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

Creating a graduated vector layer symbol renderer

A graduated vector layer symbol renderer is the vector equivalent of a raster color ramp. You can group features into similar ranges and use a limited set of colors to visually identify these ranges. In this recipe, we'll render a graduated symbol using a polygon shapefile.

Getting ready

You can download a shapefile containing a set of urban area polygons from

Extract this file to a directory named ms in your qgis_data directory.

How to do it...

We will classify each urban area by population size using a graduated symbol, as follows:

  1. First, we import the QColor object to build our color range:

            from PyQt4.QtGui import QColor 
  2. Next, we load our polygon shapefile as a vector layer:

            lyr = QgsVectorLayer("/qgis_data/ms/MS_UrbanAnC10.shp",
                                 "Urban Areas", "ogr") 
  3. Now, we build some nested Python tuples that define the symbol...