These four volumes present innovative thematic applications implemented using the open source software QGIS. These are applications that use remote sensing over continental surfaces. The volumes detail applications of remote sensing over continental surfaces, with a first one discussing applications for agriculture. A second one presents applications for forest, a third presents applications for the continental hydrology, and finally the last volume details applications for environment and risk issues.
Introduction to QGIS
QGIS (formerly known as Quantum GIS) is a free, open-source, cross-platform and scalable GIS tool with plugin development in Python and C++ languages. This is one of the official projects of the Open Source Geospatial Foundation (OSGeo, www.osgeo.org), whose mission is to help and promote the collaborative development of open-source geomatics software.
Today QGIS is a geographic information processing software suite that is popular with many users (www.qgis.org). Friendly and ergonomic, it makes it possible to collect, store, process, analyze, manage and present all types of spatial and geographic data similarly to the most highly priced software.
Initiated by Gary Sherman in 2002, QGIS was originally intended to be a data visualization tool for Geographic Resources Analysis Support System (GRASS) and PostGIS (Geographic Database Management System). It became a project of the OSGeo Foundation in 2007. The first major version 1.0 was released in 2009, version 2.0 in 2013 and version 3.0 is expected by the end of 2017. Supported by a community of developers structured into project committees, QGIS maintains a steady pace of integration of new features, resulting in a new minor release every 4 months (the most recent version is 2.18, version 3.0 is expected for 8 December 2017). In addition, the constant enrichment of the plugin library allows users to continuously benefit from new functionalities for customized uses and treatments.
The development of QGIS has been made with the contributions of a community of developers, translators and anomaly reporters who generally work on a voluntary basis but in some cases are professionals of an institution wishing to contribute to the project. A project steering committee ensures cohesion, the orientations of the project and the federation of the community.
Development is assured by a multitude of contributors, each competent in their field, forming a global QGIS community. There were 112 in the world in May 2016 at the time of the 15th international meeting of developers of QGIS in Girona. The management of contributions, exchanges with users and the diffusion of up-to-date news on the project can take place thanks to QGIS community spaces: websites, wikis, forums, mailing lists and the blog portal of the Frenchspeaking user group: http://osgeo.asso.fr/content/project/qgis-user-fr.
Technically, QGIS integrates the Geospatial Data Abstraction Library (GDAL), which allows it to read and process a large number of geographic images (free and proprietary). QGIS also supports a variety of vector data formats (PostgreSQL-PostGIS, Shapefiles, GPX, GeoJSON, SQLite, KML, MapInfo, Autocad DXF, ESRI Personal Geodatabase, Oracle Spatial, Erdas, ENVI, MBTiles, etc.). Other processing libraries such as Sextante are also integrated into QGIS.
Distributed under the GNU/GPL (General Public License) version 2, QGIS allows free access to a powerful, inexpensive GIS program that can be used on most platforms: GNU/Linux, Unix, Mac OS X and Windows.
1.2. QGIS graphical user interface
The QGIS graphical user interface (GUI) is presented below with, as illustrative data, the ROUTE 120® database available in open licensing on the site of the French National Geographic Institute (IGN): http://professionnels.ign.fr/route120.
1.2.1. Standard interface
As a standard, the interface presents (Figure 1.1):
- - the Map View;
- - the Browser, Layers and Overview Panel in the left panel;
- - toolbars containing tools grouped by functionality around the map;
- - the Status Bar with cursor coordinates, the coordinate system, scale, etc., at the bottom of the screen.
Figure 1.1. Standard QGIS GUI (v2.16) with ROUTE 120® database
The pre-setup of QGIS is done using the basic options of the software accessible via the Settings menu> Options (Figure 1.2).
The various tabs allow you to configure:
- - "General": the default options;
- - "CRS": the definition of the coordinate system;
- - "Networks": network and proxy settings for Internet access.
Figure 1.2. Settings: "Options" menu
1.2.3. Add layer
Adding layers is done using the "Manage Layers" toolbar buttons depending on the layer type (Figure 1.3).
Figure 1.3. "Manage Layers" toolbar
1.2.4. QGIS project
The QGIS working environment is saved in a project materialized by a file with the extension .qgs.
The information contained are as follows:
- - open layers with links to access them;
- - thematic analyses;
- - the requests made;
- - the semiology (style) of the layers;
- - the projection used;
- - zoom;
- - completed layouts (maps);
- - etc.
Since data are not contained in the project (Figure 1.4), care must be taken to preserve the access paths to the project when moving or transferring files.
Figure 1.4. Toolbar "project"
The "Map Navigation" toolbar allows you to move and adjust the zoom of the screen (Figure 1.5).
Figure 1.5. "Map Navigation" toolbar
1.2.6. Attributes of entities
The "Attributes" toolbar allows you to select the entities, access and query the attribute data (Figure 1.6).
Figure 1.6. "Attributes" toolbar
1.3. The processing module, the toolkit for spatial analysis
1.3.1. History and interest of the treatment module
Since the earliest versions, QGIS software has provided spatial processing and analysis functions. But until version 1.8, these functions were only accessible via the drop-down menus and buttons of the main interface of the software. This only allowed the functions to be used in a graphic mode. It was therefore impossible to repeat one or more operations without laboriously reworking them with the interface.
In addition, since the QGIS software was designed in a modular architecture, many functions were available via additional extensions that users install according to their own needs. The disadvantage was that the various processing functions were dispersed in the menus of the different extensions. For the user, it was not always very intuitive in the interface to find the various useful tools, a chain of treatments requiring the use of very different menus.
These reasons prompted QGIS developers to offer a toolkit that allows the processing functions of the software and its extensions to be integrated into a common interface. It was at the time of the release of version 2.0 of QGIS in September 2013 that this Processing Toolbox appeared. This is actually the integration and enhancement of a toolkit called Sextante that was previously available as an extension.
From the start, the developers of this module conceived it as the most open and the most configurable possible. Not only did it have to centralize the processing functions proposed by QGIS and its extensions, it also offered functions available in third-party tools. Indeed, other free software such as Orfeo Toolbox (OTB), SAGA GIS or GRASS offer spatial processing functions. It is much more interesting to propose a way of integrating them into QGIS rather than redeveloping identical functions in QGIS. The interest is twofold: to add new functions to QGIS at lower cost and to offer graphical interfaces to the user in order to use different external software, which is most often only accessible in command line.
On the other hand, this module was also designed to facilitate the automation of tasks in QGIS. It makes it possible to prepare a task that can be composed of one or more processing functions and to replay it very easily with a different dataset without having to further manipulate the interface of the QGIS GUI.
During the various updates of the QGIS version, the processing module was constantly improved by bug fixes and the addition of new functions. In the current version 2.18 of QGIS (February 2017), it proposes the following external libraries by default: OTB, GRASS, GDAL/OGR, R, SAGA, TauDEM, LAStools (LIDAR tools).
1.3.2. Presentation of the toolbox and its algorithms
The processing module is proposed by default with the QGIS software and does not require the installation of an extension in order to be used (Figure 1.7). To use it, the QGIS GUI offers a "Processing" menu. If this menu does not appear, the module is not activated in your QGIS, which should be the case with a new clean installation of QGIS. To activate the module, you must go to the QGIS extensions management menu: "Plugins> Manage and Install plugins". In the search field, type "processing" and then click the checkbox to the left of the "Processing" entry in the list. The "Processing" menu is now available in the main...