## 5 Camera Placement based on HybVOR Approach
### Hybrid Assessing of the Camera Network Coverage
#### 5.3.1 Raster-Based Viewsed Calculation  
This method allows assessing the degree of coverage of an area of interest based on a Digital Surface Model (DSM) in raster format. This DSM results from the combination of the ground elevation with buildings' height. In order to create such a DSM, a Digital Elevation Model (DEM) in raster format that represents the ground elevation has to be generated. The raster DEM is generally extracted from a Triangular Irregular Network (TIN) that represents the elevation of the ground. Then, buildings' footprints are generated either from an existing 3D Model of buildings or from topographic surveys. After that, these footprints that are in vector format are converted to a raster format using the same resolution of the original DEM. The pixel values in this resulting raster correspond to the roof elevation of each building. Thereafter, the raster file that represents the elevation of buildings' roofs is combined with the raster DEM that corresponds to the ground elevation. Here are the main steps to perform this combination:  
* For each pixel that belongs to the resulting DSM:
* If the corresponding value in the building' roof raster file is null then the pixel value is taken from the raster DEM. * Otherwise, the pixel value is taken from the corresponding pixel in the building' roof raster file. The combination of the Raster DEM and the elevation information from 3D Buildings Model will provide a Digital Surface Model in raster format for the area of interest. This DSM is mandatory in order to assess the coverage of camera network through the HybVOR approach. After generating the DSM, each camera should have a height value, which corresponds to **OFFSETA** (_c.f._ Section 3.1). This offset value will be chosen according to the height of the two buildings which are considered as generators of the Voronoi edge where the camera is placed. The camera height is mainly related to the possibility of constructing poles to place cameras. Two situations occur:  
The first case happens when the surrounding buildings of the camera pole are not very tall. In this situation, the value of the camera height will correspond to the tallest building. The advantage of this criterion is offering the possibility of monitoring the roofs in addition to entrances and facades. Otherwise, if it is not feasible (or not profitable) to construct a pole that has the same height as the taller building, a standard height for the pole to place the camera has to be chosen. However, the roof of buildings may be not visible from the camera. This will not have a great impact in our case because buildings' roofs do not have a high importance as mentioned Section 5.2. The main parameters of cameras to be deployed (which are explained in Section 3.1 and Figure 3) will be integrated in the attribute table of the point feature class representing the 2D placement of the camera network. Figure 13 shows the attribute table of the class \"Camera\". In this example, the range of all cameras is constant and equal to 100 m. As mentioned previously, the parameters **OFFSETB**,  
**AZIMUTH1**, **AZIMUTH2**, **VERT1**, **VERT2**, **RADIUS1** and **RADIUS2** are also constant. However, **SPOT** and **OFFSETA** may vary depending on the location of each camera. The attribute **ID Camera** represents the primary key of the class \"Camera\", _i.e._, that each camera will have a unique identifier. Once the raster DSM (which combines the ground elevation with the Building height) and the feature class that represents cameras (with their location and attribute) have been generated, the viewshed is calculated. The idea is to apply the methodology presented in Section 3.2 to assess each pixel if it is visible or not from one or more cameras belonging to the deployed network. The attribute table of the class \"Camera\" is used to provide the parameters of cameras that were required to calculate the Viewsheed. In Section 6.2, the results of the raster Viewsheed calculation based on a DSM that represents an area of Jeddah Seaport are presented.