Methods for mapping volcanic events overlaid across time

This article describes a number of issues encountered when developing maps of past natural phenomena in the field of volcanology. In order to enable experts to exploit geographical data related to this topic, maps showing the temporal chronology of such events are required. Developing useful maps are made more complex by the coexistence of numerous phenomena in the geographic same space over time, which entails the management of spatial overlays, and includes the difficulty of integrating temporal information in static maps. In this article, we present our approach to the overlaying and temporal ordering of natural phenomena presented as information in maps, using the example of lava flow data. These data are derived from a case study of volcanic hazard affecting La Reunion Island in the Indian Ocean. We explore different methods for mapping evolution over time, for instance 'map collections' or 'small multiple maps', the use of both color or hue and saturation to represent the dates of events, and the use of 'semi-transparency' to preserve the representation of past events overlaid one on the other. Legibility and effectiveness of the map were a prime concern in this exploratory analysis.