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- Herausgeber: Packt Publishing
- Kategorie: Fachliteratur
- Sprache: Englisch
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.
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Seitenzahl: 443
Veröffentlichungsjahr: 2015
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Table of Contents
Mastering QGIS
Mastering QGIS
Copyright © 2015 Packt Publishing
All rights reserved. No part of this book may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, without the prior written permission of the publisher, except in the case of brief quotations embedded in critical articles or reviews.
Every effort has been made in the preparation of this book to ensure the accuracy of the information presented. However, the information contained in this book is sold without warranty, either express or implied. Neither the authors, nor Packt Publishing, and its dealers and distributors will be held liable for any damages caused or alleged to be caused directly or indirectly by this book.
Packt Publishing has endeavored to provide trademark information about all of the companies and products mentioned in this book by the appropriate use of capitals. However, Packt Publishing cannot guarantee the accuracy of this information.
First published: March 2015
Production reference: 2020415
Published by Packt Publishing Ltd.
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ISBN 978-1-78439-868-2
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Cover image by NASA Earth Observatory
Credits
Authors
Kurt Menke, GISP
Dr. Richard Smith Jr., GISP
Dr. Luigi Pirelli
Dr. John Van Hoesen, GISP
Reviewers
Paolo Corti
Abdelghaffar Khorchani
Gergely Padányi-Gulyás
Commissioning Editor
Dipika Gaonkar
Acquisition Editors
Richard Harvey
Rebecca Youé
Content Development Editor
Samantha Gonsalves
Technical Editors
Ruchi Desai
Manal Pednekar
Copy Editors
Sonia Michelle Cheema
Jasmine Nadar
Project Coordinator
Kinjal Bari
Proofreaders
Martin Diver
Maria Gould
Elinor Perry-Smith
Indexer
Rekha Nair
Graphics
Valentina D'silva
Production Coordinator
Alwin Roy
Cover Work
Alwin Roy
Foreword
It has been my pleasure to witness the development of both this book, Mastering QGIS, and the QGIS software in the past 12 months. Who could have predicted the rapid development and adoption of QGIS in such a short time? QGIS is now on a par, in terms of its functionality and features, with the best of commercial GIS application software. With an aggressive code development schedule of quarterly updates, the QGIS project is adding new features faster than most GIS professionals can keep pace. To help with the dire need for professional training, this book has been created to provide you with the concise technical expertise that will serve you well, both now and in future versions of this powerful GIS software. I have enjoyed the privilege of working closely with the contributing authors of this book for the past 2 years. We have been engaged in an intense curriculum development process to create the first-ever national GIS curriculum that is based around a national standard—the U.S. Department of Labor's Geospatial Technology Competency Model (GTCM). This effort has resulted in a series of GIS courses, all based around QGIS, that provide a solid foundation upon which this book can be used to enhance your technical skills. Each of the contributing authors is a very experienced GIS professional and many of them serve as instructors for highly respected academic GIS programs. Dr. Richard (Rick) Smith, a certified GIS Professional (GISP), serves as an assistant professor for the Geographic Information Science program at Texas A&M University—Corpus Christi, Texas, USA. Rick has been onboard the curriculum effort since day one, where his expertise in GIS and cartography is highlighted. Kurt Menke is a certified GIS Professional (GISP) and operates his own GIS consulting business (Bird's Eye View GIS) in New Mexico, USA, where he teaches open source GIS software at the local college and universities. Kurt is well respected in both conservation and healthcare GIS and has completed numerous GIS projects in these disciplines. Dr. John Van Hoesen (GISP) serves as an associate professor of geology and is the Environmental Studies Community Mapping Lab Director at Green Mountain College in Vermont, USA. His passions include open source software, environmental science, and the great outdoors, where he leads students in the discovery of our natural world. Luigi Pirelli, from Spain, is a core contributor to QGIS and a contributing author of this book.
He provided us with the chapters on programming for QGIS. A huge thanks to our most capable editor, Samantha Gonsalves, for her management during the creation of this book. A former systems engineer at Infosys in Mumbai, India, and now an editor for Packt Publishing, her leadership kept the team on a tight deadline to complete Mastering QGIS while maintaining the highest editorial standards. For all of us, it has been a fascinating and rewarding experience and now you hold the results of our effort in your hands. Best wishes for success on Mastering QGIS, now and in the future!
