## 1 Introduction  
The aging and deterioration of port facilities in the Republic of Korea has become an issue that should be addressed. As of 2020, 49.4% (538 locations) of port facilities are aged over 20 years, while 13.1% (143 locations) are aged over 40 years. According to the safety inspection and precision safety diagnosis reports of port facilities, in the facilities of more than 20 years age, the A-grade ratio decreased sharply, while in the case of 40 years or more, the A- and B-grade ratios tended to decrease. Moreover, the increase in the intensity and frequency of natural disasters related to climate change increase the variability of the design external force and enhance the possibility of large-scale damage to aging port facilities [1]. Figure 1 shows the critical damage cases that have occurred in port facilities. In response, the Ministry of Oceans and Fisheries of the Republic of Korea has established a national roadmap in 2020 for the smart sensing, monitoring, analysis, evaluation, and repair of port facilities for proactive and timely maintenance. In the detailed guidelines for infrastructure safety inspection and precision safety diagnosis, the critical major defects in port facilities are defined as: foundation scour, damage and corrosion of piles, loss of internal force due to carbonation and chloride attack in concrete, corrosion of lock gate facilities, and the normal displacement and settlement of berthing structures [2, 3]. The settlement and normal displacement of berthing structures is generally evaluated by surface level surveying; the foundation scour should be evaluated by divers, and the members towards the sea should be inspected by inspectors moving in a boat. These evaluations and inspections are carried out every few years, and thus continuous monitoring is difficult. Attachment of various electric sensors is one method to monitor the behavior,such as displacement, settlement, slip, and slope, but it is complicated to organize the sensing system with consideration of the berth, salt attack, high-risk work on the members towards the sea, and facility users' route [4; 5; 6; 7]. Thus, in this paper, we present a technique for measuring the precise behavior of a berthing structure that could be caused by scouring, settlement, slip, damage, material deterioration, and so forth. The behavior of the structures using a vision-based non-contact type displacement measurement system has gained rapid developments in the past decade [8]. [PERSON] et al. (2013) presented a vision-based deflection measurement method using the digital image correlation coefficient [9]. [PERSON] et al. (2014) proposed a 6-DOF translational and rotational displacement measurement system with a vision sensor and a uniquely designed marker [10]. [PERSON] et al. (2015) proposed a multi-point displacement measurement method by use of a pattern-matching algorithm [11]. [PERSON] et al. (2015) proposed a structural displacement measurement method with a subpixel resolution using the upsampled cross correlation algorithm [12]. [PERSON] et al. (2020) proposed a videogrammetric technique for displacement monitoring that eliminates the measurement error due to the image drift induced by temperature variation [13]. Most of the aforementioned non-contact type vision-based displacement measurement systems, however, have one of following drawbacks: only estimated deflection, which is 1-DOF displacement, markers are attached on the structures for feature points detection, or the accuracy of the measurements highly depends on the camera calibration results for calculating intrinsic parameters. The 6-DOF displacement also can be measured using structured light composed of lasers and vision sensors [14; 15; 16]. The translational and rotation displacement measurement system called a paired structured light system composed of two sides facing each other, each with one or two lasers, a screen, and a camera.