# DTAG: A Methodology for Aggregating Digital Twins Using the WOTDT Ontology
## 4 DTw Aggregation Methodology  
The DTw aggregation (DTAG) methodology is a lightweight methodology for developing aggregation between DTws to obtain an aggregated DTw (DTwA). The DTws involved in aggregation can be DTwI or DTwA, with the aggregation always producing a DTwA. In addition, this DTwA can also be used for future aggregation processes. This methodology follows WoTDT ontology for aggregating the different DTws according to the dimensions conceptualised in the ontology. Therefore, the various tasks involved in the aggregation process will be distinguished according to the five dimensions [8] conceptualised in the ontology, as explained in Section 3.  
The DTAG methodology defines interactions on a basic workflow composed of the following activities, also represented in Figure 5:  
1. **DTws instantiation**, where the DTws involved in aggregation are instantiated using WoTDT ontology.
2. **DTws dimension aggregation development**, where the information contained in the different dimensions of DTw involved are aggregated.
3. **DTwA maintenance**, where the DTwA obtained from the aggregation process is updated according to the functionalities that will be performed.  
Although numerous roles may participate in the aggregation of DTws, the DTAG methodology categorises them into the following groups:  
* **DTw developer**: a DTw developer is part of the DTw development team and possesses extensive knowledge regarding the DTws used in the aggregation process.
* **Domain expert**: a domain expert possesses expertise in the fields addressed by the DTws used in the aggregation process.
* **DTwA user**: a DTwA user is the end user of the DTwA obtained from the aggregation process. This actor will also include DTw developers who will use and extend the DTwA knowledge and functions.  
The subsequent sections provide comprehensive information on each activity, encompassing detailed tasks and workflows, intermediate results, and the tools used for support. In addition, for every activity performed, an example taken from the COGITO project will be explained; therefore, they will be limited to the constraints imposed on the project.  
Furthermore, examples of aggregation involve two DTws. The first DTw refers to a construction pilot of a railway station in Munich (Germany), while the second DTw refers to a construction pilot of a railway station in Murcia (Spain). The desired result of the aggregation of these two DTws is to obtain a DTwA that defines a DTw of the entire COGITO project.  
It is worth noting that this article focuses on the investigation of the description of an ontology that allows us to describe the metadata of a DTw in its totality and offers a functionality for the possible aggregation between DTws. In turn, the described aggregation methodology focuses on offering solutions to the possible types of aggregation between the existing components in the DTws. Therefore, in each of the activities of the methodology, tools or frameworks existing in the literature are proposed to allow the aggregation or integration of the different components of DTws.  
Moreover, the aggregation methodology focuses, as the ontology, on aggregation by dimensions of the five-dimensional model. For this reason, unlike the aggregations presented in Section 2, this methodology allows the aggregation of metadata and data (semantic or non-semantic) belonging to different DTws within a DTwE, obtaining a single DTwA containing all the centralised metadata thanks to the WoTDT ontology, and all the data belonging to the DTwIs/DTwAs involved in the aggregation process.