## 6 AR-Based Construction Guidance
### BIM-AR Based Visualization Testing  
The post-earthquake retrofitting visualization of buildings described above has two major functions: viewing the BIM model, and simulating an animation of the retrofitting using mobile devices. A wall damaged by shear was taken as an example for testing these functions.  
According to the method proposed in Section 5.2, Augin(r) is chosen as the AR platform. It can operate on mobile device, web pages, and plug-in software. The mobile device for the AR application can be a tablet or a smartphone. The application system of the smartphone can be Android or Apple, but all the above devices need to be after 2017. In general, the more memory space the device has, the smoother the operation will be. Tablets have a greater advantage in terms of model loading speed, but they are larger and less portable. Smartphones are slow to load, but easy to get and easy to carry. Smartphones are chosen as the display tool to facilitate construction. This study uses the iPhone 8 as the mobile device for the AR application.  
The experimental steps are as follows.  
Construction preparation phase. Building information models and retrofitting videos are obtained and uploaded to the AR platform. Managers assign AR application accounts to construction workers and upload models and videos according to different construction areas. Workers in the same area have the same account and can see the same model. Managers upload models and videos on the plug-in software and/or web page, as shown in Figure 7A-D. To view the model on a real scale, this study uses a reference tracker to achieve fast localization of the AR model in the real world. In particular, a virtual tracking mark is placed in the model, as shown in Figure 7F. At the same time, the managers place physical trackers in advance at the corresponding locations on the construction site, and the placement locations are different for different construction areas. In addition, they upload the retrofitting video and the corresponding thumbnails on the web side. The thumbnails are printed and placed at the location where the retrofitting is needed.  
Retrofitting construction phase. Once the preparation is complete, the model and video are synchronized to the AR application on the mobile device. After scanning the pre-arranged reference tracker on the AR interface, the model appears in the corresponding position and can be viewed at a ratio of 1:1, as shown in Figure 8. After specifying the construction location based on the overall model, the crews use the iPhone 8 to view the BIM model of the local component. The integrated BIM damage information described in Section 4.1 is presented by clicking on the component, as shown in Figure 9A-C. At any stage of construction, a video of the retrofitting can be viewed. The video is played by scanning the thumbnail in the AR interface, as shown in Figure 9D,E. Workers can watch the video in an AR environment to guide the retrofitting construction.  
Figure 8: Viewing the model at 1:1 ratio: (**A**) Project list; (**B**) AR model; (**C**) Identify the reference; (**D**) View the model at 1:1 ratio.  
Figure 7: Uploading data to the AR platform.