Geovisualization applications typically organize data into layers. These layers hold different types of geographical fea-tures, describe different characteristics of the same features, or represent those features at different points in time. Lay-ers can be composited in various ways, most often employing a juxtaposition or superimposition strategy, to produce maps that users can explore interactively. From an HCI perspective, one of the main challenges is to design interactive composi-tions that optimize the legibility of the resulting map and that ease layer comparison. We characterize five representative techniques, and empirically evaluate them using a set of real-world maps in which we purposefully introduce six types of differences amenable to inter-layer visual comparison. We discuss the merits of these techniques in terms of visual in-terference, user attention and scanning strategy. Our results can help inform the design of map-based visualizations for supporting geo-analysis tasks in many application areas.