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Raw material extraction processes face problems that differ from those encountered in most production systems. Production steps are often quite simple, but material handling and inprocess inventory are complicated by a constantly changing environment. The factory that delivers raw materials must be mobile and flexible to handle variations that occur in the raw material environment. Techniques used to analyze problems common to factories at fixed locations are often inappropriate for extraction processes. A simulation model of the extraction process can be used to provide input to many production planning steps. It should consist of three components. The production process can be modeled following the traditional procedures of discrete event simulation. The raw material component will need to focus on raw material characteristcs with the greatest effect on system production. The third segment of the model represents the interface between raw material characteristics and the production process. The three model components were integrated for extraction of trees from the woods in a model called SAPLOS (Simulation APplied to LOgging Systems). SAPLOS is programmed in the GASP IV simulation language. Event subroutines were developed for subsystems of timber harvesting at critical locations in the raw material environment. Components of the system critical to cost and production were tested in detail. The raw material model was tested for the adequacy of tree size generation and relative tree placement. Sensitivity of skidding production and cost to changes in critical raw material characteristics were thoroughly investigated. Usefulness of the model in providing data for management decisions was demonstrated through a case study of cable yarding. The result of this process is a general, flexible model that can be used by practitioners, students, and researchers in the planning process for timber harvesting. Modeling procedures and uses of the model in production planning can be extended to industries where the factory must be mobile and the work environment is constantly changing.