## 2 Materials and Methods
### The Ontological Model and Its Implementation
* They have to be _coherent_ by sanctioning inferences consistent with the definitions; coherence should apply even to those concepts described informally in natural-language documentation and examples. * They have to be _extendible_, able to anticipate the uses of the shared vocabulary and able to define new terms based on the existing vocabulary. * They have to show the _minimal encoding bias_, avoiding representation choices purely made for the sake of implementation. * They should only require the _minimal ontological commitment_ by specifying the weakest theory and defining only the essential terms. Elsewhere, we have outlined [18] the importance of defining interdisciplinary categories for digital historical research and the role of the archaeological method in the normalization of those categories. The Harris Matrix [41] method is the most well-known exercise of normalization and modelling in social and human sciences, and it is the technique most commonly used in archaeology to document and explore stratigraphic formations [26]. Bearing this in mind, the archaeological method provides some underpinning concepts that open a brand new world for data management and exchange when extended and adapted to non-material vestiges. The UT/Ac model, now under development within the framework of the Horai[12] technological platform, has proved to be effective for the integrated exploitation and management of historical and archaeological information, building upon the concepts of Unit of Topography, Unit of Stratigraphy and Actor, defined as follows:  
* **Unit of Topography (UT)**: It is the evidence of an action or situation that can be located in space and time, regardless of the specificity of the information source and its biotic, non-biotic or anthropic attributes. Each UT has a specific location and date. Location can be expressed as a UTM coordinate or as an administrative delimitation that might have changed through time. * **Unit of Stratigraphy (US)**: It is the material evidence of an action that occurred in the past, representing an archaeological aspect of the cycle of time. They are of universal character and can be found on any archaeological site in the world [41] (p. 42). As a reflection of materiality, graphic and cartographic representations are essential attributes of these units. * **Actor (Ac)**: It is the individual or corporative, active or passive, protagonist of an action identified as a UT. If being an individual, its attributes are their name, gender, religion, citizenship, date of birth and death, etc. Different individual actors gathered for a given period of time with a particular purpose and under determinate conditions can act as corporative actors. As such, a Unit of Topography can be the definition of a site itself, the existence of a pottery (even though we might not know it from the archaeological record), a technological practice, a clay source, a pottery fabric informing about the provenance of vessels, etc. A Unit of Stratigraphy records the archaeological material evidence of pottery production: the structure of a kil, a layer of clay or the interface marks of the wheel location within a site, among many others. An Actor can be a potter, a customer, a non-specialized worker in a workshop, a potter's wife or children collaborating or not in the potting tasks, etc. Further details on the relations and attributes of these categories have been published in previous literature [17; 18; 19; 20; 21], together with the interesting dialectics between UT and US [16] (pp. 13-14). The most remarkable feature of the UT is that its identification is not restricted to the archaeological domain, where the Unit of Stratigraphy would mainly apply. We adopted this model as top-level ontology. The quantity of ontological knowledge available is modest, but its quality enables the integration of material analysis databases and ensures the interoperability of datasets containing archaeological and historical information.