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

Accessing the map canvas

Maps in QGIS are controlled through the map canvas. In this recipe, we'll access the canvas and then check one of its properties to ensure that we have control over the object.

Getting ready

The only thing you need to do for this recipe is to open QGIS and select Python Console from the Plugins menu.

How to do it...

We will assign the map canvas to a variable named canvas. Then, we'll check the size property of the canvas to get its size in pixels. To do this, perform the following steps:

  1. Enter the following line in the QGIS Python Console:

            canvas = qgis.utils.iface.mapCanvas() 
  2. Now, to ensure that we have properly accessed the canvas, check its size in pixels using the following line of code:

  3. Verify that QGIS returns a QSize object that contains the canvas's pixel size as width and height respectively, similar to the following format. The actual width and height will vary based on the size of your QGIS window and screen size: