## 6 AR-Based Construction Guidance
### Information Transfer in AR  
The application of the BIM-AR system in the post-earthquake retrofitting of buildings is mainly reflected as follows. First, an AR viewport is used to view a model of the damaged member; this provides a basis for determining the retrofitting solution, and for building the retrofitting model and animation. Second, during the retrofitting construction, an AR mobile device is used to superimpose the retrofitting model on the actual component, to thereby provide retrofitting guidance to the construction crews. Third, when problems such as substandard construction quality or reworking are found during project completion and acceptance, the AR platform can be used to compare the actual components with the model, e.g., to identify and correct problems on time.  
In view of the application scenario for the BIM model, the transmission process for related information in the AR environment is shown in Figure 5. The information transmission in the retrofitting process can be roughly divided into three aspects, as described below.  
* Construction preparation stage. The damage model of the building is retrieved from the model base and uploaded to the AR system. All designers can use the AR platform to view the client building to enable remote design guidance and collaboration, as well as to propose recommendations and methods for retrofitting. According to the design requirements, retrofitting models and animations are built, integrated, and stored in the BIM database. Then, all of the information needed during construction is uploaded to the AR platform for easy viewing.
* Retrofitting construction stage. The construction site is divided into several zones, based on appraisal recommendations and the retrofitting design. Each area of the model is installed with an AR reference tracker placed at the appropriate location, so as to facilitate model matching and spatial positioning on the site. Contractors can scan a reference tracker through a handheld mobile device to retrieve the model of the corresponding retrofitted area and can superimpose and compare the physical component and virtual model. This can help the contractor quickly determine the location for the retrofitting. The AR visualization of the model provides on-site views of retrofitting methods for the relevant personnel and can be used by managers to allocate crews and improve construction efficiency. Construction crews can view  
Figure 5: Information transfer flow in AR.  
retrofitting videos and learn by scanning markers arranged in advance, enabling real-time construction guidance.
* Completion acceptance stage. At the end of the construction phase, the inspectors must check the quality of the project. If the project quality deviates from the planned value, the crews mark a screenshot of the problem interface, and send it back to the system to provide a reference for the management staff (e.g., to analyze and make decisions in the next step). By retrieving the original model and comparing it with the feedback, the technicians can adjust the construction plan, and the construction crews can further strengthen and maintain the building according to their assigned tasks. Once the project has been completed and accepted, the information in the database can be updated. The technical design and construction tasks for the next stage are arranged in the system, and the above process is repeated.