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QGIS Blueprints
QGIS Blueprints

QGIS Blueprints: Develop analytical location-based web applications with QGIS

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QGIS Blueprints

Chapter 2. Identifying the Best Places

In this chapter, we will take a look at how the raster data can be analyzed, enhanced, and used for map production. Specifically, you will learn to produce a grid of the suitable locations based on the criteria values in other grids using raster analysis and map algebra. Then, using the grid, we will produce a simple click-based map. The end result will be a site suitability web application with click-based discovery capabilities. We'll be looking at the suitability for the farmland preservation selection.

In this chapter, we will cover the following topics:

  • Vector data ETL for raster analysis
  • Batch processing
  • Raster analysis concepts
  • Map algebra
  • Additive modeling
  • Proximity analysis
  • Raster data ETL for vector publication
  • Leaflet map application publication with qgis2leaf

Vector data – Extract, Transform, and Load

Our suitability analysis uses map algebra and criteria grids to give us a single value for the suitability for some activity in every place. This requires that the data be expressed in the raster (grid) format. So, let's perform the other necessary ETL steps and then convert our vector data to raster.

We will perform the following actions:

  • Ensure that our data has identical spatial reference systems. For example, we may be using a layer of the roads maintained by the state department of transportation and a layer of land use maintained by the department of natural resources. These layers must have identical spatial reference systems or be transformed to have identical systems.
  • Extract geographic objects according to their classes as defined in some attribute table field if we want to operate on them while they're still in the vector form.
  • If no further analysis is necessary, convert to raster.

Loading data and establishing the CRS...

Raster analysis

Raster data, by organizing the data in uniform grids, is useful to analyze continuous phenomena or find some information at the subobject level. We will use continuous elevation and proximity data in this case, and we will look at the subapplicant object level —at the 30 meter-square cell level. You would choose a cell size depending on the resolution of the data source (for example, from sensors roughly 30 meters apart), the roughness of the analysis (regional versus local), and any hardware limitations.

First, let's make a few notes about raster data:

  • Nodata refers to the cells that are included with the raster grid because a grid can't have completely undefined cells; however, these cells should really be considered off the layer.
  • QGIS's raster renderer is more limited than in its proprietary competitors. You will want to use the Identify tool as well as custom styles (Singleband Pseudocolor) to make sense of your outputs.
  • In this example, we will rely...

Publishing the results as a web application

Now that we have completed our modeling for the site selection of a farmland for conservation, let's take steps to publish this for the Web.

qgis2leaf

qgis2leaf allows us to export our QGIS map to web map formats (JavaScript, HTML, and CSS) using the Leaflet map API. Leaflet is a very lightweight, extensible, and responsive (and trendy) web mapping interface.

qgis2leaf converts all our vector layers to GeoJSON, which is the most common textual way to express the geographic JavaScript objects. As our operational layer is in GeoJSON, Leaflet's click interaction is supported, and we can access the information in the layers by clicking. It is a fully editable HTML and JavaScript file. You can customize and upload it to an accessible web location, as you'll understand in subsequent chapters.

qgis2leaf is very simple to use as long as the layers are prepared properly (for example, with respect to CRS) up to this point. It is also very powerful...

Summary

In this chapter, using the site selection example, we covered basic vector data ETL, raster analysis, and web map creation. We started with vector data, and after unifying CRS, we prepared the attribute tables. We then filtered and converted it to raster grids using batch processing. We also considered some fundamental raster concepts as we applied proximity and terrain analysis. Through map algebra, we combined these results for additive modeling site selection. We prepared these results, which required conversion to vector, styling, and labeling. Finally, we published the prepared vector output with qgis2leaf as a simple Leaflet web map application with a strong foundation for extension. In the next chapter, you will learn more about raster analysis and web application publishing with a hydrological modeling example.

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Description

QGIS, the world’s most popular free/open source desktop geographic information system software, enables a wide variety of use cases involving location – previously only available through expensive specialized commercial software. However, designing and executing a multi-tiered project from scratch on this complex ecosystem remains a significant challenge. This book starts with a primer on QGIS and closely related data, software, and systems. We’ll guide you through six use-case blueprints for geographic web applications. Each blueprint boils down a complex workflow into steps you can follow to reduce time lost to trial and error. By the end of this book readers should be able to build complex layered applications that visualize multiple data sets, employing different types of visualization, and give end users the ability to interact with and manipulate this data for the purpose of analysis.

