book

QGIS Python Programming Cookbook, Second Edition - Second Edition

Name: QGIS Python Programming Cookbook, Second Edition - Second Edition
Author: Joel Lawhead
ISBN: 9781787124837

by Joel Lawhead

March 2017

Intermediate to advanced

464 pages

8h 24m

English

Packt Publishing

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QGIS Python Programming Cookbook - Second Edition
QGIS Python Programming Cookbook - Second Edition
Credits
About the Author
About the Reviewer
www.PacktPub.com
Why subscribe?
Customer Feedback
Preface
What this book covers
What you need for this book
Who this book is for

Sections
Getting readyHow to do it…How it works…There's more…See also
Conventions
Reader feedback
Customer support
Downloading the example codeDownloading the color images of this book ErrataPiracyQuestions
1. Automating QGIS
Introduction
Installing QGIS 2.18 for development
Getting readyHow to do it...Installing PyQGIS using the Debian package managerInstalling PyQGIS using the RPM package managerSetting the environment variablesSetting the environment variables on WindowsSetting environment variables on LinuxHow it works...There's more...Finding the PyQGIS path on WindowsFinding the location of the QGIS Python installation on other platforms
Using the QGIS Python console for interactive control
How to do it...How it works...
Automatically starting the Python console
Getting readyHow to do it...How it works...
Using the Python's Script Runner plugin
Getting readyHow to do it...How it works...There's more...
Setting up your QGIS IDE
Getting readyHow to do it...Adding the Python interpreter to WindowsAdding the PyQGIS module paths to the interpreterAdding the PyQGIS API to the IDEAdding environment variablesHow it works...
Debugging QGIS Python scripts
How to do it...Configuring QGISConfiguring EclipseTesting the debuggerHow it works...
Navigating the PyQGIS API
Getting readyHow to do it...How it works...There's more...
Creating a traditional QGIS plugin
Getting readyHow to do it...How it works...There's more...
Creating a Processing Toolbox plugin
Getting readyHow to do it...How it works...There's more...
Distributing a plugin
Getting readyHow to do it...How it works...
Building a standalone application
Getting readyHow to do it...How it works...There's more...
Storing and reading global preferences
Getting readyHow to do it...How it works...There's more...
Storing and reading project preferences
Getting readyHow to do it...How it works...There's more...
Accessing the script path from within your script
Getting readyHow to do it...How it works...There's more...
2. Querying Vector Data
Introduction
Loading a vector layer from a file sample
Getting readyHow to do it...How it works...
Loading a vector layer from a geodatabase
Getting readyHow to do it...How it works...
Examining vector layer features
Getting readyHow to do it...How it works...
Examining vector layer attributes
Getting readyHow to do it...How it works...There's more...
Filtering a layer by geometry
Getting readyHow to do it...How it works...
Filtering a layer by attributes
Getting readyHow to do it...How it works...
Buffering a feature
Getting readyHow to do it...How it works...
Measuring the distance between two points
Getting readyHow to do it...How it works...
Measuring distance along a line
Getting readyHow to do it...How it works...
Calculating the area of a polygon
Getting readyHow to do it...How it works...
Creating a spatial index
Getting readyHow to do it...How it works...
Calculating the bearing of a line
Getting readyHow to do it...How it works...
Loading data from a spreadsheet
Getting readyHow to do it...How it works...There's more...
Accessing layer metadata
Getting readyHow to do it...How it works...
3. Editing Vector Data
Introduction
Creating a vector layer in memory
Getting readyHow to do it...How it works...
Adding a point feature to a vector layer
Getting readyHow to do it...How it works...There's more...
Adding a line feature to a vector layer
Getting readyHow to do it...How it works...
Adding a polygon feature to a vector layer
Getting readyHow to do it...How it works...
Adding a set of attributes to a vector layer
Getting ready...How to do it...How it works...
Adding a field to a vector layer
Getting readyHow to do it...How it works...
Joining a shapefile attribute table to a CSV file
Getting readyHow to do it...How it works...There's more...
Changing vector layer geometry
Getting readyHow to do it...How it works...
Changing a vector layer feature's attribute
Getting readyHow to do it...How it works...
Removing data from a vector layer
Getting readyHow to do it....How it works...
Deleting a vector layer feature's attribute
Getting readyHow to do it...How it works...
Reprojecting a vector layer
Getting readyHow to do it...How it works...
Converting a shapefile to KML or GeoJSON
Getting readyHow to do it...How it works...There's more...
Merging shapefiles
Getting readyHow to do it...How it works...There's more...
Splitting a shapefile
Getting readyHow to do it...How it works...
Generalizing a vector layer
Getting readyHow to do it...How it works...
Dissolving vector features
Getting readyHow to do it...How it works...
Performing a union on vector shapes
Getting readyHow to do it...How it works...
Rasterizing a vector layer
Getting readyHow to do it...How it works...
Exporting a layer to the GeoPackage format
Getting readyHow to do it...How it works...
4. Using Raster Data
Introduction
Loading a raster layer
Getting readyHow to do it...How it works...
Loading a NetCDF file
