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

Adding a field to a vector layer

This recipe demonstrates adding a new field to a layer. Each field represents a new column in a data set for which each feature has a new attribute. When you add a new attribute, the attribute value for all existing features are set to NULL for that field index.

Getting ready

We will use the New York City museums shapefile used in other recipes, which you can download as a ZIP file here:

Extract this shapefile to /qgis_data/nyc.

How to do it...

All data management for a layer is handled through the layer's data provider, and fields are no different. We will load the layer, access the data provider, define the new field, and finalize the change:

  1. Start QGIS.

  2. From the Plugins menu, select Python Console.

  3. First, we must import the Qt library data types that PyQGIS uses to specify layer field data types:

            from PyQt4.QtCore import QVariant 
  4. Next, load and validate the layer: