10. PyQGIS Scripting | Mastering QGIS

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Table Of Contents

Mastering QGIS

By : Kurt Menke, GISP

4.6 (12)

Mastering QGIS

4.6 (12)

By: Kurt Menke, GISP

Overview of this book

If you are a GIS professional, a consultant, a student, or perhaps a fast learner who wants to go beyond the basics of QGIS, then this book is for you. It will prepare you to realize the full potential of QGIS.

Preface

Preface

What this book covers

What you need for this book

Who this book is for

Conventions

Reader feedback

Customer support

Free Chapter

1. A Refreshing Look at QGIS

1. A Refreshing Look at QGIS

QGIS download and installation

Tour of QGIS

Loading data

Working with coordinate reference systems

Working with tables

Editing data

Composing maps

Adding functionality with plugins

Summary

2. Creating Spatial Databases

2. Creating Spatial Databases

Fundamental database concepts

Creating a spatial database

Importing data into a SpatiaLite database

Exporting tables out of SpatiaLite as a shapefile

Managing tables

Creating queries and views

Summary

3. Styling Raster and Vector Data

3. Styling Raster and Vector Data

Choosing and managing colors

Managing color ramps

Styling single band rasters

Styling multiband rasters

Creating a raster composite

Raster color rendering

Raster resampling

Styling vectors

Vector layer rendering

Using diagrams to display thematic data

Saving, loading, and setting default styles

Summary

4. Preparing Vector Data for Processing

4. Preparing Vector Data for Processing

Merging shapefiles

Creating spatial indices

Checking for geometry errors

Converting vector geometries

Using basic vector geoprocessing tools

Defining coordinate reference systems

Advanced field calculations

Complex spatial and aspatial queries

Summary

5. Preparing Raster Data for Processing

5. Preparing Raster Data for Processing

Reclassifying rasters

Rescaling rasters

Creating a raster mosaic

Generating raster overviews (pyramids)

Converting between raster and vector data models

Creating raster surfaces via interpolation

Summary

6. Advanced Data Creation and Editing

6. Advanced Data Creation and Editing

Creating points from coordinate data

Geocoding address-based data

Georeferencing imagery

Checking the topology of vector data

Repairing topological errors via topological editing

Summary

7. The Processing Toolbox

7. The Processing Toolbox

About the processing toolbox

Performing raster analyses with GRASS

SAGA

Exploring hydrologic analyses with TauDEM

R

Summary

8. Automating Workflows with the Graphical Modeler

8. Automating Workflows with the Graphical Modeler

An introduction to the graphical modeler

Opening the graphical modeler

Configuring the modeler and naming a model

Adding inputs

Adding algorithms

Running a model

Editing a model

Documenting a model

Saving, loading, and exporting models

Executing model algorithms iteratively

Nesting models

Using batch processing with models

Converting a model into a Python script

Summary

9. Creating QGIS Plugins with PyQGIS and Problem Solving

9. Creating QGIS Plugins with PyQGIS and Problem Solving

Webography - where to get API information and PyQGIS help

The Python Console

Exploring iface and QGis

Exploring a QGIS API in the Python Console

Creating a plugin structure with Plugin Builder

A simple plugin example

Setting up a debugging environment

Debugging session example

Summary

10. PyQGIS Scripting

10. PyQGIS Scripting

Where to learn Python basics

Loading layers

Vector structure

Iterating over features

Editing features

Running processing toolbox algorithms

Running an external algorithm or command

Interacting with the map canvas

Summary

Index

Index

Iterating over features

Now, it's time to discover how to get all the features or a subset of them. The main way to iterate over all features or records of myVector is by using the following code that shows the ID of each feature:

for feature in myVector.getFeatures():
    feature.id()

This will print a list of all the 653 record IDs as shown here:

0L
1L
...[cut]...
652L

It's not always necessary to parse all records to get a subset of them. In this case, we have to set the QgsFeatureRequest class parameters to instruct getFeatures and then retrieve only a subset of records; in some cases, we must also retrieve a subset of columns.

The following code will get only a subset of features and columns:

rect = QgsRectangle(  1223070.695,  2293653.357 , 9046974.211,  4184988.662)

myVector.setSubsetString(' "AREA_MI" > 1000 ')
request =  QgsFeatureRequest()
request.setSubsetOfAttributes([0, 2])
request.setFilterRect( rect )

for index, feature in enumerate( myVector.getFeatures( request ) ):
  ...

CONTINUE READING

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