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 vector contours from elevation data

Contours provide an effective visualization of terrain data by tracing isolines along the same elevation to form a loop at set intervals in the dataset. Similar to the hillshade capability in QGIS, the Contour tool is provided by GDAL, both as a menu option under the Raster menu in the Extraction category and as a Processing Toolbox algorithm.

Getting ready

This recipe uses the DEM from, which is used in the other recipes as well.

Unzip the file named dem.asc and place it in your /qgis_data/rasters directory.

How to do it...

In this recipe, we will load and validate the DEM layer, add it to the map, and then produce and load the contour vector as a layer. To do this, we need to perform the following steps:

  1. Start QGIS.

  2. From the Plugins menu, select Python Console.

  3. Import the processing module.

            import processing 
  4. Load and validate the DEM:

            rasterLyr = QgsRasterLayer("/qgis_data/rasters...