## 4 Experimental Section: An Ontology of the SEA Procedure of City Masterplans  
Within the context of spatial planning, ontologies have generally been proposed as a means to share and reuse existing information and data [20], to homogenize data and solve semantic conflicts [21], to support the modeling of spatial datasets [22], and therefore as a possible solution to the issue of making interoperability and integration possible in spite of the proliferation of data and data sets built in the absence of common standards [23]. One of the most challenging and promising field of research concerning the use of ontologies in spatial planning deals with allowing for a better understanding and awareness of programming and planning processes [24, 25]. Ontologies, in this context, do not deal with \"the specification of what exists and what does not exist, but rather with the creation of a data set that contains concepts related to the domain under inquiry\" [25]; in other words, they tackle the problem of describing a given domain of interest by identifying key concepts that define the domain, relations that connect the concepts, and existing constraints, thus making formalization and knowledge sharing within the given domain possible. There are many definitions of the word \"ontology\" in the literature regarding the field of artificial intelligence [26]. As [PERSON] and [PERSON] [27] point out, there is no single definition, and conceptual definitions \"that regard ontology as a reference system for knowledge\" coexist with \"others, more operational, which lay the grounds for their actual construction, development and use.\" One of the most used and most frequently cited definitions is [PERSON]'s [28], for whom an ontology is an \"explicit specification of a conceptualization\": this conceptualization, or in other words the construction of an abstract and simplified conceptual model of a given object, or phenomenon, or process represented by the ontology, is explicit because each concept, relationship and constraint is explicitly defined. The subsequent definition by [PERSON] _et al._[29] (\"formal, explicit specification of a shared conceptualization\"), enriches [PERSON]'s with two additional requirements: first, an ontology should be formal, that is machine-readable; in addition, the conceptual model of the object being represented needs to be agreed by a group of individuals [30], and therefore consensus of members of a given community is necessary. A significant experience, as far as ontologies for urban planning are concerned, is that of the Towntology project [31], which, among other things, aimed at developing an urban civil engineering ontology and at exploring whether ontologies can improve communication between stakeholders in the field of urban civil engineering. However, the formalization of environmental assessment procedures (either Environmental Impact Assessment or SEA or Appropriate Assessment under the Habitats directive) was not within the scope of the project. The ontology of the domain \"SEA of city Masterplans\" was developed according to the phases suggested by guidance documents and methodological reports produced by the Ordnance Survey [32, 33, 34], according to which the process whereby an ontology is built can be broken down into a series of steps, the first being the identification of the purpose of the ontology (_i.e._, the specification of needs and requirements that the ontology should be able to fulfill) and of its scope (_i.e._, a delimitation of the domain under investigation). These two aspects are crucial for ensuring both that the ontology is correctly formalized and that it is useful, meaning that it contains only those concepts, relationships and constraints that are judged to be relevant, with regard to the possible ways in which the ontology can be used.