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

Converting a raster to a vector

Raster datasets represent real-world features efficiently, but can have limited usage for geospatial analysis. Once you have classified an image into a manageable data set, you can convert those raster classes into a vector data set for more sophisticated GIS analysis. GDAL has a function for this operation called polygonize.

Getting ready

You will need to download the following classified raster and place it in your /qgis_data/rasters directory:

How to do it...

Normally, you would save the output of this recipe as a shapefile. We won't specify an output file name. The Processing Toolbox will assign it a temporary filename and return that filename. We'll simply load the temporary file into QGIS. The algorithm allows you to write to a shapefile by specifying it as the last parameter. Perform the following steps:

  1. In the QGIS Python Console, import the processing module:

            import processing...