Natural hazard studies are often based on the spatial analysis of remarkable past events. However, historical information contains imperfections in its spatial and temporal locations and in the description of events. In this paper, we want to show the benefits of a system approach, that take the context of use of the visualization into account during the early phase of storing up data in the GIS, in order to make maps with imperfect data. We base on a concrete example: making a map of the chronology of phenomena during a past flood event, for decision makers who are lay users in cartography.