Phillip Davis
Director, National Information Security & Geospatial Technology Consortium,
Del Mar College,
Texas, USA
About the Authors
Kurt Menke, a certified GIS Professional (GISP), has been working in the GIS field since 1997. Prior to this, he worked as a professional archaeologist for 10 years in the American Southwest. He earned a master's degree (MA) in geography from the University of New Mexico in 2000. That same year, he founded Bird's Eye View (www.BirdsEyeViewGIS.com) to apply his expertise with the GIS technology to the world's mounting ecological and social problems. Toward this end, Mr. Menke's work focuses largely on wildlife conservation and public health. His specialties are spatial analysis, modeling, and cartography.
He is a longtime advocate of FOSS4G. He began writing MapServer applications in 2001 and has been using QGIS since 2007. He is one of the coauthors of the curriculum at the FOSS4G Academy (http://foss4geo.org/) and has been teaching FOSS4G college courses since 2009. In 2014, Kurt began authoring an award-winning blog on FOSS4G technologies and their use in community health mapping (http://communityhealthmaps.nlm.nih.gov/).
A special thanks goes to Phil Davis for his leadership in the development of the FOSS4G Academy and for his continuing efforts to promote FOSS4G in the U.S. educational system. I would like to thank Rick Smith for being such a joy to work with. I'd also like to acknowledge Karl Benedict for introducing me to the world of FOSS4G and Jeffery Cavner for his ongoing camaraderie. Finally, I'd like to thank my beautiful wife, Sarah, for her steady support and encouragement.
Dr. Richard Smith Jr., is an assistant professor of geographic information science at the School of Engineering and Computing Sciences at Texas A&M University Corpus Christi. He has a PhD in geography from the University of Georgia and holds a master of science in computer science and a bachelor of science in geographic information science degree from Texas A&M University Corpus Christi. Richard actively does research in cartography, systems integration, and the use of geospatial technology for disaster response. Richard is an advocate of FOSS4G and building FOSS4G curriculum. He is one of the coauthors of the FOSS4G Academy (http://foss4geo.org).
Richard has collaborated with other writers in his field, but Mastering QGIS is his first book.
I would like to thank my wife and daughter for putting up with my late-night and weekend writing sessions. I would also like to thank my coauthor Kurt Menke for being patient with my edits. I would especially like to thank the editorial team; you have made my first book-writing experience an excellent one.
Outside those directly involved or affected by the writing of this book, I'd like to thank my academic and life mentors, Dr. Stacey Lyle, Dr. Thomas Hodler, Dr. Gary Jeffress, and Dr. Robin Murphy, for providing their support and good wishes as I begin my career. In addition to teaching me, you have inspired me to have the confidence to teach and write. To those of you reading this, I hope I do my mentors justice by providing a clear and useful text to assist you in mastering QGIS.
Dr. Luigi Pirelli is a freelance software analyst and developer with an honors degree in computer science from the University of Bari. He worked for 15 years in Satellite Ground Segment and Direct Ingestion for the European Space Agency. Since 2006, he has been involved with the GFOSS world, contributing to QGIS, GRASS, and MapServer core and developing and maintaining many QGIS plugins. He actively participates in QGIS Hackmeetings. He is the founder of the OSGEO Italian Local Chapter (http://gfoss.it/drupal/) and now lives in Spain and contributes to this GFOSS community. During the last few years, he started teaching PyQGIS by organizing trainings from basic to advanced levels and supporting companies to develop their specific QGIS plugins.
He is the founder of the local hackerspace group Bricolabs.cc that is focused on Open Hardware. He likes cycling, repairing everything, and trainings groups on conflict resolution. Besides this book, he has also contributed to Lonely Planet Cycling Italy.
A special thanks to the QGIS developer community and core developers as the project is managed in a really open way by allowing contributions from everyone.
I want to thank everyone with whom I have worked. I learned from each of them, and without them, I wouldn't be here giving my contribution to free software and to this book.
A special thanks to my friends and neighbors who helped me with my son during the writing of this book.
