As software applications become more and more a part of people's lives, the concepts of location and space become more important. Developers are regularly finding themselves having to work with location-based data. Maps, geospatial data, and spatial calculations are increasingly becoming just another part of the everyday programming repertoire.
A decade ago, geospatial concepts and development was limited to experts in the Geographic Information Sciences. These people spent years working with maps and the complex mathematics that underlie them. Often coming from a university background, these specialists would spend years becoming familiar with a particular Geographic Information System (GIS), and would make a career of using that system to draw maps and process geospatial data.
While the ever-popular Google Maps meant that anyone can view and manipulate a map, the more advanced custom display and processing of geospatial data was still limited to those who used a professional GIS system. All this changed with the advent of freely available (and often open source) tools for manipulating and displaying geospatial data. Now, anybody can learn the necessary concepts and start building their own mapping applications from scratch. Rather than being limited to the minimal capabilities and restrictive licensing terms of Google Maps, developers can now build their own mapping systems to meet their own requirements, and there are no limits to what can be done.
While the necessary tools and libraries are freely available, the developer still needs to put them together into a workable system. Often, this is a rather complex process and requires a lot of understanding of geospatial concepts, as well as how to compile the necessary wrappers and configure the tools to work on a particular computer.
Fortunately, now there is an even easier way to include geospatial programming tools and techniques within your Python applications. Thanks to the development of the freely available QGIS system, it is now easy to install a complete geospatial development environment, which you can use directly from within your Python code. Whether you choose to build your application as a plugin for the QGIS system, or write a standalone mapping application using QGIS as an external library, you have complete flexibility in how you use geospatial capabilities within your code.
Chapter 1, Getting Started with QGIS, shows you how to install and run the QGIS application, and introduces the three main ways in which Python can be used with QGIS.
Chapter 2, The QGIS Python Console, explores the QGIS Python Console window, and explains how it acts as a useful tool while building your own custom mapping applications. It also gives you a taste of what can be done with Python and QGIS, and improves your confidence and familiarity with the QGIS environment.
Chapter 3, Learning the QGIS Python API, introduces the Python libraries available for the QGIS Python developer, and shows how these libraries can be used to work with geospatial data and create useful and interesting maps based on your geospatial data.
Chapter 4, Creating QGIS Plugins, introduces the concept of a QGIS plugin, and explains how to write a plugin using Python. We take an in-depth look at how plugins work, and how to create a useful geospatial application as a QGIS plugin. We also look at the possibilities and limitations of QGIS plugins.
Chapter 5, Using QGIS in an External Application, completes the process of building standalone Python applications that make use of the QGIS Python libraries. You will learn how to create a wrapper script to handle platform-specific dependencies, design and build a simple but complete standalone mapping application, and learn about the structure of an application built on top of QGIS. Along the way, you will become a far more competent QGIS programmer as you build your own turnkey mapping application from scratch.
Chapter 6, Mastering the QGIS Python API, delves once more into the PyQGIS library, looking at some more advanced aspects of this library, as well as various techniques for working with QGIS using Python.
Chapter 7, Selecting and Editing Features in a PyQGIS Application, looks at how Python programs built using PyQGIS can allow the user to select, add, edit, and delete geospatial features within a map interface.
Chapter 8, Building a Complete Mapping Application Using Python and QGIS, covers the process of designing and building a complete turnkey mapping application called "ForestTrails". You will design the application, implement the overall user interface, and construct a suitable high-resolution basemap for use by the application.
Chapter 9, Completing the ForestTrails Application, covers the completion of the implementation of the "ForestTrails" mapping application by implementing the various map-editing tools, as well as writing a feature to find the shortest available path between two points on the map.
To follow through the examples in this book, you will need to install the following software on your computer:
QGIS Version 2.2 or later
Python Version 2.6 or later (but not Python 3.x)
GDAL/OGR Version 1.10 or later
PyQt4 Version 4.10 or later
Depending on your operating system, you might also need to install the Qt toolkit so that PyQt will work
All of this software can be freely downloaded, and works on Mac OS X, MS Windows, and Linux computers.
This book is aimed at experienced Python developers who have some familiarity with maps and geospatial concepts. While the necessary concepts are explained as we go along, it would help to have at least some understanding of projections, geospatial data formats, and the like.
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: "This uses the QGIS_PREFIX environment variable we set earlier to tell QGIS where to find its resources."
A block of code is set as follows:
app = QApplication(sys.argv)
viewer = MapViewer("/path/to/shapefile.shp")
viewer.show()
app.exec_()When we wish to draw your attention to a particular part of a code block, the relevant lines or items are set in bold:
def unload(self):
self.iface.removePluginMenu("Test Plugin", self.action)
self.iface.removeToolBarIcon(self.action)
Any command-line input or output is written as follows:
export PYTHONPATH="$PYTHONPATH:/Applications/QGIS.app/Contents/Resources/python"
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: "If you haven't already installed QGIS, click on the Download Now button on the main QGIS web page to download the QGIS software."
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