## 4 The AGO Domain Model  
AGO aims to cover the main elements required to support semantic labeling and the description of automotive scenarios for testing environments. Hence, the core concepts defined in the ontology correspond to those used to structure the information in VCD and OpenLABEL. Its high-level structure can be seen in Figure 1. The main superclasses of the ontology are _Object_, _Context_, _Action_, and _Event_. These elements form the first level of classes and all other classes are derived from them. Furthermore, in VCD and OpenLABEL, the _Relation_ element is required to structure the domain knowledge and semantically enrich the ontology. The RDF language model [10] defines several axioms for describing properties and relationships among named terms: \"rdfs: domain\", \"rdfs: range\", \"rdfs: subClassOf\". Additionally, some non-standard relations are proposed in AGO for the purpose of covering the needs of the description of automotive scenarios. * \"isSubjectOfAction\" and \"isObjectOfAction\": these axioms are defined for actions expressed with _transitive verbs_. Transitive actions can be naturally treated as triples from a language perspective. Nevertheless, transitive triples do not allow relating the action with other nodes of the ontology (because the action in a transitive triple is the predicate or relation between subject and object, and thus it is not a class). Therefore, transitive actions are unwrapped as two related RDF triples, where the action is a class and the relations are \"isSubjectOfAction\" and \"isObjectOfAction\". In the case of the _intransitive actions_, a unique triple is generated relating the element that performs the action and the Action itself. * \"isSubjectOfEvent\" and \"isObjectOfEvent\": as for the Actions, the Events can be also distinguished as either transitive or intransitive. Therefore, the same type of relations is defined in AGO for these elements. One describes _\"who\"_ performs the Event and the other \"_who/what_\" is affected by it. * \"causes\": Events are occurrences that happen in a time instant and usually trigger another Event or Action. Therefore, this axiom is adopted to relate Events with Actions and represent this effect. To provide the ontology with the capability of representing spatio-temporal information by relating the different classes, two additions were performed. First, [PERSON]'s temporal relations [21] were adopted (e.g., \"meets\" as a relation used to define the timeline of the scenario by relating Action and Events in temporal order). Second, for defining the spatial relations among the elements (e.g., required to construct a complete description of the road network):  
* \"isPartOf\": describes a spatial relation between the different Objects (i.e., \"Lane\"--\"isPartOf\" -> \"Road\")
* \"isConnectedTo\": this relation is defined to relate all the spatial objects describing the network of objects. Concatenating these static objects (i.e., \"Road\"--\"isConnectedTo\" -> \"Intersection\") is required to formally represent the road network in OpenDRIVE ([[https://www.asam.net/standards/detail/opendrive/](https://www.asam.net/standards/detail/opendrive/)]([https://www.asam.net/standards/detail/opendrive/](https://www.asam.net/standards/detail/opendrive/)) accessed on 13 May 2021) format. Furthermore, spatial relations among objects in the scenario can be further specified by \"behindOf\", \"inFrontOf\", \"leftOf\", \"rightOf\", and \"middleOf\". Figure 1: Schema of the domain model of AGO showing core concepts and main relationship types. For the representation of the knowledge, in the OWL Reference documentation, the W3C states that an OWL ontology is an RDF graph, which is in turn a set of RDF triples [11] (_subject-predicate -> object_). Therefore, some notations from the data-modeling vocabulary defined by the RDF Schema and the OWL language principles were adopted for the purpose of this work [10]:  
* Classes are identified by an Internationalized Resource Identifier (IRI).