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

Dissolving vector features

Dissolving a group's geometry allows you to produce a single geometry with common attributes. In this recipe, we'll dissolve some census data into a single dataset.

Getting ready

Download the GIS census tract shapefile, which contains tracts for several counties from the following URL:

Extract it to your /qgis_data directory in a directory called census.

How to do it...

We will use the processing toolbox for this recipe and specifically a native QGIS algorithm called dissolve:

  1. Start QGIS.

  2. From the Plugins menu, select Python Console.

  3. We must import the processing module:

            import processing 
  4. Next, we run the dissolve algorithm specifying the input data False to specify we don't want to dissolve all shapes into one but to use an attribute instead, the attribute we want to use, and the output file name: