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
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Querying the value of a raster at a specified point

A common remote sensing operation is to get the raster data value at a specified coordinate. In this recipe, we'll query the data value in the center of the image. It so happens that the raster layer will calculate the center coordinate of its extent for you.

Getting ready

As with many recipes in this chapter, we will again use the SatImage raster, which is available at

Place this raster in your /qgis_data/rasters directory.

How to do it...

We will load the layer, get the center coordinate, and then query the value. To do this, we need to perform the following steps:

  1. First, load and validate the layer:

            rasterLyr = QgsRasterLayer("/qgis_data/rasters/satimage.tif",
                                       "Sat Image") 
  2. Next, get the layer's center point from its QgsRectangle extent object, which will return a QGSPoint which looks and behaves like a...