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

Splitting a shapefile

Sometimes you need to split a shapefile to break a larger dataset into more manageable sizes, or you want to isolate a specific area of interest. There is a script in the Processing Toolbox that splits a shapefile by attribute. It is very useful, even though it is provided as an example on how to write processing scripts.

Getting ready

We will split a census track shapefile by county. You can download the sample zipped shapefile from the following URL:

Extract the zipped shapefile to a directory named /qgis_data/census.

How to do it...

This recipe is as simple as running the algorithm and specifying the file name and data attribute:

  1. Start QGIS.

  2. From the Plugins menu, select Python Console.

  3. Import the processing module:

            import processing 
  4. Define our data directory as a variable to shorten the processing command:

            pth = "/qgis_data/census/" 
  5. Finally, run the algorithm: