*2.1. The Methodology*

The method described exploits the capability of some TLS models to function as a line scanner and to provide the timestamp of each detected point, synchronized with the GPS time.

It is necessary to identify a significant line to be surveyed, which allows to obtain the di fference between the elastic curve of the unloaded structure (dead load) and that of the structure subject to mobile or static loads. To this purpose, it is generally chosen to scan a line at the bottom of the deck, on the longitudinal axis or parallel to it.

For this reason, the TLS must be correctly aligned. Since the instrument does not project a visible beam, which would allow to check the alignment, it is possible to use a total station and/or exploit the features present on the surface of the structure or specifically positioned targets.

As a preliminary step, a line scan of the unloaded structure is performed, in correspondence with the selected section, which allows to have the reference shape. This shape is given by the best fitting line of the points measured (usually 10 to 100 points per meter at a 100 m distance from TLS). To obtain instant displacements, line scans are performed during the normal activity of the structure, with a sampling rate of up to 120 lines per second. For each line scan, the interpolation line is obtained and the scan time t, provided by the instrument timestamp, is memorized. The displacements at time

t, are given by the deviation between the interpolation line at time t and the reference line. These displacements provide the elastic curve due to the load present at time t.

TLS is equipped with a GNSS receiver, which allows you to transform the timestamp into GPS time. The instantaneous position of a vehicle can be obtained from a GNSS receiver with which the vehicle is equipped. A video of the structure during the activity can also be used to determine the position of moving loads. Nowadays, several cameras are equipped with accessories that can provide Coordinated Universal Time (UTC) and, therefore, allow us to synchronize TLS and camera acquisitions.
