## 1 Introduction  
Among the technical parameters for assessing the structural health, the displacements are probably the most relevant. The structural suitability is verified by loading a structure with known loads and comparing the actual displacements with those expected in the design phase. As for bridges, some trucks of known weight, arranged in one or more rows, are positioned on the deck in predetermined positions. Levels or total stations are used to measure beam deflections; additional dynamic tests are performed, depending on the importance of the structure [1]. For Structural Health Monitoring (SHM), along with the deformations, different physical and mechanical parameters are measured. A broad overview of the methodologies used can be found in [2]. Below is a short list of noteworthy techniques: (a) The structural vibration data are increasingly used; a review of vibration-based methods can be found in [3], where natural frequency-based, mode shape-based and curvature/strain mode shape-base methods are described; (b) Instrumented vehicles are more and more used for indirect bridge monitoring. This technique allows measurements without the need to place sensors on the deck or on the piers; references can be found in [4]; (c) Acoustic Emission (AE) is an effective technique, used since several decades [5]; this method, as the X-ray methods,requires to roughly know the position of damages, along with the accessibility for performing tests; (d) Magnetic sensing is used in particular in structures that contain ferrous elements such as reinforced concrete [6]; (e) Ground Penetrating Radar (GPR) is increasingly adopted both to identify hidden lesions and to find discontinuities in structural materials [7]. For detecting and monitoring structural displacements, both conventional and innovative methods are used; below are some of the most used, with their pros and cons. (1) Digital levels are still among the most used tools, thanks to their high accuracy. Their main limit is the need to measure one target at a time, so they cannot be used to dynamically monitor multiple points; (2) Total robotic stations are characterized by high precision and measurement automation. Thanks to the capability to transfer data even via the Internet and be managed remotely, they are used to monitor slopes and large structures [8; 9]. The sampling frequency up to 7 Hz allows its use for dynamic measurements [10; 11], but it is not possible to track more points at the same time; (3) GNSS satellite surveying is often used for tall buildings and large span bridges [12; 13; 14]. It is possible to obtain an accuracy of a few millimeters, while the acquisition frequency reaches 20 Hz. The main limitation lies in the need to place an antenna on each point one must monitor; (4) The increase in the resolution of digital cameras facilitated the development of computer vision based systems. Among the techniques used, digital image correlation (DIC) has proved to be effective for measurements of bridge deflection [15; 16; 17; 18; 19; 20]; (5) Dial gauges, widely employed for measurements of floor slabs deflections, are used for bridges with limited height and in absence of water; (6) The use of inclination parameters obtained from microelectromechanical systems (MEMS) has been proposed to derive the deflection [21]. The high S/N ratio, typical of these devices, strongly influences the results of dynamic tests; (7) Currently, the measurement of deflections by using laser beams is widespread. Some systems for the measurement of deflections and displacements with the use of laser beams are described in [22; 23]; (8) An effective methodology for measuring displacements of large structures is offered by Ground-Based Synthetic Aperture Radar (GB-SAR).Used in RAR mode (Real Aperture Radar) it allows measurements of vibration frequency and displacements associated with dynamic loads.