Book Image

QGIS Python Programming Cookbook, Second Edition - Second Edition

By : Joel Lawhead
Book Image

QGIS Python Programming Cookbook, Second Edition - Second Edition

By: Joel Lawhead

Overview of this book

QGIS is a desktop geographic information system that facilitates data viewing, editing, and analysis. Paired with the most efficient scripting language—Python, we can write effective scripts that extend the core functionality of QGIS. Based on version QGIS 2.18, this book will teach you how to write Python code that works with spatial data to automate geoprocessing tasks in QGIS. It will cover topics such as querying and editing vector data and using raster data. You will also learn to create, edit, and optimize a vector layer for faster queries, reproject a vector layer, reduce the number of vertices in a vector layer without losing critical data, and convert a raster to a vector. Following this, you will work through recipes that will help you compose static maps, create heavily customized maps, and add specialized labels and annotations. As well as this, we’ll also share a few tips and tricks based on different aspects of QGIS.
Table of Contents (16 chapters)
QGIS Python Programming Cookbook - Second Edition
About the Author
About the Reviewer
Customer Feedback

Using a map tool to draw polygons or lines on the canvas

In this recipe, we'll create a tool to draw polygons on the canvas. This tool is an important tool because it opens the doors to even more advanced tools. Once you have a polygon on the canvas, you can do all sorts of operations that involve querying and geometry.

Getting ready

We will use the application framework from the Adding standard map tools to the canvas recipe, so complete that recipe. We will extend that application with a new tool. The complete version of this application is available in the code samples provided with this book.

How to do it...

We will add a new tool to the toolbar and also create a class that describes our polygon tool, as follows:

  1. First, we define our polygon tool's button in the actions portion of our application. Place this line after the QAction("Pan")method:

            actionPoly = QAction("Polygon", self) 
  2. In the next section, we make sure that when we click on the button, it stays selected: