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  • Book Overview & Buying QGIS Python Programming Cookbook
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QGIS Python Programming Cookbook

QGIS Python Programming Cookbook

By : Joel Lawhead
3.8 (6)
Buy this Book
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QGIS Python Programming Cookbook

QGIS Python Programming Cookbook

3.8 (6)
By: Joel Lawhead
Buy this Book

Overview of this book

If you are a geospatial analyst who wants to learn more about automating everyday GIS tasks or a programmer who is responsible for building GIS applications,this book is for you. The short, reusable recipes make concepts easy to understand. You can build larger applications that are easy to maintain when they are put together.
Table of Contents (11 chapters)
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Preface
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Preface
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What this book covers
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What you need for this book
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Who this book is for
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Sections
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Conventions
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Reader feedback
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Customer support
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1
1. Automating QGIS
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1. Automating QGIS
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Introduction
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Installing QGIS for development
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Using the QGIS Python console for interactive control
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Using the Python ScriptRunner plugin
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Setting up your QGIS IDE
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Debugging QGIS Python scripts
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Navigating the PyQGIS API
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Creating a QGIS plugin
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Distributing a plugin
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Creating a standalone application
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Storing and reading global preferences
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Storing and reading project preferences
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Accessing the script path from within your script
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2. Querying Vector Data
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2. Querying Vector Data
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Introduction
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Loading a vector layer from a file sample
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Loading a vector layer from a spatial database
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Examining vector layer features
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Examining vector layer attributes
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Filtering a layer by geometry
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Filtering a layer by attributes
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Buffering a feature intermediate
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Measuring the distance between two points
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Measuring the distance along a line sample
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Calculating the area of a polygon
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Creating a spatial index
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Calculating the bearing of a line
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Loading data from a spreadsheet
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3. Editing Vector Data
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3. Editing Vector Data
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Introduction
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Creating a vector layer in memory
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Adding a point feature to a vector layer
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Adding a line feature to a vector layer
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Adding a polygon feature to a vector layer
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Adding a set of attributes to a vector layer
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Adding a field to a vector layer
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Joining a shapefile attribute table to a CSV file
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Moving vector layer geometry
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Changing a vector layer feature's attribute
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Deleting a vector layer feature
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Deleting a vector layer attribute
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Reprojecting a vector layer
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Converting a shapefile to KML
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Merging shapefiles
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Splitting a shapefile
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Generalizing a vector layer
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Dissolving vector shapes
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Performing a union on vector shapes
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Rasterizing a vector layer
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4. Using Raster Data
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4. Using Raster Data
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Introduction
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Loading a raster layer
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Getting the cell size of a raster layer
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Obtaining the width and height of a raster
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Counting raster bands
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Swapping raster bands
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Querying the value of a raster at a specified point
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Reprojecting a raster
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Creating an elevation hillshade
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Creating vector contours from elevation data
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Sampling a raster dataset using a regular grid
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Adding elevation data to line vertices using a digital elevation model
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Creating a common extent for rasters
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Resampling raster resolution
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Counting the unique values in a raster
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Mosaicing rasters
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Converting a TIFF image to a JPEG image
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Creating pyramids for a raster
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Converting a pixel location to a map coordinate
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Converting a map coordinate to a pixel location
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Creating a KML image overlay for a raster
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Classifying a raster
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Converting a raster to a vector
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Georeferencing a raster from control points
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Clipping a raster using a shapefile
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5. Creating Dynamic Maps
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5. Creating Dynamic Maps
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Introduction
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Accessing the map canvas
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Changing the map units
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Iterating over layers
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Symbolizing a vector layer
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Rendering a single band raster using a color ramp algorithm
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Creating a complex vector layer symbol
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Using icons as vector layer symbols
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Creating a graduated vector layer symbol renderer
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Creating a categorized vector layer symbol
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Creating a map bookmark
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Navigating to a map bookmark
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Setting scale-based visibility for a layer
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Using SVG for layer symbols
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Using pie charts for symbols
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Using the OpenStreetMap service
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Using the Bing aerial image service
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Adding real-time weather data from OpenWeatherMap
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Labeling features
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Changing map layer transparency
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Adding standard map tools to the canvas
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Using a map tool to draw points on the canvas
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Using a map tool to draw polygons or lines on the canvas
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Building a custom selection tool
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Creating a mouse coordinate tracking tool
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6. Composing Static Maps
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6. Composing Static Maps
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Introduction
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Creating the simplest map renderer
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Using the map composer
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Adding labels to a map for printing
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Adding a scale bar to the map
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Adding a north arrow to the map
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Adding a logo to the map
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Adding a legend to the map
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Adding a custom shape to the map
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Adding a grid to the map
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Adding a table to the map
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Adding a world file to a map image
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Saving a map to a project
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Loading a map from a project
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7. Interacting with the User
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7. Interacting with the User
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Introduction
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Using log files
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Creating a simple message dialog
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Creating a warning dialog
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Creating an error dialog
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Displaying a progress bar
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Creating a simple text input dialog
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Creating a file input dialog
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Creating a combobox
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Creating radio buttons
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Creating checkboxes
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Creating tabs
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Stepping the user through a wizard
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Keeping dialogs on top
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8. QGIS Workflows
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8. QGIS Workflows
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Introduction
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Creating an NDVI
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Geocoding addresses
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Creating raster footprints
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Performing network analysis
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Routing along streets
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Tracking a GPS
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Creating a mapbook
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Finding the least cost path
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Performing nearest neighbor analysis
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Creating a heat map
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Creating a dot density map
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Collecting field data
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Computing road slope using elevation data
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Geolocating photos on the map
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Image change detection
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9. Other Tips and Tricks
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9. Other Tips and Tricks
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Introduction
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Creating tiles from a QGIS map
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Adding a layer to geojson.io
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Rendering map layers based on rules
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Creating a layer style file
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Using NULL values in PyQGIS
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Using generators for layer queries
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Using alpha values to show data density
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Using the __geo_interface__ protocol
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Generating points along a line
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Using expression-based labels
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Creating dynamic forms in QGIS
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Calculating length for all selected lines
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Using a different status bar CRS than the map
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Creating HTML labels in QGIS
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Using OpenStreetMap's points of interest in QGIS
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Visualizing data in 3D with WebGL
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Visualizing data on a globe
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Index
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Index