I would like to dedicate this book to my partner and especially to my son for his patience when he used to see me sitting in front of a computer for hours instead of playing with him.
Dr. John Van Hoesen is an associate professor of geology and environmental studies at Green Mountain College in rural west-central Vermont. He earned an MS in 2000 and a PhD in geology from the University of Nevada, Las Vegas, in 2003. He is a certified GIS Professional (GISP) with a broad background in geosciences and has been using some flavor of GIS to evaluate and explore geologic processes and environmental issues since 1997. He has used and taught some variant of FOSS GIS since 2003, and over the last 3 years, he has taught graduate, undergraduate, and continuing education courses using only FOSS GIS software.
About the Reviewers
Paolo Corti is an environmental engineer based in Rome, Italy. He has more than 15 years of experience in the GIS field; after working with proprietary solutions for some years, he proudly switched to open source technologies and Python almost a decade ago.
He has been working as a software architect, developer, and analyst for organizations such as the United Nations World Food Programme, the European Commission Joint Research Centre, and the Italian Government.
Currently, he is working within the GeoNode project, for which he is the core developer, in the context of emergency preparedness and response.
He is an OSGeo charter member and a member of the pyCSW and GeoNode Project Steering committees. He is the coauthor of PostGIS Cookbook by Packt Publishing, and he writes a popular blog on open source geospatial technologies at http://www.paolocorti.net.
Abdelghaffar Khorchani has a license degree in geographic information systems, a fundamental license in natural science applied in biology and geology, and a master's degree in geomatics and planning. He is also a computer engineer. Currently, he is pursuing his master's degree in planning and regional development (University of Laval—Canada) and his PhD in marine sciences (University of Milano-Biccoca – Italy).
He has prepared courses in Japan on fishery resource management approaches for young leaders and in Spain in the field use of geographic information systems for scheduling and management. He has also prepared other training modules in Tunisia on urban administration.
He has 8 years of experience in the geomatics field and has worked on several projects in the agriculture sector, environment, transport, and mapping.
Currently, he is in the Ministry of Agriculture in Tunisia and is responsible for the mapping service for project VMS (short for Vessel Monitoring System).
He is also a trainer in the mapping field of Geographic Information System, GPS, and CAD. He is particularly interested in the development of decision support tools.
A special thanks to Packt Publishing for this opportunity to participate in the review of this book. I thank my family, especially my parents, for their physical and moral support. Finally, I want to thank Cheima Ayachi, who helped me a lot when I was reviewing this book.
Gergely Padányi-Gulyás is a GIS and web developer and remote sensing analyst with over 7 years of experience. He specializes in designing and developing web mapping applications and Geographic Information Systems (GIS). He is a dedicated user/developer of open source software, and he is also an active member of the OSGeo local chapter. He is familiar both with client- and server-side programming.
For more than 4 years, he worked for archaeologists as a GIS engineer and remote sensing analyst where he contributed to laying the foundation of the Hungarian Archaeological predictive modelling. After that, he became a Java web developer for a private company. Since then, he has been working at a state nonprofit corporation as a GIS and web developer where he uses the skills he learned from his previous jobs: combining GIS with development. During the past few years, he has been involved with plugin development in different programming languages such as Java for GeoServer and Python for QGIS.
He has a website (www.gpadanyig.com).
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Preface
Welcome to Mastering QGIS. The goal of this book is to help intermediate and advanced users of GIS develop a deep understanding of the capabilities of QGIS while building the technical skills that would facilitate in making the shift from a proprietary GIS software package to QGIS.
QGIS embodies the open source community's spirit. It seamlessly works with other free and open source geospatial software, such as SAGA, GDAL, GRASS, and fTools, and supports standards and formats that are published by myriad organizations. QGIS is about freedom in the geospatial world: freedom to choose your operating system, freedom from licensing fees, freedom to customize, freedom to look under the hood, and freedom to contribute to the development of QGIS. As you work through this book, we believe that you will be amazed at how much capability and freedom QGIS provides.
