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QGIS Python Programming Cookbook

By : Joel Lawhead

3.8 (6)

QGIS Python Programming Cookbook

3.8 (6)

By: Joel Lawhead

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.

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. Automating QGIS

1. 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

2. 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

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

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

4. 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

5. 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

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 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

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 tabs

Stepping the user through a wizard

Keeping dialogs on top

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 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

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 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

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Using the QGIS Python console for interactive control

The QGIS Python console allows you to interactively control QGIS. You can test out ideas or just do some quick automation. The console is the simplest way to use the QGIS Python API.

How to do it…

In the following steps, we'll open the QGIS Python console, create a vector layer in memory, and display it on the map:

Start QGIS.
From the Plugins menu, select Python Console.

The following code will create a point on the map canvas:

layer =  QgsVectorLayer('Point?crs=epsg:4326', 'MyPoint' , 'memory')
pr = layer.dataProvider()
pt = QgsFeature()
point1 = QgsPoint(20,20)
pt.setGeometry(QgsGeometry.fromPoint(point1))
pr.addFeatures([pt])
layer.updateExtents()
QgsMapLayerRegistry.instance().addMapLayers([layer])

How it works…

This example uses a memory layer to avoid interacting with any data on disk or a network to keep things simple. Notice that when we declare the layer type, we add the parameter for the Coordinate Reference System (CRS) as EPSG:4326. Without this declaration, QGIS will prompt you to choose one. There are three parts or levels of abstraction to create even a single point on the map canvas, as shown here:

First, create a layer that is of the type geometry. Next, set up a data provider to accept the data source.
Then, create a generic feature object, followed by the point geometry.
Next, stack the objects together and add them to the map.

The layer type is memory, meaning that you can define the geometry and the attributes inline in the code rather than in an external data source. In this recipe, we just define the geometry and skip the defining of any attributes.

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