The open source geographic information system QGIS at version 2 now rivals even the most expensive commercial GIS software in both functionality and usability. It is also a showcase of the best geospatial open source technology available. It is not just a project in itself but the marriage of dozens of open source projects in a single, clean interface.
Geospatial technology is not just the application of technology to geography. It is a symphony of geography, mathematics, computer science, statistics, physics, and other fields. The underlying algorithms implemented by QGIS are so complex that only a handful of people in the world understand them all. Yet QGIS packages all of this complexity so well that school children, city managers, disease researchers, geologists, and many other professionals wield this powerful software with ease to make decisions that improve life on Earth.
But this book is about another feature of QGIS that makes it the best choice for geospatial work. QGIS has one of the most deeply integrated and well-designed Python interfaces of any software period. In the latest version, there is virtually no aspect of the program that is off-limits to Python, making it the largest geospatial Python library available. Almost without exception, the Python API, called PyQGIS, is consistent and predictable.
This book exploits the best features of QGIS to demonstrate over 170 reusable recipes that you can use to automate workflows in QGIS or build standalone GIS applications. Most recipes are very compact and have less than 20 lines of code. Even if you can’t find the exact solution you are looking for, you should be able to get very close. This book covers a lot of ground and pulls together fragmented ideas and documentation scattered throughout the Internet as well as the results of many hours of experimenting at the edges of the PyQGIS API.
Chapter 1, Automating QGIS, provides a brief overview of the different ways of using Python with QGIS, including the QGIS Python Console, standalone applications, plugins, and the Script Runner plugin. This chapter also covers setting and retrieving application settings and a few other Python-specific features.
Chapter 2, Querying Vector Data, covers extracting information from vector data without changing the data using Python. Topics include measuring, loading data from a database, filtering data, and other related processes.
Chapter 3, Editing Vector Data, introduces creating and updating data to add new information. It also teaches you how to break datasets apart based on spatial or database attributes as well as combine datasets. This chapter also teaches you how to convert data into different formats, change projections, simplify data, and more.
Chapter 4, Using Raster Data, demonstrates recipes for using and transforming raster data to create derivative products. This chapter highlights the capability of QGIS as a raster-processing engine and not just a vector GIS.
Chapter 5, Creating Dynamic Maps, transitions into recipes for controlling QGIS as a whole to control map, project, and application-level settings. It includes recipes for accessing external web services and building custom map tools.
Chapter 6, Composing Static Maps, shows you how to create printed maps using the QGIS Map Composer. You will learn to place reference elements on a map as well as design elements such as logos.
Chapter 7, Interacting with the User, teaches you to control QGIS GUI elements created by the underlying Qt framework to create interactive input widgets for scripts, plugins, or standalone applications.
Chapter 8, QGIS Workflows, contains more advanced recipes that result in a finished product or an extended capability. These recipes target actual tasks that geospatial analysts or programmers encounter on the job.
Chapter 9, Other Tips and Tricks, contains interesting recipes that fall outside the scope of previous chapters. Many of these recipes demonstrate multiple concepts within a single recipe that you may find useful for a variety of tasks.
You will need the following software to complete all of the recipes in this book. If a specific version is not available, use the most recent version.
QGIS 2.18
Python 2.7.6 (should be included with QGIS itself)
IBM Java 8 JDK
IBM Eclipse Neon 4.6.0 or higher
Eclipse PyDev 5.5.0
Google Earth 7.1.7.2602 or higher
This book is for geospatial analysts or programmers at any experience level who would like to learn more about controlling QGIS using Python. A basic knowledge of Python is essential and some experience with QGIS is helpful but not required.
In this book, you will find several headings that appear frequently (Getting ready, How to do it, How it works, There's more, and See also).
To give clear instructions on how to complete a recipe, we use these sections as follows:
This section tells you what to expect in the recipe, and describes how to set up any software or any preliminary settings required for the recipe.
This section usually consists of a detailed explanation of what happened in the previous section.
This section consists of additional information about the recipe in order to make the reader more knowledgeable about the recipe.
In this book, you will find a number of text styles that distinguish between different kinds of information. Here are some examples of these styles and an explanation of their meaning.
Code words in text, database table names, folder names, filenames, file extensions, pathnames, dummy URLs, user input, and Twitter handles are shown as follows: "Open the startup.py file you created."
A block of code is set as follows:
import os
os.getcwd()Any command-line input or output is written as follows:
sudo apt-get update
New terms and important words are shown in bold. Words that you see on the screen, for example, in menus or dialog boxes, appear in the text like this: "From the Window menu, select Preferences."
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