Who is this book for?

This book encompasses relatively experienced GIS developers who have a strong grounding in the fundamentals of GIS development. They will have used QGIS before, but are looking to understand how to develop more complex, layered map applications that expose various data sets, utilize different visualizations, and are consumable (usable) by end users

What you will learn

  • Review geographic information principles and the application of these principles in the QGIS free/open source ecosystem
  • Perform advanced analysis with site selection, hydrologic, and topological networks
  • Build performant web applications by tile caching and generating static assets
  • Provide collaborative editing capabilities for your team or community
  • Develop custom and dynamic analysis and visualization capabilities
  • Select the best components from desktop and web, for your use case
  • Integrate it with social media and crowdsourcing

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Publication date : Sep 25, 2015
Length: 258 pages
Edition : 1st
Language : English
ISBN-13 : 9781785284090
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Product Details

Publication date : Sep 25, 2015
Length: 258 pages
Edition : 1st
Language : English
ISBN-13 : 9781785284090
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Table of Contents

8 Chapters
1. Exploring Places – from Concept to Interface Chevron down icon Chevron up icon
2. Identifying the Best Places Chevron down icon Chevron up icon
3. Discovering Physical Relationships Chevron down icon Chevron up icon
4. Finding the Best Way to Get There Chevron down icon Chevron up icon
5. Demonstrating Change Chevron down icon Chevron up icon
6. Estimating Unknown Values Chevron down icon Chevron up icon
7. Mapping for Enterprises and Communities Chevron down icon Chevron up icon
Index Chevron down icon Chevron up icon

Customer reviews

Rating distribution
Full star icon Full star icon Full star icon Full star icon Half star icon 4.3
(3 Ratings)
5 star 33.3%
4 star 66.7%
3 star 0%
2 star 0%
1 star 0%
Dragons8mycat Nov 20, 2015
Full star icon Full star icon Full star icon Full star icon Full star icon 5
What a lovely surprise this book was, the title doesn't really do the book justice. The book covers some of the more advanced/intermediate concepts and tasks which GIS consultants and technicians might face.Written clearly and easy for even the new user to understand, this covers subjects such as; analysing raster data for suitability, network analysis (routing) and web mapping. The concepts are explained well and there are clear steps to reproducing results demonstrated.I've looked at using pgRouting to calculate shortest path analysis with QGIS in the past, after reading this I was able to easily draw up some network paths.In summary, like with Anita Grasers' introduction to QGIS, this is a swiss army knife of neat tips & tricks which will open up your QGIS from a meagre desktop software to a fully functioning death star.
Amazon Verified review Amazon
Gian Dec 04, 2015
Full star icon Full star icon Full star icon Full star icon Empty star icon 4
I bought this book to improve my skills in GIS analysis. I was looking for a clear practical manual concerning the use of Qgis, and I found QGIS Blueprints very useful and satisfactory to my needs. In particular, I appreciated the in-depth explanations of the matters through step-by-step instructions and the screenshots of commands and outputs. The only "wrong note" is that not all the treated arguments are at the same level. Some of them are more advanced than others (at least for me), for instance, the chapter about the "dynamic web application".
Amazon Verified review Amazon
Peter M Nov 10, 2015
Full star icon Full star icon Full star icon Full star icon Empty star icon 4
As an intermediate QGIS-user, I bought his book to explore more advanced use of QGIS and the overlap with other GIS products. For exploring the current advanced possibilities (November 2015), this book is a great source of information. Unfortunately it is written as a cookbook, without much explanation of the workflows, steps, settings and results. But, for covering so much topics in 250 pages, this may be understandable. A point of criticism is the inaccuracy of the book and accompanying files. I encountered many inaccuracies and in some occasions had to invent missing shapefiles myself to continue the workflow. Nevertheless, this book is good as a reconnaissance and I learned a lot from it.
Amazon Verified review Amazon
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