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Fri Oct 31 22:50:08 2014

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QGIS Planet

Two book recommendations

I recently finished reading two books which may be of interest to open-source GIS users – “PostGIS Cookbook” and “The PyQGIS Programmer’s Guide“, both of which I highly recommend:

PostGIS Cookbook

PostGIS CookbookI’ve been a fan of Stephen Mather’s blog for a while now, and have consistently found it to be a great source of trustworthy information and creative solutions to GIS problems. So when I first saw mention of his work on the PostGIS Cookbook I knew it would be a must-read for me. PostGIS is an essential part of my daily toolkit, and I’ll quickly devour any tutorial or guide which can lead me to better ways to put it to use. And that’s exactly what this book is! It’s full of tips and guides which has inspired me in a lot of techniques I’d never tried or even thought possible in PostGIS.

It’s important to point out that this book isn’t a training manual or beginner’s guide to PostGIS. It assumes readers are already familiar with using PostGIS and have a good understanding of GIS software in general. (If you’re looking for a book to start from scratch with PostGIS, PostGIS in Action is a better fit). I think that’s really what makes this book stand out though. There’s currently not a lot of books available covering PostGIS, and as far as I’m aware the PostGIS Cookbook is the only book available which is targeted to experienced PostGIS users.

Highlights for me are:

  • A great explanation and write up on optimised KNN filtering in PostGIS (something which often trips me up)
  • The detailed guide to topologically correct simplification of features
  • The exploration of PgRouting, which is a great introduction to PostGIS’ routing abilities
  • The “PostGIS and the web” chapter – I really wasn’t expecting this, but it’s quite eye opening (I’m going to have to do some digging into GeoDjango sometime)

The only criticism I have with this book is that it jumps around a lot between operating systems. While most of the code is provided for both Linux/OSX and Windows, there’s occasional examples which only have code for one specific operating system. It’s a little jarring and assumes the user is well versed in their particular operating system to workaround these omissions.

Overall, I strongly recommend the PostGIS Cookbook, and would consider it a must have for anyone serious about expanding their PostGIS abilities. (Also, looks like the publisher, Packt, have a two-for-one sale going at the moment, so it’s a good time to grab this title).

The PyQGIS Programmer’s Guide

The PyQGIS Programmer's GuideThe second book I’ve just finished reading is Gary Sherman’s “The PyQGIS Programmer’s Guide“. For those who are unaware, Gary was the original founder of QGIS back in 2002, so you can be confident that he knows exactly what he’s writing about. In The PyQGIS Programmer’s Guide  Gary has created an in-depth guide on how to get started with programming for QGIS using python. It takes readers all the way from simple scripts right through to developing QGIS plugins and standalone applications based on the QGIS API.

This book fills an important void in the literature available for QGIS. Previously, the PyQGIS Developer Cookbook was the only available guide for QGIS python scripting, and unfortunately it’s a little out-of-date now. PyQGIS scripting can be a steep learning curve and that’s why this book is so appreciated.

It would be valuable to have some python knowledge and experience prior to reading this book. While the “Python Basics” chapter quickly runs through an introduction to the language, the book makes no claims to be a comprehensive python tutorial. But if you’ve dabbled in the language before and have familiarity with the python way of doing things you’ll easily be able to follow along.

Highlights are:

  • The “Tips and Techniques” chapter, which is a great mini-reference for performing a range of common tasks in PyQGIS (including loading layers, changing symbol styles, editing feature attributes, etc).
  • A complete tutorial for creating a QGIS plugin
  • A guide to debugging PyQGIS code and plugins

I’d definitely recommend that anyone who wants to get started with PyQGIS start with Gary’s work – you’ll find it the perfect place to begin.

A routing script for the Processing toolbox

Did you know that there is a network analysis library in QGIS core? It’s well hidden so far, but at least it’s documented in the PyQGIS Cookbook. The code samples from the cookbook can be used in the QGIS Python console and you can play around to get a grip of what the different steps are doing.

As a first exercise, I’ve decided to write a Processing script which will use the network analysis library to create a network-based route layer from a point layer input. You can find the result on Github.

