## 6 Discussion
### Applications in AEC  
The results of our study show several placement reliability values in terms of accuracy and stability that should be judged with respect to the context of use. Indeed, the fitness of use of each placement method should be identified to guide the user in choosing the appropriate method. In the following, we highlight the applicability of each method in AECO projects. We note that the need for precise placement depends on several factors: the complexity of the project, its phase (from design to operation), the difficulty of the operation (risk and decision-making), etc.  
* Planning and design In the planning phase of construction, it is crucial to ensure that the project plan aligns with the client's requirements [63]. By using AR in conjunction with a 3D model, architects and construction companies can truly present working models to their clients before construction begins. For this purpose, high accuracy of the placement of a building model is not required; an error range of a few meters remains tolerable. Marker-less methods or other solutions, such as the SiteVision system, can be used in this context.
* Site supervision Precise placement in site tracking ensures that virtual objects are properly aligned with the physical world. Marker-based methods are often used for AR applications inside buildings [64](centimeter accuracy). By equipping the site with beacons, the monitoring of the progress of the work can be the most precise compared to methods based on sensors which lack precision in the covered areas (example of GNSS). However, marker-based AR technology suffers from placement issues, which sometimes dramatically increase setup time. For example, depending on the lighting conditions, the paper marker must be moved so that the model is accurately superimposed on the built environment. In larger sites, the use of several markers may be necessary with an update of the database of markers set up according to the progress of the work. Additionally, the marker installation time could impact the overall value provided by this technology when used in an active construction site with tight time constraints.  
For outdoor, the method used will need to meet site requirements such as larger working environments and environmental conditions. The hybrid approach, which combines IMU sensors and a GNSS receiver with computer vision to improve GNSS accuracy, can be one of the methods to use in this context.  
* Construction inspection The use of AR solutions for construction inspection allows identifying potential accidents that could threaten the entire project, such as a pipe collision or a defect on a wall, for example. The exact placement on site is a critical factor for this type of application. To meet these needs, marker-based methods are the most adaptable to this application [65]. As demonstrated by our study, these methods provide placement accuracy down to a few centimeters (example of ARcore marker-based methods).
* Management of public services AR solutions are recommended to identify buried networks whose location should be determined before any digging [66]. Here, the accuracy of the locations of underground networks is essential. Therefore, the use of a smartphone connected to an external GNSS device (i.e., SiteVision with VRS network) in an open environment is recommended to minimize errors related to positioning by GNSS (multipath, the presence of a power line). In an environment where conditions do not allow precise positioning by GNSS, the use of pre-defined marker placement methods on site can ensure centimeter accuracy.