Keywords: setting out, layout, augmented reality, construction sites.

The setting out of a building site is the process that ensures that each element of the works (e.g. building corners, wall lines or boundaries) is constructed in the right position and to the correct level from the location information provided by design plans. This marking out process is a very accurate procedure subject to all sorts of problems, which are checked by surveyors using existing permanent control points. The purpose of these control points is to avoid gross errors and to maintain the current setting out accuracy of the construction to below a predefined acceptance criterion.

This paper presents a new low-cost system oriented to the management of the setting out in construction sites based on augmented reality technology. The term augmented reality (AR) is often used to define some computer graphic procedures where the real-world view is superimposed by computer-generated objects in real-time. Unlike virtual reality, where the user is provided with a completely natural experience in realistic simulated worlds, the goal of augmented reality is to realistically enhance the user's view of the real-world with visual information provided by virtual objects. Following this general approach, the proposed setting-out system consists of a hardware platform based on commercial off-the-shelf (COTS) components and an augmented reality application.

The hardware device of the proposed system includes a high performance PTZ network camera, a reliable 3G-WiFi router and an AC voltage stabilizer in a 15-inch watertight box. This box is made of fiberglass and is located at the top of the main crane to obtain a high resolution top view of the overall construction site. The AR application establishes a connection to this wireless hardware device and obtains top-view images of the construction site in real time. In this sense, the software application of the proposed system acts as a remote monitoring device and augmentates the received images with the information included in the design plans.

In order to evaluate the system performance, accuracy and usability, several experiments have been conducted comparing the proposed approach to not only traditional, but also to high-end, surveying equipment. The results of this performance evaluation shows that the presented approach can be considered an efficient quality/cost trade-off for setting out purposes on construction sites. In this sense, the proposed approach obtains a setting out accuracy of approximately 3-5 centimeters, in most of the tested construction sites, by means of low-cost equipment. Moreover, the application can be executed remotely avoiding the need to send personnel to the construction site for this kind of purpose.

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