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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

Installing QGIS for development

QGIS has a set of Python modules and libraries that can be accessed from the Python console within QGIS. However, they can also be accessed from outside QGIS to write standalone applications. First, you must make sure that PyQGIS is installed for your platform, and then set up some required system environment variables.

In this recipe, we will walk you through the additional steps required beyond the normal QGIS installation to prepare your system for development. The steps for each platform are provided, which also include the different styles of Linux package managers.

Getting ready

QGIS uses slightly different installation methods for Windows, GNU/Linux, and Mac OS X. The Windows installers install everything you need for Python development, including Python itself.

However, on Linux distributions and Mac OS X, you may need to manually install the Python modules for the system installation of Python. On Mac OS X, you can download installers for some of the commonly used Python modules with QGIS from http://www.kyngchaos.com/software/python.

How to do it

On Linux, you have the option to compile from the source or you can just specify the Python QGIS interface to be installed through your package manager.

Installing PyQGIS using the Debian package manager

  1. For Linux distributions based on the Debian Linux package manager, which includes Ubuntu and Debian, use the following command in a shell:

    sudo apt-get update

  2. Next, install the QGIS, PyQGIS, and QGIS GRASS plugins:

    sudo apt-get install qgis python-qgis qgis-plugin-grass

Installing PyQGIS using the RPM package manager

  1. For Linux distributions based on the Red Hat Package Manager (RPM), first update the package manager, as follows:

    sudo yum update

  2. Then, install the packages for the QGIS, PyQGIS, and QGIS GRASS plugins:

    sudo yum install qgis qgis-python qgis-grass

Setting the environment variables

Now, we must set the PYTHONPATH to the PyQGIS directory. At the same time, append the path to this directory to the PATH variable so that you can use the PyQGIS modules with an external IDE.

Setting the environment variables on Windows

  1. Set the PYTHONPATH variable in a command prompt to the bin directory of the QGIS installation:

    set PYTHONPATH="C:\Program Files\QGIS Brighton\bin"

  2. Next, append QGIS's bin directories to the system's PATH variable:

    set PATH="C:\Program Files\QGIS Brighton\bin";"C:\Program Files\QGIS Brighton\bin\apps\qgis\bin";%PATH%
Setting the environment variables on Linux

  1. Set the PYTHONPATH variable in a command prompt to the bin directory of the QGIS installation:

    export PYTHONPATH=/usr/share/qgis/python

  2. Now, append the QGIS shared library directory to the runtime search path. Note that this location can vary depending on your particular system configuration:

    export LD_LIBRARY_PATH=/usr/share/qgis/python

How it works…

The QGIS installation process and package managers set up the Python module's configuration internal to QGIS. When you use the Python console inside QGIS, it knows where all the PyQGIS modules are. However, if you want to use the PyQGIS API outside QGIS, using a system Python installation on Windows or Linux, it is necessary to set some system variables so that Python can find the required PyQGIS modules.

There's more…

This recipe uses the default QGIS paths on each platform. If you aren't sure which PyQGIS path is for your system, you can figure this out from the Python console in QGIS.

Finding the PyQGIS path on Windows

The libraries on Windows are stored in a different location than in the case of other platforms. To locate the path, you can check the current working directory of the Python console:

  1. Start QGIS.

  2. Select Python Console from the Plugins menu, which appears in the lower-right corner of the QGIS application window, as shown in the following screenshot:

  3. Use the os module to get the current working directory:

    import os
os.getcwd()

  4. Verify that the current working directory of the Python console is returned.

Finding the location of the QGIS Python installation on other platforms

Perform the following steps to find the path needed for this recipe on all the platforms besides Windows:

  1. Start QGIS.

  2. Start the QGIS Python Console.

  3. Use the sys module to locate the PyQGIS path:

    import sys
sys.path

  4. Python will return a list of paths.

  5. Find the path that ends in /python, which is the location of the Python installation used by QGIS

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