## 2 Materials and Methods
### The Ontological Model and Its Implementation
The schema shown in Figure 1 represents the integration structure of top-level, domain/task and application ontologies used for the design and implementation of the Greyware research system. The Horai UT/Ac model is considered here as a top-level ontology, describing very general concepts but useful for a large community of users [33]. The archaeological method--mostly built after [PERSON]'s [41] and [PERSON]'s [42] proposals--describes the vocabulary related to the archaeological domain, while specific and precise approaches to material analysis or field record management are developed in accordance with data models considered here as application ontologies [33]. Accordingly, the Greyware information system, as will be described in the results section, establishes different degrees of commitment depending on the nature and origin of the sources analyzed and the sphere of interest addressed by the research project. Thus, databases designed for archaeological pottery analysis are object-oriented and detailed and do not permit wide interoperability at this level, but significant clusters representing specific productions or distribution areas are considered as Units of Topography. Taking into account that data gathered from written and ethnographic sources are recorded as Units of Topography as well, both kind of data are accessible from the same upper level of the ontology, which allows them to be compared spatiotemporally. Data gathering from archival or published references might commit to a detailed application ontology, which is far from Greyware's sphere of interest. When datasets gathered from these sources are integrated within Greyware, they commit directly to the top-level ontology, but the information included in those databases is still interoperable and reusable by experts in this domain, who might adopt the Horai UT/Ac model to interoperate with their own specific datasets, as far as they follow the main categories. These categories and their relations within the top-level ontology are highlighted in the UML diagram for the Horai UT/Ac model represented in Figure 2.  
Figure 1: Graphic representation of the Greyware ontological commitment to the Horai UT/Ac model as a top-level ontology. As shown in Figure 2, ontological differences between UT and US have to be considered, even though these never compromise unit interoperability. Both of them are evidence of the recent or remote past, for which a spatiotemporal context has to be provided more precisely or generically. Place and time are essential ontological attributes in all cases, regardless of the origin of the source--vestige or reflection, its materiality, the scientific discipline that produces it, the methodological specificities of data gathering or the reliability of the information [43] (pp. 65-75). The Horai UT/Ac data model has proved both its extendibility and commitment to upper data modelling and ontologies. Understanding Units of Stratigraphy as a specific class of Units of Topography, and considering the chance of grouping several US in one single UT explanatory of a specific process, the model is extendible to ontologies in the archaeological domain. In this sense, other experiences have evidenced the model's adjustment to specific archaeology-related domains such as restoration [44]. Similarly, defining Units of Topography as classes of Entities and Events makes the ontology commit to archival-science-based proposals [35; 36] or to the CIDOC CRM ontology [11]. To that extent, the Unit of Topography category could be considered a class of either Entities or Events as defined by the CIDOC CRM classes (E1 and E5 respectively), while Actors would be a clear equivalent of class E39 [11] (p. 35).