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QGIS Python Programming Cookbook

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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

Setting up your QGIS IDE

The Eclipse IDE with the PyDev plugin is cross-platform, has advanced debugging tools, and is free.

Note

You can refer to http://pydev.org/manual_101_install.html in order to install PyDev correctly.

This tool makes an excellent PyQGIS IDE. Eclipse allows you to have multiple Python interpreters configured for different Python environments. When you install PyDev, it automatically finds the installed system Python installations. On Windows, you must also add the Python interpreter installed with PyQGIS. On all platforms, you must tell PyDev where the PyQGIS libraries are.

Getting ready

This recipe uses Eclipse and PyDev. You can use the latest version of either package that is supported by your operating system. All platforms besides Windows rely on the system Python interpreter. So, there is an extra step in Windows to add the QGIS Python interpreter.

How to do it…

The following steps will walk you through how to add the QGIS-specific Python interpreter to Eclipse in order to support the running standalone QGIS applications or to debug QGIS plugins.

Adding the QGIS Python interpreter on Windows

The process used to add the QGIS Python interpreter to Eclipse on Windows is different from the process used on Linux. The following steps describe how to set up the interpreter on the Windows version of Eclipse:

  1. Open Eclipse.

  2. From the Window menu, select Preferences. On OS X, you must click on the Eclipse menu to find the preferences menu.

  3. In the pane on the left-hand side of the Preferences window, click on the plus sign next to PyDev.

  4. From the list of PyDev preferences, select Interpreter Python.

  5. In the pane labelled Python Interpreters, click on the New button.

  6. In the Select interpreter dialog, name the interpreter PyQGIS.

  7. Browse to the location of the QGIS Python interpreter called python.exe within the bin folder of the QGIS program folder. On OS X and Linux, you use can use the system Python installation. On Windows, Python is included with QGIS. The default location on Windows is C:\Program Files\QGIS Brighton\bin\python.exe, as shown in the following screenshot:

  8. When you click on the OK button, Eclipse will attempt to automatically add every Python library it finds to the Python path for this interpreter configuration. We need to control which libraries are added to prevent conflicts. Click on the Deselect All button and then click on OK:

  9. Eclipse will issue a warning dialog because you haven't selected any core libraries. Click on the Proceed anyways button, as shown here:

Adding the PyQGIS module paths to the interpreter

Apart from adding the Python interpreter, you must also add the module paths needed by PyQGIS using the following steps. These steps will require you to switch back and forth between QGIS and Eclipse:

  1. Start QGIS.

  2. Start the QGIS Python Console from the Plugins menu.

  3. Use the sys module to locate the PyQGIS Python path, as described in the previous recipe, Setting the environment variables:

    import sys
sys.path

  4. We also want to add the PyQGIS API. Next, find that path using the QGIS Python Console by typing the following command:

    qgis

  5. For each path in the returned lists, click on the New Folder button in Eclipse's Libraries pane for your QGIS interpreter, and browse to that folder until all the paths have been added. If a given folder does not exist on your system, simply ignore it, as shown here:

  6. Click on the OK button in the Preferences dialog.

Adding the PyQGIS API to the IDE

To take full advantage of Eclipse's features, including code completion, we will add the QGIS and Qt4 modules to the PyQGIS Eclipse interpreter preferences. The following steps will allow Eclipse to suggest the possible methods and properties of QGIS objects as you type; this feature is known as autocomplete:

  1. In the PyDev preferences for the PyQGIS Interpreter, select the Forced Builtins tab, as shown in the following screenshot:

  2. Click on the New button.

  3. In the Builtin to add dialog, type qgis:

  4. Click on the OK button.

Adding environment variables

You will also need to create a PATH variable, which points to the QGIS binary libraries, DLLs on Windows, and other libraries needed by QGIS at runtime on all platforms.

  1. In the PyDev preferences dialog, ensure that the PyQGIS interpreter is selected in the list of interpreters.

  2. Select the Environment tab.

  3. Click on the New button.

In the Name field, enter PATH.

  1. For the Value field, add the path to the QGIS program directory and to any QGIS directories containing binaries separated by a semicolon. The following is an example from a Windows machine:

    C:\Program Files\QGIS Brighton;C:\Program Files\QGIS Brighton\bin;C:\Program Files\QGIS Brighton\apps\qgis\bin;C:\Program Files\QGIS Brighton\apps\Python27\DLLs

How it works…

Eclipse and PyDev use only the information you provide to run a script in the Eclipse workspace. This approach is very similar to the popular Python tool virtualenv, which provides a clean environment when writing and debugging code to ensure that you don't waste time troubleshooting issues caused by the environment.

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