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according to the selected system. Therefore, to calculate efficiency or cost coefficients of the decision variables, previously conducted studies were used (Aykut et al., 1997; Acar, 1997; Acar, 1998; Çağlar, 2002; Öztürk and ªentürk, 2006; ªentürk et al., 2007). Technical parameters of selected systems as man power, animal power (oxen), skidder (MB Trac), small size cable crane (Koller K 300), medium size cable crane (Urus MIII) and sledge yarder (Gantner), used in above mentioned literature are given in Table 1 and Table 2.
In order to calculate the required technical parameter information for each stand, Geographic Information Systems (GIS) were used with ArcGIS 10.2 TM software. Geographic data used in this study were acquired from topographical maps produced by the General Command of Mapping of Turkey containing 10 m contour interval elevation data and digital stand type map. Forest road network, which is an important technical limitation for timber extraction systems, was also incorporated into the model. GIS queries or functions such as „TIN” (Triangulated Irregular Network) data „average slope” or „Generate Near Feature” were also handled in the determination of the working direction and calculating the maximum distances from the nearest forest roads. Similarly, it is important to keep the cost, time, loss of the amount of timber as low as possible. Moreover, it is generally known that forest soil, standing trees, and wild life were natively affected depending on the selected timber extraction system. These results or deterioration in forest lands have adverse effects on forest soils, erosion and environmental destruction (Fairweather, 1991; Ampoorter et al., 2007). Therefore, it is so essential to integrate environmental parameters into the harvesting planning process as well as efficiency and financial considerations. Considered parameters were taken from the mentioned literature given for technical parameters (Table 2).
Damage degree values ranges between 0 and 4, taken from Eroğlu et al. (2009). Damage degree of animal power system were not put into account in the study which has already been mentioned, therefore, the mean values of man power and skidder were used in animal power damage degree in this study. Timber loss quantities used in the model were taken from literature. For instance, timber loss from man power was taken as 17% (Gürtan, 1975). Other values were taken from Acar et al., (2000) previously estimated as 5%, 3% and 1% respectively for the animal power, skidder and cable cranes.
General Structure of the Timber Extraction Model – Generalna struktura modela privlaèenja drva
In order to determine the best suitable timber extraction system, Integer Linear Programming (ILP) was used, as it is a powerful tool for generating an optimal solution which enables further sensitivity analyses (Oborn, 1996; Eker and Acar, 2006; Bont et al., 2015; Çalıºkan and Karahalil, 2015). Different factors affecting timber extraction systems were integrated, and a number of operational planning strategies were developed to evaluate the trade-offs among timber extraction systems. Different planning strategies were developed with various characteristics and solved with LINDO™ software (LINDO, 2016).
Objective Functions:                           Zmax= Profit; Zmin= Timber Loss;                                Zmin= Time; Zmin= Damage                (Eq. 9)
Subject to:                                                                                           (Eq. 10)
                                                                                          (Eq. 11)
                                  Income-Cost-Profit=0                  (Eq. 12)