Navigating the PyQGIS API

The QGIS Python API, also known as PyQGIS, allows you to control virtually every aspect of QGIS. The ability to find the PyQGIS object you need in order to access a particular feature of QGIS is critical to automation.

Getting ready

The PyQGIS API is based on the QGIS C++ API. The C++ API is kept up to date online and is well-documented.

Note

The QGIS API's web page is located at http://qgis.org/api/2.6/modules.html.

Notice the version number, 2.2, in the URL. You can change this version number to the version of QGIS you are using in order to find the appropriate documentation.

The PyQGIS API documentation is not updated frequently because it is nearly identical to the structure of the C++ API. However, the QGIS project on github.com maintains a list of all the PyQGIS classes for the latest version. The PyQGIS 2.6 API is located at https://github.com/qgis/QGIS/blob/master/python/qsci_apis/Python-2.6.api.

You can locate the documented class in the main C++ API and read about it. Then, look up the corresponding Python module and class using the PyQGIS API listing. In most cases, the C++ API name for a class is identical in Python.

In this recipe, we'll locate the PyQGIS class that controls labels in QGIS.

How to do it…

We will perform the following steps to see in which PyQGIS module the QGIS Label object and QgsLabel are located in:

  1. Go to the QGIS API page at http://qgis.org/api/2.6/index.html.
  2. Click on the Modules tab.
  3. Click on the link QGIS Core Library.
  4. Scroll down the list of modules in alphabetical order until you see QgsLabel.
  5. Click on the QgsLabel link to access the label object documentation.
  6. Now, go to the PyQGIS API listing at https://github.com/qgis/QGIS/blob/master/python/qsci_apis/Python-2.6.api.
  7. Scroll down the alphabetical class listing until you see qgis.core.QgsLabel.LabelField.

How it works…

The QGIS API is divided into five distinct categories, as follows:

  • Core
  • GUI
  • Analysis
  • Map composer
  • Network analysis

Most of the time, it's easy to find the class that targets the functionality you need with most of QGIS being contained in the catch-all Core module. The more you use the API, the quicker you'll be able to locate the objects you need for your scripts.

There's more…

If you're having trouble locating a class containing the keyword you need, you can use the search engine on the QGIS API website.

Tip

Beware, however, that the results returned by this search engine may contain items you don't need and can even send you looking in the wrong direction because of the similar keywords in different modules.

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