in the status bar
Pushing messages to the message bar
Pushing widgets to the message bar
Creating tabs
Stepping the user through a wizard
Keeping dialogs on top
8
8. QGIS Workflows
8. QGIS Workflows
Introduction
Creating an NDVI
Geocoding addresses
Creating raster footprints
Performing network analysis
Routing along streets
Tracking a GPS
Creating a mapbook
Finding the least cost path
Performing nearest neighbor analysis
Creating a DEM from LIDAR
Creating a heat map
Creating a dot density map
Collecting field data
Computing road slope using elevation data
Geolocating photos on the map
Image change detection
Adjusting imprecise building footprints
Visualizing multitemporal data
9
9. Other Tips and Tricks
9. Other Tips and Tricks
Introduction
Creating tiles from a QGIS map
Adding a layer to geojson.io
Rendering map layers based on rules
Creating a layer-definition file
Using NULL values in PyQGIS
Using generators for layer queries
Using alpha values to show data density
Using the __geo_interface__ protocol
Getting the output file names from processing algorithms
Generating points along a line
Using expression-based labels
Creating dynamic forms in QGIS
Calculating length for all selected lines
Using a different status bar CRS than the map
Creating HTML labels in QGIS
Using OpenStreetMap points of interest in QGIS
Changing the QGIS web proxy
Running QGIS scripts as scheduled tasks
Visualizing data in 3D with WebGL
Visualizing data on a globe
Make a globe-like azimuthal orthographic projection
Animating a layer

Debugging QGIS Python scripts

In this recipe, we will configure Eclipse to debug QGIS Python scripts. This setup will allow you to interactively watch the execution of programs as well as pause the execution as needed. This kind of interactive debugging is particularly useful in a GUI program such as QGIS because in addition to the user interface, there are program functions happening behind the scenes. Debugging programs that have processes in the foreground and background can be extremely difficult. This interactive debugging approach makes the development of complex applications such as these much more efficient by stepping through each part of the program as it is executed. You can see what is happening in real time and troubleshoot more easily.

How to do it...

Both QGIS and Eclipse must be configured for debugging so that the two pieces of software can communicate. Eclipse attaches itself to QGIS in order to give you insights into the Python scripts running in QGIS. This approach allows you to run scripts in a controlled way that can pause the execution while you monitor the program to catch bugs as they occur.

Configuring QGIS

The following steps will add two plugins to QGIS; this will allow Eclipse to communicate with QGIS. The first plugin, Plugin Reloader, allows you to reload a QGIS plugin into the memory without restarting QGIS for faster testing. The second plugin, Remote Debug, connects QGIS to Eclipse. Remote Debug is an experimental plugin, so we must ensure that experimental plugins are visible to the QGIS plugin manager in the list of the available plugins. This can be done in the following way:

  1. Start QGIS.
  2. Under the Plugins menu, select Manage and Install Plugins...
  3. In the left pane of the Plugins dialog, select the Settings tab.
  4. Scroll down the Settings window and ensure that the Show also experimental plugins checkbox is checked, as shown in the following screenshot:

    Configuring QGIS

  5. Click on the OK button.
  6. Select the tab labeled All in the pane on the left-hand side of the Plugins window.
  7. In the Search dialog at the top of the window, search for Plugin Reloader.
  8. Select Plugin Reloader from the search results and then click on the Install Plugin button.
  9. Next, search for the Remote Debug plugin and install it as well.
  10. Finally, install the HelloWorld plugin too.

Configuring Eclipse

Now that QGIS is configured for debugging in Eclipse, we will configure Eclipse to complete the debugging communication loop, as shown in the following steps:

  1. Start Eclipse.
  2. In the File menu, select New and then click on Project.
  3. Select General and then click on Project from the New Project dialog.
  4. Click on the Next> button.
  5. Give the project the name HelloWorld Plugin.
  6. Click on the Finish button.
  7. Select the new HelloWorld plugin project in the project explorer window and select New. Then, click on Folder from the File menu.
  8. In the New Folder dialog, click on the Advanced>> button.
  9. Choose the Link to alternate location (Linked Folder) radio button.
  10. Click on the Browse button and browse to the location of the HelloWorld plugin folder.

    Tip

    You can find the location of the HelloWorld plugin from within the QGIS plugin manager, as shown in the following screenshot:

    Configuring Eclipse

  11. Click on the Finish button.

Testing the debugger

In the previous parts of this recipe, we configured Eclipse and QGIS so they could work together to debug QGIS plugins. In this section, we will test the configuration using the simplest possible plugin, HelloWorld, to run Eclipse using the Debug Perspective. We will set up a breakpoint in the plugin to pause the execution and then monitor the plugins execution from within Eclipse, as follows:

  1. Under the HelloWorld folder, open the HelloWorld.py file.
  2. From the Eclipse Window menu, select Open Perspective and then click on Other....
  3. From the Open Perspective dialog, select Debug.
  4. Click on the OK button.
  5. Scroll to the first line of the hello_world() function and double-click on the left-hand side of the line number to set a breakpoint, which is displayed as a green-colored icon:

    Testing the debugger

  6. From the Pydev menu, select Start Debug Server.
  7. Verify that the server is running by looking for a message in the Debug console at the bottom of the window, similar to the following:
    Debug Server at port: 5678
  8. Switch over to QGIS.
  9. From the QGIS Plugins menu, select Remote Debug and then select the Remote Debug command.
  10. Verify that the QGIS status bar in the lower-left corner of the window displays the following message:
    Python Debugging Active
  11. Now select HelloWorld from the QGIS Plugins menu and then select HelloWorld.
  12. Switch back to Eclipse.
  13. Verify that the hello_world() function is highlighted at the breakpoint.
  14. From the Run menu, select Resume.
  15. Switch back to QGIS.
  16. Verify that the HelloWorld dialog box has appeared.

How it works...

The Remote Debug plugin acts as a client to the PyDev debug server in order to send the Python script's execution status from QGIS to Eclipse. While it has been around for several versions of QGIS now, it is still considered experimental.

The Plugin Reloader plugin can reset plugins that maintain the state as they run. The HelloWorld plugin is so simple that reloading is not needed to test it repeatedly. However, as you debug more complex plugins, you will need to run it in order to reset it before each test. This method is far more efficient and easier to use than closing QGIS, editing the plugin code, and then restarting it.

Note

You can find out more about debugging QGIS, including using other IDEs, at http://docs.qgis.org/testing/en/docs/pyqgis_developer_cookbook/ide_debugging.html.

End of Section 8
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