## 2 Materials and Methods
### The Global Human Settlement Layer Principles  
The GHSL concept was introduced by the European Commission (EC), Joint Research Centre (JRC) during the years 2008-2011, in the frame of the program named \"_Information Support for Effective and Rapid External Action_\", developing new image information mining technologies that are in support of geo-spatial information analysis for global security and stability [20; 21]. At that time, the application areas setting the requirements for the GHSL were framed inside the post-natural-disaster and post-conflict damage, needs and reconstruction assessment, including refugee camps and temporary, rapidly-changing human settlement monitoring [21; 22; 23]. In this frame, the notion of \"built-up area\" was introduced after a critical revision of the available satellite-derived land-use/land-cover information categories. [24; 25]. The data and the semantic requirements of these initial application areas strongly influenced the design of the GHSL information production system. In particular, they were influencing the design of the automatic satellite image information mining methods that were outlined under a set of pragmatic principles, as summarized below:  
* robustness against real-world Big Earth Data scenarios [26; 27; 28] involving large-volume, largely heterogeneous/unstructured data sources, and rapidly changing data specifications [29],
* enhanced semantic interoperability and robustness against multi-stakeholder international information decision support scenarios, and
* effectiveness in time-critical image-derived analytics requirements that are set by crisis management applications. Early GHSL proof-of-concept was provided using European medium resolution ENVISAT ASAR data--_Mapping Human Settlements Globally_, European Union Science Hub News, 21/3/2011, an extensive global sample of image data that were collected from optical sensors ranging from 0.5 to 10 meters of spatial resolution [25], and a seamless pan-European image data layer at 2.5 m of spatial resolution in support to the European Cohesion policy [30]. During 2014, the process of applying the GHSL concept in support to post-2015 international frameworks was initialized. They included: the UN Third Conference on Housing and Sustainable Urban Development (Habitat III, 2016), the 2030 Agenda on sustainable development goals (SDGs), the UN Framework Convention on Climate Change, and the Sendai Framework for Disaster Risk Reduction (DRR) 2015-2030. The setting of this new area of application introduced three new general requirements in the GHSL concept, aligned with the principles that were published in 2014 by the United Nations Secretary-General's Independent Expert Advisory Group on a Data Revolution for Sustainable Development (IEAG) [7]:  
* operates in an open and free data and methods access policy (open input, open method, open output),
* facilitates reproducible, scientifically defendable, fine-scale, synoptic, complete, planetary-size, and cost-effective information production, and
* facilitates information sharing and multilateral democratization of the information production and collective knowledge building. The first and second requirements are linked to the need of public control of the data-driven policy decisions, and they are aligned to the Data Sharing Principles of the inter-ministerial Group of Earth Observation (GEO) and the Global Earth Observation System of Systems (GEOSS) [31]. The second and third requirements are necessary to process fine-scale global data, lowering the information production cost and thus enlarging the floor of the potential information producers, and consequently improving the inclusiveness and multi-laterality of the global information production ecosystem. Nowadays, information is abundant, redundant, and also (in general) largely contradictory--typically not being harmonized. It is rapidly changing, heterogeneous, and only partially structured, because multilateral actors with a multiplicity of different objectives create it.