# Visibility Assessment of New Photovoltaic Power Plants in Areas with Special Landscape Value
## 2 Methodology
### Accumulated Perception Time
#### 2.1.2 Calculation Process
By using GIS tools, it is possible to determine the Euclidean distance \(d_{i,n}\) between the area represented by cell \(i\) and the observation point represented by node \(n\) of route \(k\). Subsequently, the weighting factor \(w_{i,n}\) is obtained for each cell \(i\), as a function of the distance \(d_{i,n}\), using the expression previously shown in (1);
5. Determination of the visible height factor (\(f\hbar v_{i,n}\)). This represents the visible part (in terms of height) of the future PV plant, placed in the cell \(i\), when the observer is in the observation point represented by node \(n\). Previously, the maximum height (\(H_{pv}\)) of the PV plant was divided into segments of equal length (\(h_{seg}\)). To determine \(f\hbar v_{i,n}\), for each node \(n\), several repeated analyses are performed, following the steps outlined below: * To carry out a correct \"visibility analysis\" with a DSM, a new DSM must be generated in which the elevation of the observer in node \(n\) and of the PV plant placed in the cell \(i\) must correspond to the elevation values for bare ground in that position or cell, collected from the DTM; * Let \(h\) be the analysed height of the PV plant. In each analysis, \(h\) is increased by one segment of length \(h_{seg}\), i.e., \(h\) ranges \(h_{seg}\) to the total height \(H_{pv}\) of the PV facility; * Using GIS tools, visibility analyses are performed to evaluate the visibility factor (\(f\
u_{i,n}^{h}\)) of the PV plant with a height \(h\), placed in the cell \(i\), when the observer is in node \(n\). The result obtained for \(f\
u_{i,n}^{h}\) will take the value 0, if from node \(n\) it is not possible to see the PV plant in cell \(i\). On the contrary, \(f\
u_{i,n}^{h}\) will take the value 1, if from node \(n\) it is possible to see the PV facility with a height of \(h\) meters. The results, after applying the analysis to all the cells in the study area, are collected in a binary raster, which only stores ones and zeros; * The value of \(h\) is increased by one segment and the visibility analysis is subsequently repeated from the same node \(n\). The last analysis will be when h reaches the value of \(H_{pv}\). As a result of each analysis for each value of \(h\), a binary raster dataset of \(fv_{i,n}^{h}\) is obtained; * The values of \(fv_{i,n}^{h}\) obtained for each value of \(h\) are then summed. The result corresponds to the portion in meters of the PV plant height placed in the area represented by cell \(i\) that can be seen by the observer in node \(n\). Finally, it is multiplied by the term \(h_{seg}/H_{pv}\), as shown in expression (3), obtaining the visible height factor \(f\hbar v_{i,n}\), which represents the value per unit of the height \(H_{pv}\) seen from the observation point \(n\). \[f\hbar v_{i,n}=\frac{h_{seg}}{H_{pv}}\sum_{h=h_{seg}}^{H_{pv}}fv_{i,n}^{h}\ \ \ \ for\ \ H_{pv}>0\] (3)Evaluation of the skyline index. Generally, facilities above the skyline (with the sky in the background) will have a higher colour contrast and, therefore, higher visibility than facilities below the skyline (with the terrain in the background) [37]. In order to take this aspect into account, a skyline index (_Isk_) is calculated. Considering a PV facility in cell \(i\) and an observer in node \(n\), with the \(Isk_{i,n}\) index it is possible to give more weight to the visibility of the facility which is seen partially or completely above the horizon. Therefore, it is necessary to determine what fraction, in terms of height, of the PV facility is visible from node \(n\) above the horizon line. This index is calculated using the expression shown in (4), where only positive values are considered:  
\[\left\{ \begin{array}{ccc}Isk_{i,n}=\frac{d_{i}(\tan\theta f_{i,n}-\tan\theta h _{i,n})}{H_{pv}}&for&\theta f_{i,n}>\theta h_{i,n}\\ Isk_{i,n}=0&for&\theta f_{i,n}\leq\theta h_{i,n}\end{array} \right.