**4. Discussion**

The three tests carried out allows us to make the following remarks.

The reliability of the method, if complete information on the materials and on the structural scheme is available, has been demonstrated with the first test. The displacements obtained under static loading conditions, approximated using a cubic polynomial, differ by approximately 2.5% from those obtained from the FEM analysis and by about 5% from the results provided by independent precise measurements.

The applicability of the method to structures more complex and with less information was verified with the second test, carried out on a bridge characterized by relaxation and creep phenomena, in normal traffic conditions. The difference between the results obtained with the method described and those of a FEM analysis is about 16%. Compared to the outcomes of the aforementioned measurements carried out by the technicians of ANAS, the difference decreases to about 10%. In any case, the results obtained are acceptable for an estimate of the behavior of the structures.

As regards the third test, the comparison with the results obtained with other techniques and instrumentations (total station and inclination sensors) confirmed the validity and precision of the methodology.

In all the tests, the use of interpolating lines allowed to ge<sup>t</sup> higher precision with respect to the unmodeled observations. The working hypotheses, therefore, proved to be reliable.

A peculiarity of the technique described is the possibility of obtaining the 3D model of the monitored structure and the dynamic linear survey with the same instrument and reference system (Figure 18).

Possible fields of application of the methodology presented are both road and railway infrastructures, tall structures and buildings. The method proves particularly valid for constructions characterized by remarkable spans and/or grea<sup>t</sup> heights.

A limit to its usability is given by the need to have a point to position the TLS, with a clear view of the whole or at least of most of the structure to be monitored.

The integration with different techniques opens up the prospect of interesting uses. In the immediate future, activities are planned to exploit the high acquisition rate of TLS, in order to determine the natural frequencies of structures. To this end, acquisitions will be performed simultaneously using TLS and Ground Based Radar Interferometry.

**Author Contributions:** Conceptualization, S.A.; methodology, S.A.; software S.A.; validation, S.A. and R.Z.; structural investigation, R.Z.; resources, R.Z.; data curation, S.A.; writing—original draft preparation, S.A.; writing—review and editing, S.A. and R.Z.; visualization, S.A. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.
