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.
QGIS 2 Cookbook
Credits
www.PacktPub.com
Preface
Free Chapter
Data Input and Output
Data Management
Common Data Preprocessing Steps
Classic Vector Analysis
Raster Analysis I
Raster Analysis II
QGIS and the Web
Extending QGIS
Up and Coming
Index

The extent of a layer can be set using a second layer, which acts as a mask. This recipe shows you how to do this.

How to do it…

1. Open the `watershed.tif` layer and the `dem.tif` layer.

2. Open the Raster Calculator algorithm present in the Processing Toolbox menu.

3. In Main input layer, select the DEM, and in the Additional layers field, select the watershed layer.

4. In the Formula field, enter the formula, a*b.

5. Click on Run to run the algorithm. You will get a masked DEM, as follows:

How it works…

When using the raster calculator, all operations involving a no-data value will result in another no-data value. This means that, when multiplying the DEM layer and the mask layer, in the cells that contain no-data values in the mask layer, the value in the resulting layer will be a no-data value, no matter which elevation value is found in the DEM layer for this cell.

As cells inside the watershed in the mask layer have a value...