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QGIS Python Programming Cookbook

Book Image

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

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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