Book Image

QGIS 2 Cookbook

By : Mandel, Olaya Ferrero, Anita Graser
Book Image

QGIS 2 Cookbook

By: Mandel, Olaya Ferrero, Anita Graser

Overview of this book

QGIS is a user-friendly, cross-platform desktop geographic information system used to make maps and analyze spatial data. QGIS allows users to understand, question, interpret, and visualize spatial data in many ways that reveal relationships, patterns, and trends in the form of maps. This book is a collection of simple to advanced techniques that are needed in everyday geospatial work, and shows how to accomplish them with QGIS. You will begin by understanding the different types of data management techniques, as well as how data exploration works. You will then learn how to perform classic vector and raster analysis with QGIS, apart from creating time-based visualizations. Finally, you will learn how to create interactive and visually appealing maps with custom cartography. By the end of this book, you will have all the necessary knowledge to handle spatial data management, exploration, and visualization tasks in QGIS.
Table of Contents (14 chapters)
13
Index

Writing Processing algorithms

You can extend the capabilities of QGIS by adding scripts that can be used within the Processing framework. This will allow you to create your own analysis algorithms and then run them efficiently from the toolbox or from any of the productivity tools, such as the batch processing interface or the graphical modeler.

This recipe covers basic ideas about how to create a Processing algorithm.

Getting ready

A basic knowledge of Python is needed to understand this recipe. Also, as it uses the Processing framework, you should be familiar with it before studying this recipe.

How to do it…

We are going to add a new process to filter the polygons of a layer, generating a new layer that just contains the ones with an area larger than a given value. Here's how to do this:

  1. In the Processing Toolbox menu, go to the Scripts/Tools group and double-click on the Create new script item. You will see the following dialog:
    How to do it…
  2. In the text editor of the dialog, paste the following...