Book Image

QGIS Python Programming Cookbook

Book Image

QGIS Python Programming Cookbook

Overview of this book

Related Content you might be interested in

Current Title:

Small book image
QGIS Python Programming Cookbook
Mar, 2015 | 340 pages
No titles found
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

Creating a QGIS plugin

Plugins are the best way to extend QGIS, as they can be easily updated and reused by other people.

Getting ready

The easiest approach to creating a plugin is to use the Plugin Builder plugin to jumpstart 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 under the submenu.

  3. In the QGIS Plugin Builder dialog, name the class MyPlugin.

  4. Name the plugin My Plugin.

  5. Type a short description, such as A demonstration on building a QGIS Plugin.

  6. Enter myplugin for the Module name.

  7. Leave the default version numbers as they are.

  8. Enter My Plugin in the Text for the menu item field.

  9. Enter your name and email address for author information.

  10. Ensure that the checkbox labelled Flag the plugin as experimental is checked, as shown in the following screenshot:

  11. Click on the OK button.

  12. 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 in the main program directory:

    C:\Documents and Settings\Joel\.qgis2\python\plugins
C:\Program Files\QGIS Brighton\apps\qgis\python\plugins

  13. Close the follow-on Plugin Builder information dialog by clicking on the OK button.

  14. Using the command prompt, navigate to your new plugin template folder.

  15. Use the pyrcc4 command to compile the resource file:

    pyrcc4 –o resources_rc.py resources.qrc

    Tip

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

  16. In a text editor, such as Windows Notepad or vi on Linux, open the user interface XML file named myplugin_dialog_base.ui.

  17. Insert the following XMLfor a custom label near line 31 and 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>

  18. Now, compile the ui file using the pyuic4 tool:

    pyuic4 –o ui_myplugin.py ui_myplugin.ui

  19. Your plugin is now ready. Restart QGIS.

  20. 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 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://geospatialpython.googlecode.com/svn/MyPlugin.zip.

Note

You can find out more about QGIS plugins, including the purpose of the other files in the directory, in 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 by hand 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, which 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 http://qt-project.org/downloads.

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

Previous Section
End of Section 9
Next Section