QGIS has rapidly risen from the initial version written by Gary Sherman in 2002 to become a globally used and developed volunteer-led project. In 2009, QGIS version 1.0 was released as an Open Source Geospatial Foundation (OSGeo) project and continues to be rapidly adopted worldwide. The enduring support of the open source community has really delivered QGIS to a point where it is now a top-shelf product that should be in all GIS users' toolboxes, and we want this book to be your tour guide and reference as you learn, use, and contribute to QGIS.
What this book covers
Chapter 1, A Refreshing Look at QGIS, reviews the installation and basic functionality of QGIS that will be the assumed knowledge for the remainder of the book.
Chapter 2, Creating Spatial Databases, covers how to create and edit spatial databases using QGIS. While QGIS supports many spatial databases, SpatiaLite will be used in this chapter. First, core database concepts will be covered, followed by the creation of a spatial database. Next, importing, exporting, and editing data will be covered. The chapter will conclude with queries and view creation.
Chapter 3, Styling Raster and Vector Data, covers styling raster and vector data for display. First, color selection and color ramp management are covered. Next, singleband and multiband raster data are styled using custom color ramps and blending modes. Next, complex vector styles and vector layer rendering are covered. Rounding out the chapter is the use of diagrams to display thematic map data.
Chapter 4, Preparing Vector Data for Processing, covers techniques useful for turning raw vector data into a more usable form. The chapter will start with data massaging and modification techniques such as merging, creating indices, checking for geometry errors, and basic geoprocessing tools. Next, advanced field calculations will be covered, followed by complex spatial and aspatial queries. The chapter will end by defining new or editing existing coordinate reference systems.
Chapter 5, Preparing Raster Data for Processing, covers the preparation of raster data for further processing using the GDAL menu tools and the Processing Toolbox algorithms. Specifically, these include reclassification, resampling, rescaling, mosaics, generating pyramids, and interpolation. The chapter will conclude by converting raster to vector.
Chapter 6, Advanced Data Creation and Editing, provides advanced ways to create vector data. As there is a great deal of data in tabular format, this chapter will cover mapping coordinates and addresses from tables. Next, georeferencing of imagery into a target coordinate reference system will be covered. The final portion of the chapter will cover testing topological relationships in vector data and correcting any errors via topological editing.
Chapter 7, The Processing Toolbox, begins with an explanation and exploration of the QGIS Processing Toolbox. Various algorithms and tools, available in the toolbox, will be used to complete common spatial analyses and geoprocessing tasks for both raster and vector formats. To illustrate how these processing tools might be applied to real-world questions, two hypothetical scenarios are illustrated by relying heavily on GRASS and SAGA tools.
Chapter 8, Automating Workflows with the Graphical Modeler, covers the purpose and use of the graphical modeler to automate analysis workflows. In the chapter, you will develop an automated tool/model that can be added to the Processing Toolbox.
Chapter 9, Creating QGIS Plugins with PyQGIS and Problem Solving, covers the foundational information to create a Python plugin for QGIS. Information about the API and PyQGIS help will be covered first, followed by an introduction to the iface and QGis classes. Next, the steps required to create and structure a plugin will be covered. The chapter will be wrapped up after providing you with information on creating graphical user interfaces and setting up debugging environments to debug code easily.
Chapter 10, PyQGIS Scripting, provides topics for integrating Python analysis scripts with QGIS outside of the Processing Toolbox. Layer loading and management are first covered, followed by an exploration of the vector data structure. Next, programmatic launching of other tools and external programs are covered. Lastly, the QGIS map canvas is covered with respect to how a script can interact with the map canvas and layers within.
What you need for this book
To get the most from this book, it is recommended that you install QGIS and follow the explanations. If you choose to do so, you will need a reasonably modern computer with access to the Internet to download and install QGIS, read documentation, and install plugins. QGIS can run on Windows, Mac OS X, and many Linux distributions.
Who this book is for
This book is for intermediate to advanced GIS users, developers, and consultants who are familiar with QGIS but want to look deeper into the software to unleash its full potential. The reader is expected to be comfortable with common GIS functions and concepts, as possession of this knowledge is assumed throughout the book. This book focuses on how to use QGIS and its functions beyond the basics.
Conventions
In this book, you will find a number of styles of text that distinguish between different kinds of information. Here are some examples of these styles, and an explanation of their meaning.