Getting readyHow to do it...How it works...
Getting the cell size of a raster layer
Getting readyHow to do it...How it works...
Obtaining the width and height of a raster
Getting readyHow to do it...How it works...
Counting raster bands
Getting readyHow to do it...How it works...
Swapping raster bands
Getting readyHow to do it...How it works...There's more...
Querying the value of a raster at a specified point
Getting readyHow to do it...How it works...
Reprojecting a raster
Getting readyHow to do it...How it works...There's more...
Creating an elevation hillshade
Getting readyHow to do it...How it works...There's more...
Creating vector contours from elevation data
Getting readyHow to do it...How it works...
Sampling a raster dataset using a regular grid
Getting readyHow to do it...How it works...There's more...
Adding elevation data to a line vertices using a digital elevation model
Getting readyHow to do it...How it works...There's more...
Creating a common extent for rasters
Getting readyHow to do it...How it works...
Resampling raster resolution
Getting readyHow to do it...How it works...
Counting the unique values in a raster
Getting readyHow to do it...How it works...
Mosaicing rasters
Getting readyHow to do it...How it works...
Converting a TIFF image to a JPEG image
Getting readyHow to do it...How it works...
Creating pyramids for a raster
Getting readyHow to do it...How it works...
Converting a pixel location to a map coordinate
Getting readyHow to do it...How it works...
Converting a map coordinate to a pixel location
Getting readyHow to do it...How it works...
Creating a KML image overlay for a raster
Getting readyHow to do it...How it works...There's more...
Classifying a raster
Getting readyHow to do it...How it works...
Converting a raster to a vector
Getting readyHow to do it...How it works...
Georeferencing a raster from control points
Getting readyHow to do it...How it works...
Clipping a raster using a shapefile
Getting readyHow to do it...How it works...
5. Creating Dynamic Maps
Introduction
Accessing the map canvas
Getting readyHow to do it...How it works...
Changing the map units
Getting readyHow to do it...How it works...
Iterating over layers
Getting readyHow to do it...How it works...
Symbolizing a vector layer
Getting readyHow to do it...How it works...
Setting a transparent layer fill
Getting readyHow to do it...How it works...
Using a filled marker symbol
Getting readyHow to do it...
Rendering a single band raster using a color ramp algorithm
Getting readyHow to do it...How it works...
Setting a feature's color using a column in a CSV file
Getting readyHow to do it...How it works...
Creating a complex vector layer symbol
Getting readyHow to do it...How it works...
Using icons as vector layer symbols
Getting readyHow to do it...How it works...
Using an outline for font markers
Getting readyHow to do it...How it works...
Using arrow symbols
Getting readyHow to do it...How it works...
Creating a graduated vector layer symbol renderer
Getting readyHow to do it...How it works...
Creating a categorized vector layer symbol
Getting readyHow to do it...How it works...
Using live layer effects
Getting readyHow to do it...How it works...
Creating inverted polygon shapeburst fills
Getting readyHow to do it...How it works...
Creating a map bookmark
Getting readyHow to do it...How it works...
Navigating to a map bookmark
Getting readyHow to do it...How it works...
Setting scale-based visibility for a layer
Getting readyHow to do it...How it works...
Using SVG for layer symbols
Getting readyHow to do it...How it works...
Using pie charts for symbols
Getting readyHow to do it...How it works...There's more...
Using the 2.5D renderer
Getting readyHow to do it...How it works...
Using XYZ tiled map services
Getting readyHow to do it...How it works...
Using Esri ArcGIS map services
Getting ReadyHow to do it...How it works...
Labeling a feature
Getting readyHow to do it...How it works...
Using null symbols
Getting readyHow to do it...How it works...
Change map layer transparency
Getting readyHow to do it...How it works...
Adding standard map tools to the canvas
Getting readyHow to do it...How it works...
Using a map tool to draw points on the canvas
Getting readyHow to do it...How it works...
Using a map tool to draw polygons or lines on the canvas
Getting readyHow to do it...How it works...
Building a custom selection tool
Getting readyHow to do it...How it works...
Creating a mouse coordinate tracking tool
Getting readyHow to do it...How it works...
6. Composing Static Maps
Introduction
Creating the simplest map renderer
Getting readyHow to do it...How it works...There's more...
Using the Map Composer
Getting readyHow to do it...How it works...There's more...
Adding labels to a map for printing
Getting readyHow to do it...How it works...
Adding a scale bar to a map
Getting readyHow to do it...How it works...
Adding a North arrow to the map
Getting readyHow to do it...How it works...There's more...
Adding a logo to the map
Getting readyHow to do it...How it works...
Adding a vertical legend to the map
Getting readyHow to do it...How it works...
Adding a horizontal legend
Getting readyHow to do it...
Adding a custom shape to the map
Getting readyHow to do it...How it works...There's more...
Adding a grid to the map
Getting readyHow to do it...How it works...
Adding a table to the map
Getting readyHow to do it...How it works...
Adding a world file to a map image
Getting readyHow to do it...How it works...
Saving a map to a project
Getting readyHow to do it...How it works...
