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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

Creating a standalone application

QGIS is a complete desktop GIS application. However, with PyQGIS, it can also be a comprehensive geospatial Python library to build standalone applications. In this recipe, we will build a simple standalone script that creates a map with a line on it.

Getting ready

All you need to do to get ready is ensure that you have configured Eclipse and PyDev for PyQGIS development, as described in the Setting up your QGIS IDE recipe.

How to do it…

In PyDev, create a new project called MyMap with a Python script called MyMap.py, as follows:

  1. In the Eclipse File menu, select New and then click on PyDev Project.
  2. In the PyDev project's Name field, enter MyMap.
  3. Next, select the Python radio button from the Project Type list.
  4. From the Interpreter pull-down menu, select PyQGIS.
  5. Leave the radio button checked for Add project directory to the PYTHONPATH.
  6. Click on the Finish button.
  7. Now, select the project in the PyDev package explorer.
  8. From the File menu, select New and then click on File.
  9. Name the file myMap.py.
  10. Click on the Finish button.
  11. Add the following code to the file that is open in the editor:
    from qgis.core import *
from qgis.gui import *
from qgis.utils import *
from PyQt4.QtCore import *
from PyQt4.QtGui import *

app = QgsApplication([], True)
app.setPrefixPath("C:/Program Files/QGIS Brighton/apps/qgis", True)
app.initQgis()
canvas = QgsMapCanvas()
canvas.setWindowTitle("PyQGIS Standalone Application Example")
canvas.setCanvasColor(Qt.white)
layer =  QgsVectorLayer('LineString?crs=epsg:4326', 'MyLine' , "memory")
pr = layer.dataProvider()
linstr = QgsFeature()
geom = QgsGeometry.fromWkt("LINESTRING (1 1, 10 15, 40 35)")
linstr.setGeometry(geom)
pr.addFeatures([linstr])
layer.updateExtents()
QgsMapLayerRegistry.instance().addMapLayer(layer)
canvas.setExtent(layer.extent())
canvas.setLayerSet([QgsMapCanvasLayer(layer)])
canvas.zoomToFullExtent()
canvas.freeze(True)
canvas.show()
canvas.refresh()
canvas.freeze(False)
canvas.repaint()
exitcode = app._exec()
QgsApplication.exitQgis()
sys.exit(exitcode)
  12. From the Run menu, select Run.
  13. Verify that the standalone QGIS map appears in a new window, as shown here:
    How to do it…

How it works…

This recipe uses as little code as possible to create a map canvas and to draw a line in order to demonstrate the skeleton of a standalone application, which can be built up further to add more functionality.

To create the line geometry, we use Well-Known Text (WKT), which provides a simple way to define the line vertices without creating a bunch of objects. Towards the end of this code, we use a workaround for a bug in QGIS 2.2 by freezing the canvas. When the canvas is frozen, it does not respond to any events which, in the case of this bug, prevent the canvas from updating. Once we refresh the canvas, we unfreeze it and then repaint it to draw the line. This workaround will still work in QGIS 2.4 and 2.6 but is not necessary.

There's more...

The standalone application can be compiled into an executable that can be distributed without installing QGIS, using py2exe or PyInstaller:

You can find our more about py2exe at http://www.py2exe.org.

You can learn more about PyInstaller at https://github.com/pyinstaller/pyinstaller/wiki.

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QGIS Python Programming Cookbook
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