Main technical features and functionalities
QGIS is a professional software to build Geographic Information Systems (GIS), which are computer systems for capturing, storing, checking, editing, analyzing, composing, and displaying geo-referred data.
Data can be organized using raster or vector formats. Vector features use geometry (points, polylines, and polygons) to represent the real world, link to tables of attributes. Raster data takes a different approach. Rasters comprise a matrix of pixels (also called cells), each containing values (a value every band) representing specific conditions for the area covered by that cell.
The tables of vector features can also be processed using the R software.
Qgis has core functions and plugins able to organize analyses and decisions support systems in various disciplines.
QGIS is free and Open Source for Windows, Mac, Linux, BSD, and Android. QGIS is updating continuously, so it is possible to download the latest versions (or the previous ones) at: http://www.qgis.org/en/site/forusers/download.html<br /> Every version has an online manual and tutorials available online. The latest manual is: https://docs.qgis.org/3.10/en/docs/user_manual/<br />

Examples on how to use them to analyse Complex Systems
The software QGIS and its plugins are helpful to perform network analysis, statistical assessments, geoprocessing, morphologic analysis, and simulations.
Georeferred information is grouped by thematic layers that can be processed separately or integrated, evaluating relationships and linkages between internal and external variables and building dedicated queries. Furthermore, the data can be analyzed at different spatial and temporal resolutions, contributing to outlining the emergent proprieties.
The QGIS community publishes QGIS case studies (user stories) on the QGIS website.
The case studies are available at: http://www.qgis.org/en/site/about/case_studies/index.html<br /> For instance, the following are references regarding two papers using QGIS to develop response methodologies to very different queries:
- Stella et al. 2019. Towards a National Food Sovereignty Plan: Application of a new Decision Support System for food planning and governance. Land Use Policy 89: https://doi.org/10.1016/j.landusepol.2019.104216
In a GIS environment, the paper develops an index to simulate the level of food autonomy reachable for a territory, varying the diets of its community, and considering its suitability to different agricultural uses. The index considers the complex relationship between a place's physical and climatic characteristics, its inhabitants, the current agricultural land uses, and suitability to different crops.
- Pelorosso et al., 2017. PANDORA 3.0 plugin: A new biodiversity-ecosystem service assessment tool for urban green infrastructure connectivity planning. Ecosystem Services 26(B): 476-482. https://doi.org/10.1016/j.ecoser.2017.05.016<br /> - Pelorosso et al. 2012. Landscape development scenario analysis by Pandora model: an application in Umbria region (Italy). In "International Conference of Agricultural Engineering CIGR-AGENG 2012" 8-12 July 2012. Valencia, Spain ISBN: 9788461599288
The PANDORA model, developed as a QGIS plugin, aims for an integrated evaluation of ecosystem services in urban contexts. The plugin aims to be a versatile and innovative tool for assessing green infrastructure value in terms of ecological connectivity and biodiversity, valid for planning sustainable and resilient landscapes and cities.