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
Credits
About the Author
About the Reviewer
www.PacktPub.com
Customer Feedback
Preface

Rendering a single band raster using a color ramp algorithm


A color ramp allows you to render a raster using just a few colors to represent different ranges of cell values that have a similar meaning in order to group them. The approach that will be used in this recipe is the most common way to render elevation data.

Getting ready

You can download a sample DEM from https://github.com/GeospatialPython/Learn/raw/master/dem.zip, which you can unzip to a directory named rasters in your qgis_data directory.

How to do it...

In the following steps, we will set up objects to color a raster, create a list establishing the color ramp ranges, apply the ramp to the layer renderer, and finally, add the layer to the map. To do this, we need to perform the following:

  1. First, we import the QtGui library for color objects in the Python Console:

            from PyQt4 import QtGui 
    
  2. Next, we load the raster layer, as follows:

            lyr = QgsRasterLayer("/qgis_data/rasters/dem.asc", "DEM") 
    
  3. Now, we create a...