## 3 Experimental Test Procedure
### Testing Setup and Measurements  
For the test material, wooden beams were chosen. Wood was selected because of its excellent elastic properties, and because it exhibits large displacements under a small load.  
The wooden beam was supported on both sides, and the pressure was applied in the middle of the beam, as shown in schematic diagram in Figure 2. The pressure distribution was not measured because it has no relevance for this type of test and subsequent calculations, but the overall force is known for every step of load increment using a small load cell integrated into the loading device. This setup was necessary to determine inputs for an independent numerical model of beam displacements. Small black shapes were added as additional targets on the wood beams to increase the number of detected features with the feature matching and detecting algorithms.  
Displacements were measured with a LVDT, which is a common type of electromechanical transducer that can convert the linear motion of an object to which it is coupled mechanically into a corresponding electrical signal. LVDT typical sources of uncertainty are due to nonlinear electrical responses, mechanical positioning and orientation errors, electrical transmission noises, and digitalization errors. In the described measurement setup, the HBM WA100 LVDT sensor [30] with a nominal measuring range of 0-100 mm was used. The used sensor's nonlinearity is in the range of 0.2-1% of its entire measurement range. For reduced measuring range 0-20 mm used in this setup, overall accuracy is \(\pm\)0.02 mm (obtained by previous laboratory calibration). The LVDT measuring accuracy is higher than the achievable accuracy of measurements obtained by Trimble SX10 by a factor of 100. The displacements obtained by LVDT were taken as a reference in this research for testing the accuracy of determined displacements from RGB+D images. LVDT was positioned in the middle of the beam where the pressure was applied to, as shown in Figure 3 right.  
MS instrument Trimble SX10 was used for the testing. The instrument was positioned roughly 5 m from the wood element, as shown on Figure 3 left. The distance was chosen based on the cameras field of view (FOV) so that the whole test field is visible within one image from the integrated primary camera. For the primary camera, the image size is 2592 \(\times\) 1944 pixels, and at a 5 m distance, the ground sampling distance (GSD) is 0.3 mm. Location of the primary camera, in relation to the SX10 instrument body, is shown in Figure 4.  
The instrument station was set up in a local coordinate system. Control reference points were positioned around the test site and were used to check the stability of the instrument between the epochs.  
The final test was conducted on two different wooden beams. For each element, four or five measuring epochs were conducted. Due to changing light conditions, only four measuring epochs were tested for the BEAM 1, in comparison to five for BEAM 2. The first and last epoch were without any pressure. For epochs two to five, rising pressures were applied so that different displacements were obtained and then measured with the MS and LVDT, as shown in Table 2. In each epoch, a picture was taken with the MS primary camera. For each image theodolite angles (_Hz_, _V_) measured from the MS are stored. Using the stored theodolite angles and camera parameters, external orientation of the camera can be easily calculated. The test area was then scanned in the scanner's FINE resolution mode, which resulted in a measurement density of 1 mm. Data from the LVDT were continuously logged and stored.  
Figure 2: Schematic diagram of the loading for BEAM 1, all dimensions are in cm (Source: Own compilation).  
Figure 4: Trimble SX10 camera configuration (Source: Own compilation).  
Figure 3: Test field (**left**) and test configuration (**right**) representing the wooden beam and LVDT (Source: Own compilation).