## 5 DT Improvements Brought about by eXtended Reality Technologies
### Management and Data Visualisation  
One of the first characteristics of these devices is their ability to overlay information to the user in front of his or her surrounding environment. These visualised data can be called _augmentations_, because they provide deeper information to the operator concerning the performed operation. While benefits have already been observed in manufacturing processes, observations in the construction industry are still limited (see Section 7.1). Through the use of BIM data concerning the equipment and its position, an augmentation \begin{table}
\begin{tabular}{c c c} \hline \hline \multicolumn{1}{c}{**Functionalities**} & \multicolumn{1}{c}{**Features**} & \multicolumn{1}{c}{**Reference**} \\ \cline{3-4} and & Real-time and historical display & [26; 43; 53; 55; 64; 67; 68; 70; 72; 73; 77; 78] \\ \hline \multirow{3}{*}{Management and data visualisation} & Situated display & [25; 72] \\  & Asset highlight & [25; 43; 68; 72; 77] \\  & 3D visualisation & [43; 53; 67; 70; 72; 75; 76; 79] \\ \hline \multirow{3}{*}{Interaction with the model} & Gesture and voice recognition & [25; 26; 53; 73; 80] \\  & Standard inputs & [25; 68; 70; 76; 77; 78] \\  & Position tracking & [25; 75; 76] \\  & Head/gaze tracking & [25; 66; 71; 72; 73] \\  & MR control & [26; 55; 80] \\ \hline \multirow{3}{*}{Main devices used for visualisation} & See-through HMD/ Smart-glasses (e.g., & [25; 26; 43; 76; 67; 70; 73; 75; 78; 80] \\  & Holdeus) (used for AR and MR) & [25; 26; 43; 76; 67; 70; 73; 75; 78; 80] \\ \cline{1-1}  & Occluded HMD (e.g., HTC Vive) (used for VR) & [25; 47; 75; 78; 79] \\ \cline{1-1}  & HHD (e.g., Tablet; Smartphone) (used for AR) & [22; 47; 73; 55; 68; 69; 75; 77; 81] \\ \hline \multirow{3}{*}{Model update and collaboration} & Update digital twin & [66; 71; 81] \\  & Collaboration & [22; 25; 65; 78] \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Categorical review of the improvements brought by XR. Figure 4: Double Helix model [74]. can be displayed near or over its related equipment [25; 72], helping the operator to better understand their link. The data displayed can either be static or collected in real-time from the system (e.g., technical documentation of an equipment, time series of data collected by smart sensors, aand instructions for maintenance procedures) [26; 43; 53; 55; 64; 67; 68; 70; 72; 73; 77; 78]. Most of the time, the selection of the data is done manually by an expert [25; 53; 69; 73]. Once the selection is done, the data can be filtered, either using specific interactions through the XR device or thanks to a role specification of the operator [47] (see Figure 5a). A BIM-based DT contains the 3D models of the different equipment of a building. Some authors observed that they can be used to create immersive environments to promote collaboration between different distant users, such as for collaborative analysis of equipment through its digital representation [65; 78]. These environments can be used to help with the creation of the augmentations [25; 67; 73]. One example is the _Corsican Twin_, where the user can create and attach the augmentations (e.g., specific documentation and time series) to 3D models of the equipment through the use of a virtual environment and immersive devices. It also allows specific areas to be defined near equipment so that when the operator using the AR or the MR device approaches the equipment, the augmentations will be displayed. This system is considered _\"intuitive\"_ by its users because of the simulation of natural interaction that the immersive environment allows (e.g., drag-and-drop and click on an object). The data displayed here are time series as they are the most common in building management [25]. With the use of an AR or an MR device, the operator can see these 3D models overlaying their physical counterpart. This allows the operator to benefit from both the information needed for the procedure (e.g., real-time data feed and historical maintenance) and the contextual information from his or her surrounding environment.