# DTAG: A Methodology for Aggregating Digital Twins Using the WOTDT Ontology
## 4 DTw Aggregation Methodology
### DTws Instantiation
#### 4.1.1 DTw Metadata Extraction by Dimension  
The goal of DTw metadata extraction by dimension activity is to obtain all the metadata information of the DTw to instantiate and centralise it in a single place. This activity involves DTws developers and domain experts of DTws involved in the aggregation process. The output of the activity is a table that contains the metadata information that refers to the different dimensions of the DTws, which describes all the functionalities and accesses of DTws. The described table should contain the following columns:  
* **DTw**: DTw where the metadata are extracted.
* **DTw Identifier**: identifier that represents the DTw.
* **DTw Title**: title that describes the DTw specifically.
* **Entity**: entity or asset that is represented by the DTw.
* **Entity Identifier**: identifier of the entity or asset represented by the DTw.
* **Entity Title**: title that specifically describes the entity or asset that is represented by the DTw.
* **Models**: models used by the DTw to model its data.
* **Model Type**: type of model described as subclasses in the model class of WoTDT ontology.
* **Model Identifier**: identifier of the model.
* **Model Title**: title that describes specifically the model.
* **Model Description*
* (_optional_): description that describes the model.
* **Model Formats**: formats in which the model can be found.
* **Model Access URL**: URL where the model can be accessed.
* **Resources**: data resources used by the DTw.
* **Resource Type**: type of data resource described as subclasses in the resource class of the reused DCAT ontology in the WoTDT ontology.
* **Resource Identifier**: identifier of the resource.
* **Resource Title**: title that describes specifically the resource.
* **Resource Description*
* (_optional_): description that describes the resource.
* **Resource Access URL**: URL where the resource can be accessed.
* **Resource Download URL*
* (_optional_): URL where the resource can be retrieved.
* **Related Resources Identifier*
* (_optional_): relations with other data resources, such as distributions or data services where a dataset is stored.
* **Services**: services used by the DTw to execute processes.
* **Service Type**: if it is a property, action, or event affordance from the thing description ontology.  
Figure 6: Workflow proposed for the DTw instantiation activity.  
* **Service Identifier**: identifier of the service.
* **Service Title**: title that describes the service specifically.
* **Service Description*
* (_optional_): description that describes the service.
* **Service Access URL**: URL where the service can be accessed.
* **Service Content Type*
* (_optional_): content type that returns the service, such as \"application/json\".
* **Connections**: existing connections between the different dimensions of the DTw.
* **Connection Type**: type of connection described as subclasses in the connection class of WoTDT ontology.
* **Connection Identifier**: identifier of the connection.
* **Connection Title**: title that describes specifically the connection.
* **Connection Description*
* (_optional_): description that describes the connection.
* **Connection Provider Identifier**: identifier of the connection provider.
* **Connection Consumer Identifier**: identifier of the connection consumer.  
An example of the extraction of the DTw metadata from the COGITO Munich pilot is shown in Tables 2-4.