Code words in text, database table names, folder names, filenames, file extensions, pathnames, dummy URLs, and user input are shown as follows: "Type a comma after $now, and enter 'dd/MM/yyyy' followed by a close parenthesis."
A block of code is set as follows:
When we wish to draw your attention to a particular part of a code block, the relevant lines or items are set in bold:
Any command-line input or output is written as follows:
New terms and important words are shown in bold. Words that you see on the screen, in menus or dialog boxes for example, appear in the text like this: "You can explore the QGIS plugin ecosystem by navigating to Plugins | Manage and Install Plugins."
Note
Warnings or important notes appear in a box like this.
Tip
Tips and tricks appear like this.
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Chapter 1. A Refreshing Look at QGIS
QGIS is a volunteer-led development project licensed under the GNU General Public License. It was started by Gary Sherman in 2002. The project was incubated with the Open Source Geospatial Foundation (OSGeo) in 2007. Version 1.0 was released in 2009. At the time of writing this book, QGIS 2.6 was the stable version and new versions are released every four months.
In this chapter we will review the basic functionality of QGIS, which will be assumed knowledge for the remaining chapters in this book. If you need a refresher on QGIS or a quick-start guide to QGIS, you should read this chapter. The topics we will cover in this chapter are as follows:
QGIS download and installation
QGIS can be installed on Windows, Mac OS X, Unix, Linux, and Android operating systems, making it a very flexible software package. Both the binary installers and source code can be downloaded from download.qgis.org. In this section, we will briefly cover how to install QGIS on Windows, Mac OS X, and Ubuntu Linux. For the most up-to-date installation instructions, refer to the QGIS website.
Installing QGIS on Windows
For Windows, there are two installation options, which are as follows:
Installing QGIS on Mac OS X
To install QGIS on Mac OS X, the Geospatial Data Abstraction Library (GDAL) framework and matplotlib Python module must be installed first, followed by the QGIS installation. The installation files for GDAL, matplotlib, and QGIS are available at http://www.kyngchaos.com/software/qgis.
Installing QGIS on Ubuntu Linux
There are two options when installing QGIS on Ubuntu: installing QGIS only, or installing QGIS as well as other FOSSGIS packages. Either of these methods requires the use of the command line, sudo rights, and the apt-get package manager.
Installing QGIS only
Depending on whether you want to install a stable release or an experimental release, you will need to add the appropriate repository to the /etc/apt/sources.list file.
With sudo access, edit /etc/apt/sources.list and add the following line to install the current stable release or current release's source code respectively:
Depending on the release version of Ubuntu you are using, you will need to specify the release name as trusty, saucy, or precise. For the latest list of QGIS releases for Ubuntu versions, visit download.qgis.org.
With the appropriate repository added, you can proceed with the QGIS installation by running the following commands:
To install the GRASS plugin (recommended), install the optional package by running this command:
Installing QGIS and other FOSSGIS Packages
The ubuntugis project installs QGIS and other FOSSGIS packages, such as GRASS on Ubuntu. To install theubuntugis package, remove the http://qgis.org/debian lines from the /etc/apt/sources.list file, and run the following commands:
Note
QGIS is also available for Android. We have not provided detailed installation instructions because it is in alpha testing at the moment. However, there are plans to have a normalized installation process in a future release. You can find more information about this at http://hub.qgis.org/projects/android-qgis.
The download page is available at http://qgis.org/downloads/android/.
A related app has recently been announced and it is named QField for QGIS. For a short time, it was named QGIS Mobile. It is described as a field data capture and management app that is compatible with QGIS. At the time of writing this, it was in invite-only alpha testing. It is eventually expected to be available in the Android Play Store. You can find more information on this app at http://www.opengis.ch/tech-blog/.
Tour of QGIS
QGIS is composed of two programs: QGIS Desktop and QGIS Browser. Desktop is used for managing, displaying, analyzing, and styling data. Browser is used to manage and preview data. This section will give you a brief tour of the graphical user interface components of both QGIS Desktop and QGIS Browser.
