In studying the performance of any matching technique for image registration it is important to select an appropriate reference image. In this paper a set of a priori probabilities of detection based on explicit information about the materials which surround the edges is used in a strategy for reference selection. This set of a priori probabilities of detection is obtained by finding the frequency of occurence of material edges. The correlation results for three edges, each belonging to a different material class, are presented. The results of two criteria for optimum reference selection, namely the Neyman-Pearson and the ideal observer, are used to show that the best choice appears to be a compromise. This minimizes the total probability of error while giving a higher probability of correct acquisition for a fixed probability of false alarm.