I. Introduction
With the om-nipresence of smart devices, research in the field of Internet-of-Things (IoT) and in its areas of application, such as intelligent transportation [1], smart cities [2] or smart agriculture [3], has become a massive boost in industry and academic. IoT applications are characterized by massive heterogeneity of “things” [4] [5], ranging from low-level devices, such as actuators and sensors [6], via network communication gateways to high-level applications and services [7]. Thus, interoperability on various levels (e.g., device-, network-, syntactical-, semantic, and platform interoperability) is a key challenge in IoT research [4]. In order to achieve adequate interoperability levels, semantic technologies in terms of ontologies are a promising paradigm strongly focused in state-of-the-art IoT research [7]. Although a plethora of ontologies for representing IoT systems exist, “no existing ontology is comprehensive enough to document all the concepts required for semantically annotating an end-to-end IoT application” [8]. Hence, many proposed ontologies are a composition of other existing ones designed for application-specific IoT systems (e.g., smart home, transportation and logistic, or e-health applications). Several survey publications discuss and compare a vast number of already existing IoT ontologies, e.g., [8] [9]. It is, however, nearly impossible to get an overall and up-to-date view of the IoT ontology landscape, since individual survey papers pursue partly different goals, thus, the rationale behind their selection criteria for discussed ontologies as well as their comparison dimensions used, are different. Hence, ontology engineers aiming at reusing existing ontology concepts for their particular areas of application, easily get lost in the ontology jungle, due to the lack of an overall and up-to-date view of the IoT ontology landscape.