## 1 Introduction  
During underground excavation at a construction site, retaining walls are constructed in order to prevent adjacent ground from collapsing [1]. Since the collapse of the constructed retaining wall considerably impacts the community and causes damages regardless of the construction scale, appropriate preventive measures are required on construction sites. Also, because the deformation in retaining walls generally increases before its collapse, it must be continuously managed to prevent any collapse [2, 3]. Measuring instruments, such as inclinometers, can determine the displacement of retaining walls, which cannot be done with the natural eyes. Inclinometers are generally used to determine the displacement of retaining walls and are among the main measuring instruments used in evaluating the stability of retaining walls. However, inclinometers have the following problems: (1) the inclinometer casing must be installed near the retaining wall in advance using boring equipment; (2) selecting a cross-section of the retaining wall where large deformation occurs is important. This can be achieved by consulting measurement experts and installing multiple inclinometers at the cross-section, since these experts can only identify the displacement of the local two-dimensional (2D) cross-section of the retaining wall [4]; (3) acquiring considerable human resources can be challenging, since measurement experts collect, analyze, and interpret measurement data only on construction sites depending on the measurement cycle [5].  
To address the problems inclinometers have, several studies and techniques have been conducted and developed, respectively. These techniques are divided into contact (which use conventional inclinometers) [6, 7, 8, 9, 10, 11] and non-contact measurement methods (which use the latest technologies such as laser and image sensors) [12, 13, 14, 15, 16, 17, 1, 12, 13, 1] depending on the measurement method. On the one hand, contact measurement methods reduce the frequency at which measurement experts visit construction sites, since they can automatically collect measurement data using wireless network technologies and evaluate the stability of the retaining wall using said data. However, using wireless network technologies is more expensive than conventional inclinometers. Additionally, installing and selecting inclinometers and optimal measurement positions, respectively, is difficult. On the other hand, non-contact measurement methods determine three-dimensional (3D) displacement in different areas, such as tunnels [18], dams [19], landslides [20], and bridges [21]. This is because they are easier to install compared to contact measurement methods and can measure the overall displacement of the retaining wall. However, regarding terrestrial, laser scanner (TLS)-based non-contact measurement methods, the scanner is expensive, and the measuring personnel must install and dismantle the equipment for each measurement, because it cannot be installed permanently outdoors. Vision-based displacement measurement is less expensive than TLS. Therefore, a target plate is required to increase the accuracy of the deformation analysis results. This method has limited applicability in inclement weather such as rain and snow. Consequently, the objective of this study is to develop a retaining wall displacement measurement system that can resolve inclinometer limitations and previously developed methods for retaining walls that were constructed during underground excavation at construction sites. In order to evaluate the performance and applicability of the developed system, its displacement measurement performance and economic feasibility were analyzed. If the developed system is commercialized and used for measuring retaining walls, it is expected that retaining wall displacement management will be cost-effectively performed vis-a-vis the conventional measurement method.