In this study, we investigate the correlation between security and spatial resolution -a measure of service quality- in cluster-based wireless sensor networks (WSN). We analyze the tradeoff between these two concepts resulting from the limited channel capacity. We propose a method based on combinatorial optimization to determine the best tradeoff between security and spatial resolution for cases where network capacity is not sufficient to support required levels. The proposed method involves formulation of an optimization problem by utility maximizing approach and its solution by a heuristic algorithm providing minimal operational complexity. We also present some numerical results obtained by application of the proposed method to a sample sensor network. The numerical results demonstrate the effect of utility function parameters on the optimal solution.