## 5 Discussion  
The performed study allowed identifying the strain measurement as the major component of instrumental measurement uncertainty related to the modulus of elasticity and the Poisson ratio for both methods. In the case of the optical method, this uncertainty component reflects the influence of the spatial resolution of the acquired images and the dispersion of the scale coefficient related to the effect of the lens distortion and planar misalignments between the specimen surface, the measurement referential, and the image plane. In the case of the contact method, the LVDT sensor repeatability and linearity have a strong impact on the dimensional measurement uncertainty and are mainly due to its assembly onto the masonry specimen and the use of installation accessories. Regarding the modulus of elasticity, the instrumental measurement uncertainty related to the optical approach (between 0.07 and 0.60 GPa, as mentioned in Section 4.2) has a slightly reduced magnitude when compared with the instrumental measurement uncertainty of the contact measurement method (between 0.18 and 0.78 GPa). In both cases, the dispersion of values related to the modulus of elasticity is higher that the measurement sample dispersion, between 0.08 and 0.30 GPa, as shown in Table 5.  
Regarding the Poisson ratio, both measurement methods show high values of measurement uncertainty, namely, the optical method due to the reduced spatial resolution affecting the measurement of the horizontal strain, which is characterized by a low magnitude. Poisson ratio standard uncertainties are comprised between 0.19 and 0.50, in the case of the optical method (see Section 4.2), while for the case of the contact method, this interval corresponds to 0.10 and 0.32 (see Section 4.3). In a global perspective, the mentioned dispersion of values has a higher magnitude than the measurement sample dispersion, between 0.03 and 0.09 (see Table 5). This study showed that the suitability of the new approach proposed, based on non-invasive optical measurements applied to the destructive compressive mechanical testing of masonry specimens, is able to provide comparable results to conventional invasive measurements obtained using LVDT sensors. However, the scope of the validation process will need to be further studied, considering that the described mechanical tests were performed in a real case scenario, using a reduced dimension sample  
Figure 16: Estimates and standard uncertainties obtained from the optical and the contact measurement systems for (**a**) the modulus of elasticity; (**b**) and the Poisson ratio. of masonry specimens, each one characterized by a high variability in terms of the record strain spatial distribution, and different measurement points were defined for the optical and contact measurement methods. In this context, the development of reference materials and the use of advanced optical systems based on PIV (particle image velocimetry) [13; 14; 15] are recommended as future validation work. Based on the analysis of the results, the increase of confidence in the studied compressive mechanical properties of masonry specimens should be focused on the sampling process, e.g., by increasing the number of local strain measurements in the specimen (thus reducing the average standard deviation related to the strain spatial variability) and also the number of specimens of a certain type (hollow or solid). [PERSON] and [PERSON] conceived, designed and performed the experiments and analyzed the data. [PERSON] and [PERSON] validated the obtained results. [PERSON] and [PERSON] also wrote the paper, which was reviewed by [PERSON] and [PERSON]; [PERSON], [PERSON] and [PERSON] contributed to the experiments by scientific supervision. All authors have read and agreed to the published version of the manuscript. This research received the financial funding of LNEC--National Laboratory for Civil Engineering.