## 5 Results
### Survey Coverage  
In order to evaluate the performance of the system with respect to its intended scope, which is the collection of visual information on the built-up environment, a visibility analysis has been conducted on the collected data. The purpose of the analysis was to assess the expected coverage in terms of buildings and policies, and to highlight possible issues connected with the use of the proposed mobile mapping system. A cumulative viewshed analysis [27] along the path followed by the mobile mapping system has been carried out to select all buildings that are potentially visible from the acquired image locations (Figure 11). The algorithm iteratively loops through a set of input points and calculates \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Survey** & **Distance** & **Elapsed** & **Buildings Covered** & **\% of the Total** & **Policies Covered** & **\% of the Total** \\
**Stage** & **Covered (km)** & **Time (h)** & **(Viewshed)** & **Buildings** & **(Viewshed)** & **Policies** \\ \hline day-1 & 72.5 & 6 & 4922 & 25.7 & 9327 & 31.2 \\ day-2 & 45.9 & 4 & 2270 & 11.8 & 3812 & 12.8 \\ totals & 118.4 & 10 & 7192 & 37.5 & 19,945 & 44.8 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Summary of the two-day survey conducted in Besiktas. Figure 10: The color of the trace is proportional to the absolute speed of the mobile mapping system (**left**). Distribution of the speed during the survey (logarithmic scale) in Km/h (**right**). viewshed maps, which depict the geographical area that is visible from the input point given digital elevation information. As input elevation information, the DSM of the study area has been used, observer points have been set along the survey path at 3 m above ground (the actual height of the omnidirectional camera mounted on the car) with an equal spacing of 20 m and a maximum viewing distance of 30 m. The output is a cumulative viewshed map with raster cells being assigned the number of input locations that have visibility. A spatial intersection with the building footprints provides an estimate of the buildings that are potentially visible from the surveyed path. The estimated number of buildings covered by the survey and the related number of policies involved are presented in Table 4. Based on the viewshed analysis, it is estimated that more than 7000 buildings, equating to almost 20,000 policies, have been captured by the mobile mapping system. This equals 37.5% of the total building stock and 44% of the total number of policies in the study area. In order to further analyze the visibility under consideration of occlusion factors that could not be considered in the viewshed analysis (e.g., vegetation, cars, walls, _etc._), a set of 1000 buildings has been randomly sampled from the ones identified as being potentially visible by the viewshed analysis. An operator has been asked to manually locate the selected buildings in the omnidirectional stream, and to assess whether they are actually visible (at least partly) or hidden. The operator had to report the type of occlusion, even when the selected building was considered visible for the purpose of the survey, and to write down any other issues related to the requested analysis. At least 50% of a building should be visible in order to be assigned the label visible. A Remote Rapid Visual Screening (RRVS) desktop environment has been set up to perform the image analysis, as is described in [28]. In Table 5 the results of the detailed occlusion analysis are reported. Figure 11: Visibility analysis based on viewshed estimation for a subset of the buildings theoretically imaged by the mobile mapping system. The operator was a university student with experience in remote sensing and desktop mapping. The desktop assessment was carried out in one week, considering an average of 5-6 h a day. In the practical application, different operator profiles could be employed to fulfill different tasks.