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QGIS Python Programming Cookbook

Book Image

QGIS Python Programming Cookbook

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QGIS Python Programming Cookbook
Mar, 2015 | 340 pages
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Table of Contents (16 chapters)
QGIS Python Programming Cookbook
QGIS Python Programming Cookbook
Credits
Credits
About the Author
About the Author
About the Reviewers
About the Reviewers
www.PacktPub.com
www.PacktPub.com
Preface
Preface
Free Chapter
1
Automating QGIS
Automating QGIS
Introduction
Installing QGIS for development
Using the QGIS Python console for interactive control
Using the Python ScriptRunner plugin
Setting up your QGIS IDE
Debugging QGIS Python scripts
Navigating the PyQGIS API
Creating a QGIS plugin
Distributing a plugin
Creating a standalone application
Storing and reading global preferences
Storing and reading project preferences
Accessing the script path from within your script
2
Querying Vector Data
Querying Vector Data
Introduction
Loading a vector layer from a file sample
Loading a vector layer from a spatial database
Examining vector layer features
Examining vector layer attributes
Filtering a layer by geometry
Filtering a layer by attributes
Buffering a feature intermediate
Measuring the distance between two points
Measuring the distance along a line sample
Calculating the area of a polygon
Creating a spatial index
Calculating the bearing of a line
Loading data from a spreadsheet
3
Editing Vector Data
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
Moving vector layer geometry
Changing a vector layer feature's attribute
Deleting a vector layer feature
Deleting a vector layer attribute
Reprojecting a vector layer
Converting a shapefile to KML
Merging shapefiles
Splitting a shapefile
Generalizing a vector layer
Dissolving vector shapes
Performing a union on vector shapes
Rasterizing a vector layer
4
Using Raster Data
Using Raster Data
Introduction
Loading a raster layer
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 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
Creating Dynamic Maps
Creating Dynamic Maps
Introduction
Accessing the map canvas
Changing the map units
Iterating over layers
Symbolizing a vector layer
Rendering a single band raster using a color ramp algorithm
Creating a complex vector layer symbol
Using icons as vector layer symbols
Creating a graduated vector layer symbol renderer
Creating a categorized vector layer symbol
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 OpenStreetMap service
Using the Bing aerial image service
Adding real-time weather data from OpenWeatherMap
Labeling features
Changing 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
Composing Static Maps
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 the map
Adding a north arrow to the map
Adding a logo to the map
Adding a legend to the map
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
Interacting with the User
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 tabs
Stepping the user through a wizard
Keeping dialogs on top
8
QGIS Workflows
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 heat map
Creating a dot density map
Collecting field data
Computing road slope using elevation data
Geolocating photos on the map
Image change detection
9
Other Tips and Tricks
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 style file
Using NULL values in PyQGIS
Using generators for layer queries
Using alpha values to show data density
Using the __geo_interface__ protocol
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's points of interest in QGIS
Visualizing data in 3D with WebGL
Visualizing data on a globe
Index
Index

Debugging QGIS Python scripts

In this recipe, we will configure Eclipse to debug QGIS Python scripts.

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 execution while you monitor the program to catch bugs as they occur.

Configuring QGIS

The following steps will add two plugins to QGIS, which allows Eclipse to communicate with QGIS. One plugin, Plugin Reloader, allows you to reload a QGIS plugin into memory without restarting QGIS for faster testing. The second plugin, Remote Debug, connects QGIS to Eclipse.

Remote Debug is an experimental plugin, so you must ensure that experimental plugins are visible to the QGIS plugin manager in the list of available plugins.

  1. Start QGIS.

  2. Under the Plugins menu, select ManageandInstallPlugins…

  3. In the left pane of the Plugins dialog, select the Settings tab.

  4. Scroll down in the Settings window and ensure that the Show also experimental plugins checkbox is checked, as shown in the following screesnhot:

  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 as well.

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 NewProject dialog.

  4. Click on the Next> button.

  5. Give the project the name HelloWorldPlugin.

  6. Click on the Finish button.

  7. Select the new HelloWorldPlugin project in project explorer 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 HelloWorldPlugin folder, as shown in the following screenshot:

    Tip

    You can find the location of the HelloWorld plugin from within the QGIS plugin manager.

  11. Click on the Finish button.

Testing the debugger

The previous parts of this recipe configured Eclipse and QGIS to work together in order 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 break point in the plugin to pause the execution and then monitor plugin execution from within Eclipse, as follows:

  1. Under the HelloWorld folder, open the file HelloWorld.py.

  2. From the Eclipse Window menu, select OpenPerspective and then click on Other…

  3. From the OpenPerspective 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 break point, which is displayed as a green-icon:

  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 RemoteDebug and then select the RemoteDebug 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 break point.

  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 RemoteDebug 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 PluginReloader plugin can reset plugins that maintain 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.

Note

You can find out more about debugging QGIS, including using other IDEs, at http://docs.qgis.org/2.6/en/docs/pyqgis_developer_cookbook/ide_debugging.html.

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