# Review of Microsoft HoloLens Applications over the Past Five Years
## 3 Results
### Applications in Engineering Fields
#### 3.2.2 Architecture and Civil Engineering  
Many cities are rapidly developing into smart cities--smartly organizing information on various infrastructures by introducing ICT and the Internet of things. With this rapid development, AR technology and 3D reconstruction technology are also being developed and are being widely used in architecture and civil engineering. In this field, AR technology is widely used to map spaces or visualize data. Microsoft HoloLens is equipped with various sensors, including four tracking cameras and a ToF range camera. The sensor images and their poses, which are estimated by the built-in tracking system, can be accessed by the user. This makes HoloLens potentially interesting as an indoor mapping device [61]. [PERSON] et al. [61] introduced various sensors mounted on the HoloLens and evaluated the complete system in relation to indoor environment mapping. They evaluated the performance of the HoloLens depth sensor and its tracking system separately and evaluated the complete system for its ability to map multi-room environments. Based on the evaluation, HoloLens is an easy-to-use, relatively inexpensive device that can easily capture the geometric structure of large indoor environments. [PERSON] et al. [62] proposed a method for visualizing 3D city models in Toronto, Canada and various types of city data using a HoloLens, to utilize AR technology. The city model and dataset are displayed as augmented content in the real world through a lens. The user can obtain an interactive view of Toronto city data and check the details of the selected city data object. By using the proposed method, the users can efficiently obtain and observe various types of city data. Moreover, because the data can be easily manipulated and interacted with using HoloLens, the city data can be better explored and managed. [PERSON] et al. [63] proposed an efficient use of MR technology to manage furniture in a room using HoloLens, to create a building information modeling (BIM) system. The MR systems can interact with BIM data, from a virtual to real environment. They mapped the occupied and empty spaces of the room using a 3D spatial mapping method. The developed system is designed to be able to move virtual items (furniture) in real time and to rotate and resize objects. [PERSON] et al. [64] proposed a novel control system for the path planning of omnidirectional mobile robots based on MR. The proposed system can control the movement of a mobile robot in a real environment and the interaction between the movement of the mobile robot and virtual objects that can be added to the real environment. An interactive interface is provided through HoloLens, which displays maps, paths, control commands and other information related to the mobile robot. Furthermore, by adding a virtual object to a real map, real-time interaction between the mobile robot and virtual object can be realized. The interface of the proposed system has a virtual panel and some buttons created with the Unity3D software combined with HoloToolKit, as shown in Figure 9(a), and the communication mode of the system shows the flow, as shown in Figure 9(b). According to the verification performed in an indoor environment using the proposed system, it was found that the proposed method can generate the movement path of the mobile robot according to the specific requirements of the operator, while demonstrating excellent obstacle avoidance performance. [PERSON] et al. [65] described how HoloLens could be used in advanced control education for localization, mapping and control experiments of autonomous vehicles. HoloLens can provide 3D position information using a spatial mapping feature and solve the problem of simultaneous localization and mapping. They used HoloLens technology and a Raspberry Pi computer, connected with standard remote-control car components, to create a suitable experimental platform for control education.