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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

Installing QGIS 2.18 for development

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

In this recipe, we will walk you through the additional steps required beyond the normal installation process to prepare your system for development. The steps for each platform are provided, which also include the different styles of Linux package managers. The version of QGIS in your Linux package manager may be older. You can find additional detailed installation instructions at https://www.qgis.org/en/site/forusers/alldownloads.html.

Getting ready

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

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

How to do it...

On Linux, you have an option to either compile from the source or just specify the Python 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 need to set 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 2.18\bin"
  2. Next, append QGIS's bin directories to the system's PATH variable:
      set PATH="C:\Program Files\QGIS 2.18\bin";"C:\Program Files\QGIS
      2.18";%PATH%

Setting 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 a runtime search path. Note that this location can vary depending on your 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 in a way that it becomes 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 it by using a system Python installation on either Windows or Linux, it would be necessary for you set some system variables so that Python could 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, as opposed to other platforms, are stored in a different location. To locate the path, you need to check the current working directory of the Python console:

  1. Start QGIS.
  2. Select Python Console from the Plugins menu, which will appear in the lower-right corner of the QGIS application window, as shown in the following screenshot. You can also use the keyboard shortcut Control + Alt + P to do this:

    Finding the PyQGIS path on Windows

  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 other platforms:

  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.
End of Section 3
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