Loading a map from a project
Getting readyHow to do it...How it works...
7. Interacting with the User
Introduction
Using log files
Getting readyHow to do it...How it works...There's more...
Creating a simple message dialog
Getting readyHow to do it...How it works...There's more...
Creating a warning dialog
Getting readyHow to do it...How it works...
Creating an error dialog
Getting readyHow to do it...How it works...
Displaying a progress bar
Getting readyHow to do it...How it works...There's more...
Creating a simple text input dialog
Getting readyHow to do it...How it works...
Creating a file input dialog
Getting readyHow to do it...How it works...There's more...
Creating a combobox
Getting readyHow to do it...How it works...
Creating radio buttons
Getting readyHow to do it...How it works...
Creating checkboxes
Getting readyHow to do it...How it works...
Creating a dock widget
Getting readyHow to do it...How it works...
Displaying a message in the status bar
Getting readyHow to do it...How it works...
Pushing messages to the message bar
Getting readyHow to do it...How it works...
Pushing widgets to the message bar
Getting readyHow to do it...How it works...
Creating tabs
Getting readyHow to do it...How it works...
Stepping the user through a wizard
Getting readyHow to do it...How it works...
Keeping dialogs on top
Getting readyHow to do it...How it works...
8. QGIS Workflows
Introduction
Creating an NDVI
Getting readyHow to do it...How it works...
Geocoding addresses
Getting readyHow to do it...How it works...
Creating raster footprints
Getting readyHow to do it...How it works...There's more...
Performing network analysis
Getting readyHow to do it...How it works...There's more...
Routing along streets
Getting readyHow to do it...How it works...
Tracking a GPS
Getting readyHow to do it...How it works...There's more...
Creating a mapbook
Getting readyHow to do it...How it works...
Finding the least cost path
Getting readyHow to do it...How it works...
Performing nearest neighbor analysis
Getting readyHow to do it...How it works...
Creating a DEM from LIDAR
Getting readyHow to do it...How it works...
Creating a heat map
Getting readyHow to do it...How it works...
Creating a dot density map
Getting readyHow to do it...How it works...
Collecting field data
Getting readyHow to do it...How it works...
Computing road slope using elevation data
Getting readyHow to do it...How it works...
Geolocating photos on the map
Getting readyHow to do it...How it works...There's more...
Image change detection
Getting readyHow to do it...How it works...
Adjusting imprecise building footprints
Getting readyHow to do it...How it works...
Visualizing multitemporal data
Getting readyHow to do it...How it works...
9. Other Tips and Tricks
Introduction
Creating tiles from a QGIS map
Getting readyHow to do it...How it works...There's more...
Adding a layer to geojson.io
Getting readyHow to do it...How it works...There's more...
Rendering map layers based on rules
Getting readyHow to do it...How it works...
Creating a layer-definition file
Getting readyHow to do it...How it works...
Using NULL values in PyQGIS
Getting readyHow to do it...How it works...
Using generators for layer queries
Getting readyHow to do it...How it works...
Using alpha values to show data density
Getting readyHow to do it...How it works...
Using the __geo_interface__ protocol
Getting readyHow to do it...How it works...
Getting the output file names from processing algorithms
Getting readyHow to do it...How it works...
Generating points along a line
Getting readyHow to do it...How it works...There's more...
Using expression-based labels
Getting readyHow to do it...How it works...
Creating dynamic forms in QGIS
Getting readyHow to do it...How it works...There's more...
Calculating length for all selected lines
Getting readyHow to do it...How it works...
Using a different status bar CRS than the map
Getting readyHow to do it...How it works...
Creating HTML labels in QGIS
Getting readyHow to do it...How it works...There's more...
Using OpenStreetMap points of interest in QGIS
Getting readyHow to do it...How it works...
Changing the QGIS web proxy
Getting readyHow to do it...How it works...There's more...
Running QGIS scripts as scheduled tasks
Getting readyHow to do it...How it works...
Visualizing data in 3D with WebGL
Getting readyHow to do it...How it works...
Visualizing data on a globe
Getting readyHow to do it...How it works...
Make a globe-like azimuthal orthographic projection
Getting readyHow to do it...How it works...
Animating a layer
Getting readyHow to do it...

Content preview from QGIS Python Programming Cookbook, Second Edition - Second Edition

Calculating the area of a polygon

This recipe measures the area of a polygon. Area calculation can be an end in itself to measure the size of a plot of land or a building. It can also be the input to other calculations such as land use maps.

Getting ready

For this recipe, we'll use a single-feature polygon shapefile, which you can download from the following link:

https://github.com/GeospatialPython/Learn/raw/master/Mississippi.zip

Unzip the shapefile and put it in a directory named qgis_data/ms within your root or home directory.

How to do it...

Perform the following steps to measure the area of a large polygon:

First, import the QGIS constants library, as follows:
```
        from qgis.core import QGis 
```

Load the layer:

 lyr = QgsVectorLayer("/qgis_data/ms/mississippi.shp", ...

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