## 4 3D Laser Terrestrial Scanning: Knowledge Base
### Terrestrial Laser Scanning: Realization Phase
To achieve higher accuracy by scanning without reference balls and targets, it would be necessary to place the scanning positions at a very short distance, which would require a much larger number of final scanner positions and much more time spent on site. On the other hand, the use of reference balls and targets will only slightly prolong the time spent on site compared to the aforementioned previous case, and will also simplify the registration process in the software environment. Ball positions are used in data processing, where the software automatically recognizes them as reference elements. Based on them, it automatically combines scans from different angles and thus creates a complex point cloud of the scanned object. It is also important to note that there had to be at least three common reference points between two scan positions. The harmonization and coordination of individual scans within the postprocessing is enabled by the integrated GPS receiver. The most challenging scenery in terms of terrestrial laser scanning within the entire building was the basement of the castle. There were several reasons for this, such as the unavailability of electric power, very poor lighting conditions, high humidity, areas of fragmentation, transitions between light and dark parts of the building, the occurrence of dangerous shafts, surface condensation, falling plaster, etc. It was necessary to reduce the time required to obtain data. The scanner settings were adjusted and scanning was performed without RGB to ensure that the time required for data collection was minimal. Due to the fragmentation of the premises, 61 scanning positions were performed in the mansion basement. Figure 4 shows the correspondence view and the specific locations of the scan positions after the individual scans were registered in the postprocessing. Correspondence is the relationship between the same common reference object in multiple scans. Faro Scene software (version 2019.1, FARO Technologies, Inc., Sarasota, FL, USA) uses these objects to create correspondences. The result can then be visualized and evaluated in the Correspondence View, the 3D View or the Structure View tool. Postprocessing took place also in the FARO SCENE software environment, where the individual scenarios were registered separately in individual clusters. Then, these clusters were registered in the final point cloud. Finally, filters were applied in the software environment, and unwanted points in the point clouds of individual scenarios were removed. During the scanning itself, cleaning work also took place on the object and its surroundings, which led to minor time plan changes and various noises. These externalities had to be filtered out in the final process, as shown in Figure 5. It is important to mention that during the use of the technology of terrestrial laser scanning, so-called death spots arise where the laser beam does not hit the measured point on the object. These places must be added later in the following photogrammetry process. The resulting point cloud in this phase met the requirement of LOD 300 in terms of shape, orientation and location. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{2}{c}{**Reference Point**} & \multicolumn{3}{c}{**Average Scan Time**} \\ \cline{2-7}
**Quality** & \multicolumn{2}{c}{**Distance**} & \multirow{2}{*}{**5 m**} & \multirow{2}{*}{**15 m**} & \multirow{2}{*}{**30 m**} & **with** & **without RGB** \\ \cline{2-7}
**Settings** & & & & & **RGB (min)** & **(min)** \\ \hline \multirow{2}{*}{\(2\times\)} & 1/4 & 1150 & 120 & 25 & 5:01 & 2:47 \\ \cline{2-7}  & 1/8 & 235 & 22 & - & 3:40 & 1:26 \\ \hline \multirow{2}{*}{\(3\times\)} & 1/4 & 1175 & 126 & 27 & 6:48 & 4:34 \\ \cline{2-7}  & 1/8 & 239 & 25 & - & 4:07 & 1:53 \\ \hline \multirow{2}{*}{\(4\times\)} & 1/4 & 1390 & 135 & 25 & 10:23 & 8:09 \\ \cline{2-7}  & 1/8 & 200 & 27 & - & 5:01 & 2:47 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison of quality and resolution parameters and average scan times.