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 expression-based labels

Expressions are a kind of mini-programming language or SQL-like language found throughout different QGIS functions to selected features. One important use of expressions is controlling labels. Maps easily become cluttered if you label every single feature. Expressions make it easy to limit labels to important features. You can filter labels using expressions from within Python, as we will do in this recipe.

Getting ready

You will need to download the following zipped shapefile and decompress it to a directory named ms in your qgis_data directory:

How to do it...

We'll use the QGIS PAL labeling engine to filter labels based on a field name. After loading the layer, we'll create our PAL settings and write it to the layer. Finally, we'll add the layer to the map.

  1. First, we'll set up the path to our shapefile:

            pth = "/Users/joellawhead/qgis_data/ms/MS_UrbanAnC10.shp" 
  2. Next, we'll load...