QGIS Desktop
The QGIS interface is divided into four interface types: menu bar, toolbars, panels, and map display. The following screenshot shows QGIS Desktop with all four interface types displayed:
The map display shows the styled data added to the QGIS project and, by default, takes up the majority of the space in QGIS Desktop. The menu bar, displayed across the top, provides access to most of QGIS Desktop's functionality. The toolbars provide quick access to QGIS Desktop functionality. The toolbars can be arranged to either float independently or dock at the top, bottom, left, or right sides of the application. The panels, such as Browser and Layers, provide a variety of functionality and can be arranged to either float independently or dock above, below, right, or left of the map display.
There are four toolbars that are particularly useful, and it is recommended that you enable them:
QGIS Desktop offers a number of customization options. You can toggle the visibility of toolbars by navigating to View | Toolbars, or by right-clicking on the menu bar or the enabled toolbar button, which will open a context menu allowing you to toggle the toolbar and panel visibility. You can assign shortcut keys to operations by navigating to Settings | Configure shortcuts. You can also change application options, such as interface language and rendering options by navigating to Settings | Options.
QGIS Browser
The QGIS Browser interface (shown in the following screenshot) is composed of three parts: toolbar, data tree view, and information panel.
The data tree view is an expandable tree listing of all geospatial data files on your computer and through connections. The information display, which takes most of the space on the application, contains four tabs that provide different views of the selected data in the data tree listing, and they are as follows:
The toolbar provides access to four functions. The Refresh function reloads the data tree view while the Manage WMS function opens the WMS management screen allowing you to manage the WMS connections. The New Shapefile function opens the new vector layer dialog allowing new shapefiles to be created. Finally, the Set layer CRS function allows you to define the coordinate reference system of the geospatial data file that is selected in the data tree view.
Loading data
One strength of QGIS is its ability to load a large number of data types. In this section, we will cover loading various types of data into QGIS Desktop.
In general, data can be loaded in four ways. The first way, which will be covered in detail in this section, is to use the Add Layer menu under Layer and select the appropriate type of data that you wish to load. The second way is to open the Browser panel, navigate to the data you wish to load, and then drag the data onto the map display or onto the Layers panel. The third way to load data is to enable the Manage Layers toolbar and click on the button representing the data type that you wish to load. The fourth way is to locate the data in QGIS Browser, drag the data, and drop it onto the QGIS Desktop map display or the Layers panel.
Loading vector data
To load vector files, click on Add Vector Layer by navigating to Layer | Add Layer. This will open the Add Vector Layer dialog that will allow us to choose the source type and source of the dataset that we wish to load.
The source type contains four options: File, Directory, Database, and Protocol. When you choose a source type, the source interface will change to display the appropriate options. Let's take a moment to discuss what type of data these four source types can load:
Loading raster data
To load raster data into QGIS, click on Add Raster Layer by navigating to Layer | Add Layer. This will open a file browser window and allow you to choose a GDAL-supported raster file. The commonly used raster types supported by GDAL are as follows:
To add an Oracle GeoRaster, click on Add Oracle GeoRaster Layer by navigating to Layer | Add Layer, then connect to an Oracle database to load the raster. More information about loading database layers is in the following section.
Note
The Geospatial Data Abstraction Library (GDAL) is a free and open source library that translates and processes vector and raster geospatial data formats. QGIS, as well as many other programs, use GDAL to handle many geospatial data processing tasks.
You may see references to OGR or GDAL/OGR as you work with QGIS and GDAL. OGR is short for OGR Simple Features Library and references the vector processing parts of GDAL. OGR is not really a standalone project, as it is part of the GDAL code now; however, for historical reasons, OGR is still used.
More information about GDAL and OGR can be found at http://gdal.org. GDAL is an Open Source Geospatial Foundation (http://osgeo.org) project.
Loading databases
QGIS supports PostGIS, SpatiaLite, MSSQL, and Oracle databases. Regardless of the type of database you wish to load, the loading sequence is very similar. Therefore, instead of covering specific examples, the general sequence will be covered.
First, click on Add Layer under Layer and then choose the database type you wish to load. This will open a window with options for adding the data stored in a database. As an example, the following screenshot shows the window that opens when you navigate to Layer | Add Layer | Add SpatiaLite Layer:
Note
Note that the window will look the same for any database you choose, except for the window name.
