DIGITALNA ARHIVA ŠUMARSKOG LISTA
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|ŠUMARSKI LIST 7-8/2020 str. 68 <-- 68 --> PDF
as 6 meters. OffsetB is the height of the smoke that an observatory at the tower is expected to recognize. Usually horizontal scanning (Azimuth) in full circle in the range of 0°-360° and maximum vertical angle range (Vert.) are used at fire lookout towers (Figure 3).
Scanning is performed with a rotation of 360° at the designated scanning radius from the point where a lookout tower is located. The atmospheric condition changes the horizontal range where lookout from a tower will be effective. When there is mist and fog in the air, the range of vision decreases whereas it increases when the sky is totally clear (Catry et al., 2007). In a scientific study conducted in collaboration with the forestry authority to plan the networks of fire lookout towers in Turkey, the lookout radius of the towers was reported to be 18 km (Çanakçıoğlu, 1993). Therefore, 18 km was used as scanning radius in this study. The theoretical horizontal scanning capacity of a tower is 1017 km2 which is the surface area of a circle with a radius of 18 km. This area also contains some places that are invisible due to the topographic structure. For this reason, each of two towers has its own visibility performance. To evaluate the individual performance of the towers, the visibility index values were calculated with the following equation (1):
VI = 100 (1)
Where VI is Visibility index, VA is Visible area from tower, SA is Scanned area. SA is the maximum scan ability and responsibility area for each tower. In the borderline, some towers can detect different management areas. The visible, invisible and areas that were outside the detection radius of each tower were identified on the data layer obtained from the viewshed analysis of the towers. After overlapping the data with the forest areas and fire risk zones, the degree of visibility of forests in the study area from the towers were determined and the fire risk of the invisible forest areas was assessed.
Viewshed analysis was also performed for the lines on the asphalt roads in the region where there was heavy traffic because the fire statistics held by the forestry authority revealed that fires had been reported over the years by ordinary people rather than fire lookout towers. The forest areas that could be scanned by people in vehicles moving on the road were determined to understand which parts of the forest areas invisible from the towers could be viewed in that way. This was done to obtain a finding to support the idea that fire lookout towers were necessary and their performance should be assessed.
In order to measure the contribution of the roads to the lookout system, the roads with heavy traffic in the study area were identified. They were the asphalt roads that connected the settlement areas. This road layer was transferred to the geographic database. Points were placed on the road line with a spacing of 100 meters using the road lines layer in the geographic database. The viewshed analysis of these points was performed. For this analysis, the variables used in the viewshed analysis of the towers (Table 2) apart from OffsetA and the abovementioned method were used. OffsetA value was set as 1.4 meters in order to simulate the person traveling in a vehicle.
Individual viewshed analysis of each tower was conducted and it was tested if they achieved visibility rate of 70% that the forestry authority expected from the towers in rugged terrain (Figure 4). Areas that were visible and invisible from each of 28 towers in the study area within the detection radius of 18 km were identified. The statistics obtained from the visibility analysis of the towers are given in Table 3. The viewshed analysis results revealed that Çağlayan (2: number 2 in Figure 4), Hisartepe (4) and Çakmak (26) towers had the highest percentage of visible areas (>mean 80%). Geledost (9), Harmancık (24) and Manastır (25) towers, however, had the lowest percentage of visible areas (<mean 30%). The mean percentage of visible areas of the towers