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ŠUMARSKI LIST 5-6/2020 str. 61     <-- 61 -->        PDF

to extract DTM and slope layers specific to the Köyceğiz FED.
The digital data layer indicating the locations of current fire lookout towers in the study area was generated by using provided UTM coordinates (Table 2). The road layer of the study area, obtained from Köyceğiz FED, was also included into the GIS database to be used in the process of searching for possible new lookout towers. 
Visibility Analysis of Current Towers – Analiza vidljivosti sadašnjih tornjeva
Visibility analysis measures the visibility of viewing capability of a certain location to see the surrounding area. In this study, the visibility performance of five fire lookout towers was analyzed by using the “Observer Points” method in ArcGIS 10.4 program. Figure 2 indicates the flowchart of the methodology. The “Observer Points” method requires data inputs in the attribute table of fire lookout towers. These required inputs include elevation, tower height, smoke visibility height, horizontal view angle, visibility range, and vertical view angle. Table 3 indicates the required data to be entered in the attribute table of the lookout towers layer.
The elevation data from Table 2 were included in the attribute table of the towers. The height of each lookout tower was 10 m and smoke visibility height was estimated as 100 m to see smokes rising from the ground as well as flames on the ground during a forest fire. In order to observe all of the forested areas around the lookout towers, angle of vision and angle of view was set to 360 degree and +/-90 degree, respectively.
In this study, the visibility ranges of fire lookout towers were set from 10 km to 20 km based on the information obtained from Köyceğiz FED. The visibility range of 10 km scanning radius is widely preferred distance for rough terrain for detecting the smoke under optimal weather conditions (Kucuk et al., 2017). However, different scanning radiuses have been used in other locations. In the USA, scanning radiuses ranging from 13 to 32 km have been used while a radius of 24 km is typical in a large part of western USA. In the southern and southeaster US, where visibility is poorer due to humidity, a 10 to 13 km radius is most often used (Davis, 1959). 
After inputting the required data in the attribute table of fire lookout towers, the “Observer Points” method in the “Spatial Analyst” tools was implemented. This identified the portion of forestlands that were visible and nonvisible from the fire lookout towers in the study area. In addition, for all forest lands that were determined to be visible, visibility was categorized as to whether areas could be observed by one, two, three, four, or five towers.
Suitability Analysis of New Towers – Analiza prikladnosti novih tornjeva
After evaluating the visibility capabilities of current lookout towers, alternative towers were investigated to increase the total area of forest lands visible from the existing lookout towers. The appropriate locations for new towers must be determined not only based on their visibility capabilities but also the suitability of their potential locations. Suitability analysis using GIS techniques allows users to qualify, compare, and rank alternative sites based on a set of criteria. In this study, suitable locations for new lookout towers were searched by considering specified factors including distance to roads for accessibility, minimum elevation, low ground slope for easy construction, and topographical feature (i.e. ridges) to ensure a 360° angle of vision for finished construction (Harvey, 2015).
In the first stage of the suitability analysis, the “Proximity” tool in ArcGIS 10.4 was used to delineate accessible areas near roadways (i.e. within the 100 meters) by generating buffer polygons around a spatial road network (Clark, 2005). Then, other factors (minimum elevation, maximum ground slope, and ridges) were addressed within the border of buffer polygons. The minimum elevation of potential towers was set to 500 meters for better visibility of the forest land in the study area. Thus, the areas that were equal to or more than 500 meters in elevation were extracted from the DTM of the forest land in the study area as a starting point for analysis.