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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

Creating a traditional QGIS plugin

Plugins are the best way to extend QGIS, as they can be easily updated and reused by other people. And as we'll see throughout this book, you can use Python to create plugins. When you create a Python-based plugin, you can usually access that plugin's functionality through the PyQGIS API.

Getting ready

The easiest approach to creating a plugin is to use the Plugin Builder plugin to jump-start development. You can find it in the main QGIS plugin repository and install it.

How to do it...

Perform the following steps to create a simple plugin that displays a dialog box with a custom message:

  1. Start QGIS.
  2. From the Plugins menu, select Plugin Builder and then click on Plugin Builder in the submenu.
  3. In the QGIS Plugin Builder dialog, name the class MyPlugin.
  4. Name the plugin MyPlugin.
  5. Type a short description, such as A demonstration Plugin.
  6. Enter myplugin as the Module name.
  7. Leave the default version numbers as they are.
  8. Enter your name and e-mail address for author information.
  9. Click Next.
  10. Enter a description of the plugin in the About field.
  11. Click Next.
  12. In the Text for menu item field, enter My Plugin.
  13. Click Next and then on the next dialog click Next again.
  14. For the Bug Tracker field, enter https://github.com/GeospatialPython/Learn/issues.
  15. For the Repository field, https://github.com/GeospatialPython/Learn/.
  16. Ensure that the checkbox labeled Flag the plugin as experimental is checked.
  17. Click on the OK button.
  18. A file browser dialog will appear. You can choose a folder in which you want to create your plugin. Select one of the folders called plugins within the python folder in either the main user directory or the QGIS program directory. The following examples are from a Windows machine. You should use the folder in your user directory, which is the preferred place for third-party plugins. QGIS standard plugins go to the main program directory:
      C:\Users\<username>\.qgis2\python\plugins or the %USERPROFILE%
      environment variable

C:\Program Files\QGIS2.18\apps\qgis\python\plugins

    On OS X or Linux machines, the .qgis2 directory will be in your home directory.

  19. Close the Plugin Builder information dialog by clicking on the OK button.
  20. Using Command Prompt, navigate to your new plugin template folder.
  21. Use the pyrcc4 command to compile the resource file:
          pyrcc4 -o resources_rc.py resources.qrc

    Tip

    If you are on Windows, it is easier to use the OSGEO4W shell, which is installed along with QGIS for the Qt compilation tools to work properly.

  22. In a text editor, such as Windows Notepad or vi on Linux, open the user interface XML file named myplugin_dialog_base.ui.
  23. Insert the following XML for a custom label near line 31, just before the last </widget> tag. Save the file after this edit:
            <widget class="QLabel" name="label"> 
          <property name="geometry"> 
            <rect> 
              <x>120</x> 
              <y>80</y> 
              <width>201</width> 
              <height>20</height> 
            </rect> 
          </property> 
          <property name="font"> 
            <font> 
              <pointsize>14</pointsize> 
            </font> 
          </property> 
          <property name="text"> 
            <string>Geospatial Python Rocks!</string> 
          </property> 
        </widget>
  24. Now compile the ui file using the pyuic4 tool:
    pyuic4 -o ui_myplugin.py ui_myplugin.ui
  25. Your plugin is now ready. Restart QGIS.
  26. Select My Plugin from the Plugins menu and then select My Plugin from the submenu to see the dialog you created within QGIS, as shown here:

    How to do it...

How it works...

This recipe shows you the bare bones needed to make a working plugin. Although we haven't altered it, the code for the plugin's behavior is contained in myplugin.py. You can change the icon and the GUI and just recompile any time you want. Note that we must compile the Qt4 portion of the plugin, which creates the dialog box. The entire QGIS GUI is built on the Qt4 library, so the pyrrc4 compiler and pyuic4 is included to compile the GUI widgets.

You can download the completed plugin with both the source and compiled UI and resource files at https://github.com/GeospatialPython/Learn/raw/master/MyPlugin.zip.

Note

You can find out more about QGIS plugins, including the purpose of the other files in the directory, from the QGIS documentation at http://docs.qgis.org/testing/en/docs/pyqgis_developer_cookbook/plugins.html.

There's more...

We have edited the myplugin_dialog_base.ui XML file manually to make a small change. However, there is a better way to use Qt Creator. Qt Creator is a fully fledged open source GUI designer for the Qt framework. It is an easy what-you-see-is-what-you-get editor for Qt Widgets, including PyQGIS plugins, that uses the included Qt Designer interface. On Windows, Qt Designer can be found in the QGIS program directory within the bin directory. It is named designer.exe. On other platforms, Qt Designer is included as part of the qt4-devel package.

Note

You can also download Qt Creator, which includes Qt Designer, from https://www.qt.io/download/.

When you run the installer, you can uncheck all the installation options, except the Tools category, to install just the IDE.

End of Section 10
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