# An Operational Image-Based Digital Twin for Large-Scale Structures
## 4 3D Geometry Reconstruction  
The 3D reconstructed model of the transition piece is the visual part of the Digital Twin. This reconstruction model was based on both images and LiDAR scans. The images were collected during the drone flights; ContextCapture from the Bentley Institute was used to generate the 3D reconstruction models. These 3D models are textured meshes with a large number of faces and vertices. Figure 5a shows the CAD model of the TP from Bladt Industries and Figure 5b shows the corresponding reconstructed 3D model. This reconstruction is based on 445 images. Small holes can be observed on the cylindrical sides of the reconstructed model. These small holes in the reconstruction mesh appear in the areas where there is a low number of keypoints. A larger number of images can in some cases mitigate the number of small holes in the reconstruction. The overlapping between the CAD model and the reconstructed model is shown in Figure 6. The critical step of accurately aligning the reconstructed and the CAD model meshes was done with the use of the Meshlab tool [15]. The grey areas come from the CAD model, while the yellow areas come from the reconstructed model. It is seen that the reconstructed model, in general, is a very good representation of the CAD model, with a few exceptions. The biggest difference between the two models is the different positions of the crane. The different heights of the posts on the upper platform is another difference between the two models. The height of the posts is lower, on the tower that was built, than what it should have been according to the CAD model. Figure 7a shows the differences in distance between these models as colors superimposed on the CAD model. crane, corresponding to large values of the distance differences, are due to the different positions of the crane in the two models. The different crane position is also the reason for the second peak in the histogram for high values of geometric differences, as shown in Figure 7b. Appendix A shows how the dimension of objects can be measured from the model reconstruction. Figure 6: Comparison between design and reconstruction. The mesh of the CAD model has been aligned with the mesh of the reconstructed model. The yellow areas come from the reconstructed model, while the grey areas come from the CAD model. It is seen that the crane is not placed in the same position in the two models. Information that possibly leads to the identification of the TP owner is removed from the images. Figure 7: Quantification of geometric deviation. (**a**) The color contour represents the distance between the CAD model and the reconstructed model. (**b**) The histogram shows this relation, together with the probability of the distance differences measured in meters. Green color: distances close to zero. Red and blue colors correspond to positive and negative distances, respectively. Note that the probability is expressed using a logarithmic scale, thus making it possible to see the color bars for the distances with a very small relative probability. The histogram peaks for higher values of geometric differences correspond to real differences between the two models. The dark red peak is the result of the different positions of the crane in the two models. The reconstructed 3D model of the TP can also be based on LiDAR scans. These results are shown in Figures 8 and 9. The reconstructed model in Figure 8 is based on six static LiDAR scans of the TP, corresponding to a point cloud of 63 million points. Shiny surfaces can cause problems for photogrammetry reconstruction based not only on images but also on LiDAR scans. Blockade of the scanning LiDAR beams is another type of problem that reduces the quality of the reconstruction. We encountered all of these obstacles during the drone inspection of the TPs. A lack of information in a certain area of the tower can cause holes to appear in the reconstructed 3D model. Smaller holes can be mitigated in the photogrammetry software ContextCapture from the Bentley Institute.