Sketch maps are drawn from memories and they are in general schematized and distorted. However, the schematizations and distortions are not random. They are a consequence during the cognitive process of perceiving, memorizing and producing spatial layout. This paper describes an empirical study to investigate the impact of distortions on similarity perception. The study is designed as a human-subjects experiment of similarity ranking with two scenarios. Subjects were presented with 45 sketch

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... s and one reference map in each scenario, and they were asked to rank the sketch maps according to their similarities with the reference map. The results of the experiment are used to develop a cognitively motivated alignment strategy for computer-based comparison of sketch maps and metric maps.