## 2 Materials and Methods
### Research Methodology
#### 2.2.1 Phase 1: 3D Survey by Smartphone Depth Sensors  
In phase 1, 3D surveys of several objects in both indoor and outdoor environments were carried out. In particular, four case studies were taken into consideration. The first case study focused on the scanning of some building material samples. The tests were performed to evaluate the scanning performance of depth sensors under controlled laboratory conditions and reduced sensor movement to a minimum. This test was carried out in the geomatics laboratory of the Polytechnic University of Bari, Taranto, Italy (\(\varphi\): 40\({}^{\circ}\)31\({}^{\prime}\)36\({}^{\prime\prime}\) N; \(\lambda\): 17\({}^{\circ}\)16\({}^{\prime}\)60\({}^{\prime\prime}\) E). We chose to scan several samples of building materials in an urban environment. In this test, we considered albedo as a parameter characterising the material. The measurement of albedo was documented in [15]. We performed the scans by keeping the mobile devices in the nadir direction to the samples and moving them as little as possible (near-static conditions). We scanned the samples from distances of 0.5 m and 1.5 m and with resolutions of 2 cm (Android), 1.5 cm (iOS) and 1 cm (iOS). By combining scan distances (2) and scan resolutions (3), we obtained 6 different point clouds for each sample. With 10 samples available, a data set of 60 point clouds of samples differing in acquisition distance, resolution and material type (albedo) was obtained. In Table 2, we report the features of each sample--dimensions, area, material and albedo--while in Figure 2, an overview of the scanned samples is reported. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sample Material** & **Dimensions [m]** & **Area [sqm]** & **Albedo** \\ \hline Smooth cement plaster & 0.4 \(\times\) 0.45 \(\times\) 0.05 & 0.180 & 0.514 \\ Raw cement plaster & 0.4 \(\times\) 0.45 \(\times\) 0.05 & 0.180 & 0.524 \\ White lime plaster & 0.505 \(\times\) 0.365 \(\times\) 0.01 & 0.184 & 0.518 \\ Coloured lime plaster & 0.525 \(\times\) 0.47 \(\times\) 0.01 & 0.247 & 0.510 \\ Tetafluoroethylene (TFE) & 0.245 \(\times\) 0.19 \(\times\) 0.001 & 0.046 & 0.455 \\ Methacrylate (PMMA) & 0.305 \(\times\) 0.12 \(\times\) 0.005 & 0.037 & 0.433 \\ High-density polyethylene (HDPE) & 0.23 \(\times\) 0.22 \(\times\) 0.002 & 0.051 & 0.623 \\ Frosted glass & 0.3 \(\times\) 0.3 \(\times\) 0.005 & 0.090 & 0.517 \\ Steel & 0.205 \(\times\) 0.3 \(\times\) 0.008 & 0.061 & 0.606 \\ Brass & 0.105 \(\times\) 0.303 \(\times\) 0.009 & 0.032 & 0.661 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Features of the material samples studied. Figure 1: Pipeline of the research methodology. In the second case study, a statue in public gardens was scanned. The test was performed to investigate how sensor movement could generate aberrations in the scans. The second test was performed in the public gardens of \"Villa Peripato\" in the city of Taranto, Italy (\(\varphi\): 40\({}^{\circ}\)28\({}^{\prime}\)20\({}^{\prime}\) N; \(\lambda\): 17\({}^{\circ}\)14\({}^{\prime}\)80\({}^{\prime\prime}\) E). In the Villa Peripato, we scanned a statue representing an animal (Figure 3). The scans were realised by performing concentric circles to the statue at fixed distances of 2 m and 3 m. During the scans, the smartphones were maintained parallel to the statue and worked at resolutions of 2 cm (Android) and 1.5 cm (iOS). From this test, we obtained a dataset consisting of four point clouds that differed in acquisition distance and resolution. Figure 3: Survey of the statue (second test). Figure 2: Set of scanned samples in the first test: smooth cement plaster (**a**), raw cement plaster (**b**), white lime plaster (**c**), coloured lime plaster (**d**), tetrafluoroethylene (TFE) (**e**), methacrylate (PMMA) (**f**), high-density polyethylene (HDPE) (**g**), frosted glass (**h**), steel (**i**), brass (**j**). In the third test, we scanned a room inside the geomatics laboratory ([16]--Figure 4) to verify the performance of depth sensors in scanning an object from the inside.