## 7 Discussion and Challenges
### Future Usage: DT Interaction and Enhanced Maintenance Operations
#### 7.3.2 Data Management  
Other challenges that have been observed in the literature concern the definition of DT and data management. As observed by [PERSON] et al., the definition of DT varies between different authors [21]. [PERSON] et al., to name but a few, consider BIM as being a DT, even without the automatic exchange of data [32; 37; 38]. It has also been observed that the problem of DT maintenance is often overlooked. [PERSON] et al. proposed a methodology to perform a continuous update of the DT [74], but their methodology assumed that all the needed data exist. Some authors have already observed that, even at its creation, there is a clear lack of necessary data to create the DT [49; 61]. Therefore, the data needed to update it can be missing or could be impossible to retrieve.  
One of the first causes of difficulty in the retrieval of data is the different proprietary formats that exist. Each IoT system may have its own proprietary format, which can complicate exchanges between them [35]. Multiple authors have observed that, for better interoperability, a common format needs to be defined for data exchange between the different stakeholders [28; 33; 35]. The IFC format has already been defined as one of the main formats to structure a BIM, but it is still needed to adapt the data coming from smart systems and smart sensors. Some authors, such as [PERSON] et al., have proposed an open-source methodology based on IFC format to improve the IoT integration, even with incomplete data, into a BIM [34]. Their method used predefined common metadata attributes to add data from heterogeneous sources homogeneously through various IFC objects. Such data standardisation may also be necessary to perform inspection feedback in an efficient way. Using DT and XR technologies, the on-site operator may provide advanced inspection feedback, through both textual and vocal information, but also with real-time data provided by IoT devices. For example, [PERSON] et al. proposed a methodology to add inspection data directly to the 3D model of equipment using gaze-tracking and image recognition, which can facilitate future retrieval of information [66]. However, there is still a need to define a methodology to help feed back professional knowledge specific to maintenance operations within the DT, such as maintenance procedures or personal observations [66; 95].  
Another challenge that has been observed in the literature is that there is no clear approach concerning data and DT ownership. Multiple authors have asked and studied this question in recent years [26; 31; 37; 51; 54; 92]. This question is particularly important as it defines who is supposed to oversee the implementation, the maintenance and the update of the DT. Throughout the building lifecycle, several different companies may be involved, from its design, through its exploitation, to its disposal or renovation. Thus, there is a question of who owns the data collected during the building's lifecycle. Furthermore, as a BIM-based DT is supposed to allow access to all of the building information, the question of its owner can also be asked [26]. It is also necessary to define which data can be shared and which cannot, as some companies can collect sensitive data [31; 47; 54]. This point brings up the issue concerning the data security, especially during the exchange of data between the systems and its visualisation by the users [31; 35; 46; 54; 72]. It may be interesting to develop an encryption protocol for communications between a DT's different components, for example. Some authors have also observed that the use of blockchain technology, initially created for Bitcoin, is developing in the context of Industry 4.0 and could address the issue of privacy and security of data exchange with a DT. The ownership of the data and the communication protocols must therefore be defined early enough to allow a clear and smooth development of the DT but also to allow the overall cost of its creation to be defined [5; 6; 53; 96].