You can get a Spatialite file with testdata from Github as well. It contains a network and a routepoints1 layer:

points_to_route1

The interface of the points_to_route tool is very simple. All it needs as an input is information about which layer should be used as a network and which layer contains the route points:

points_to_route0

The input points are considered to be ordered. The tool always routes between consecutive points.

The result is a line layer with one line feature for each point pair:

points_to_route2

The network analysis library is a really great new feature and I hope we will see a lot of tools built on top of it.


Generating chainage (distance) nodes in QGIS

Something that I need to do now and then is generate points along a line at supplied distance.  I had never really looked into doing it in QGIS until this question poped up on gis.stackexchange.com.  This is a quick blog post because I thought it was a pretty handy little thing to do.

In the development version there is a new method on QgsGeometry called interpolate. This method takes a distance and returns a point along a line at that distance. Perfect! We can then just wrap this in a loop and generate a point increasing the distance as we move along

from qgis.core import (QgsFeature, QgsGeometry,
                       QgsVectorLayer, QgsMapLayerRegistry,
                       QgsField)
from PyQt4.QtCore import QVariant
from qgis.utils import iface

def createPointsAt(distance, geom):
    length = geom.length()
    currentdistance = distance
    feats = []

    while currentdistance < length:
        # Get a point along the line at the current distance
        point = geom.interpolate(currentdistance)
        # Create a new QgsFeature and assign it the new geometry
        fet = QgsFeature()
        fet.setAttributeMap( { 0 : currentdistance } )
        fet.setGeometry(point)
        feats.append(fet)
        # Increase the distance
        currentdistance = currentdistance + distance

    return feats

def pointsAlongLine(distance):
    # Create a new memory layer and add a distance attribute
    vl = QgsVectorLayer("Point", "distance nodes", "memory")
    pr = vl.dataProvider()
    pr.addAttributes( [ QgsField("distance", QVariant.Int) ] )
    layer = iface.mapCanvas().currentLayer()
    # Loop though all the selected features
    for feature in layer.selectedFeatures():
        geom = feature.geometry()
        features = createPointsAt(distance, geom)
        pr.addFeatures(features)
        vl.updateExtents()

    QgsMapLayerRegistry.instance().addMapLayer(vl)

The above code might look a bit scary at first if you have never done any Python/pyqgis but hopefully the comments will ease the pain a little. The main bit is the createPointsAt function.

Cool! If we want to use this we can just stick it in a file in the .qgis/python folder (lets call it pointtools.py) and then run import pointtools in the python console.

So lets have a go. First select some objects then run the following in the Python Console

import pointtools
pointtools.pointsAlongLine(40)

That will generate a point every 40 meters along then selected lines

Distance nodes along line in qgis-dev

To generate nodes along different lines, select the new features, then call pointtools.pointsAlongLine(40) in the Python console.

Simple as that!

(Who knows, someone (maybe me) might even add this as a core object function in QGIS in the future)


Filed under: Open Source, qgis Tagged: language python, Open Source, osgeo, pyqgis, python, qgis, qgis-tips, Quantum GIS, tips

How to Install Sphinx

This post summarizes how to install sphinx on Windows to contribute to PyQGIS Cookbook. I’m writing this as I go, so this most likely won’t be perfect.

I used my Python 2.6 stand-alone installation (not the one in OSGeo4W).

  1. Get the Sphinx egg from http://pypi.python.org/pypi/Sphinx
  2. If you don’t have it, install setuptools to get the easy_install script http://pypi.python.org/pypi/setuptools
  3. In C:\Python26\Scripts run easy_install -U sphinx
  4. Get the PyQGIS source from https://github.com/qgis/QGIS-Developer-Cookbook
  5. Create a build folder inside the QGIS-Developer-Cookbook
  6. Now you can build the Cookbook: sphinx-build "C:\Users\Anita\QGIS\QGIS-Developer-Cookbook\source" "C:\Users\Anita\QGIS\QGIS-Developer-Cookbook\build"

The build folder should now contain the Cookbook .html files.


How to Add a Legend to Your PyQGIS Application

In his post “Layer list widget for PyQGIS applications”, German Carrillo describes how to add a legend to your own PyQGIS application. You get to download the source code for the legend widget to include into your project.


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