To load data from a database, we must first create a connection to the database. To create a new connection, click on the New button to open a connection information window. Depending on the database type you are connecting to, different connection options will be shown. Once you have created a database connection, select it from the drop-down list and click on Connect; you will see a list of all layers contained within the database display. If there are a large number of tables, you can select Search options and perform a search on the database. To load a layer, select it in the list and click on Add. If you only wish to load a portion of the layer, select the layer and then click on Set Filter to open the query builder. If you set a query and then add the layer, only the filtered features will be added.
Web services
QGIS supports the loading of OGC-compliant web services such as WMS/WMTS, WCS, and WFS. Loading a web service is similar to loading a database service. In general, you will create a new server connection, connect to the server to list the available services, and add the service to the QGIS project.
Working with coordinate reference systems
When working with spatial data, it is important that a coordinate reference system (CRS) is assigned to the data and the QGIS project. To view the CRS for the QGIS project, click on Project Properties under Project and choose the CRS tab.
It is recommended that all data added to a QGIS project be projected into the same CRS as the QGIS project. However, if this is not possible or convenient, QGIS can project layers "on the fly" to the project's CRS.
Note
If you want to quickly search for a CRS, you can enter the EPSG code to quickly filter through the CRS list. An EPSG code refers to a specific CRS stored in the EPSG Geodetic Parameter Dataset online registry that contains numerous global, regional, and local CRS. An example of a commonly used EPSG code is 4326 that refers to WGS 84. The EPSG online registry is available at http://www.epsg-registry.org/.
To enable the "on the fly" projection, perform the following steps:
To view the CRS for a layer, perform the following steps:
To project a layer to a different CRS, perform the following steps:
To create a new CRS or modify an existing CRS, perform the following steps:
Tip
Proj.4 is another Open Source Geospatial Foundation (http://osgeo.org) project used by QGIS, and it is similar to OGR and GDAL. This project is for managing coordinate systems and projections. For a detailed user manual for the Proj.4 format used to specify the CRS parameters in QGIS, download it from ftp://ftp.remotesensing.org/proj/OF90-284.pdf.
Working with tables
There are two types of tables you can work with in QGIS: attribute tables and standalone tables. Whether they are from a database or associated with a shapefile or a flat file, they are all treated the same. Standalone tables can be added by clicking on the Add Vector Layer menu by navigating to Layer | Add Layer. QGIS supports the table formats supported by OGR along with database tables. Tables are treated like any other GIS layer; they simply have no geometry. Both types of tables can be opened within Desktop by selecting the layer/table in the Layers panel, and then by either clicking on Open Attribute Table under Layer or by right-clicking on the data layer, and choosing Open Attribute Table from the context menu. They can also be previewed in Browser by choosing the Attribute tab.
The table opens in a new window that displays the number of table rows and selected records in the title bar. Below the title bar are a series of buttons that allow you to toggle between editing, managing selections, and adding and deleting columns. Most of the window is filled with the table body. The table can be sorted by clicking on the column names. An arrow will appear in the column header, indicating either an ascending or a descending sort. Rows can be selected by clicking on the row number on the left-hand side. In the lower-left corner is a Tables menu that allows you to manage what portions of the table should be displayed. You can choose Show All Features (default setting), Show Selected Features, Show Features Visible on Map (only available when you view an attribute table), Show Edited and New Features, create column filters, and advanced filters (expression). The lower-right corner has a toggle between the default table view and a forms view of the table.
Note
Attribute tables are associated with the features of a GIS layer. Typically, one record in the attribute table corresponds to one feature in the GIS layer. The exception to this is multipart features, which have multiple geometries linked to a single record in the attribute table. Standalone tables are not associated with GIS data layers. However, they may have data of a spatial nature from which a spatial data layer can be generated (for more information, see Chapter 6, Advanced Data Creation and Editing. They may also contain data that you wish to join to an existing attribute table with a table join.
Table joins
Let's say that you need to make a map of the total population by county. However, the counties' GIS layer does not have population as an attribute. Instead, this data is contained in an Excel spreadsheet. It is possible to join additional tabular data to an existing attribute table.
