The Eldgjá eruption in Iceland in the late 930s CE seems to have had tremendous repercussions. Twenty-eight historical documents mention hardships in the aftermath of the eruption in Europe. However, only a few documents were written during the time in question. Interdisciplinary research means stepping “outside” of your own field. Evaluating the quality of “foreign” material is essential. A GIS map offers a useful methodological approach in this case: more details can be added to the data whereby the reliability of historical documents and the impact of events like the Eldgjá eruption and its regional scope become easily visible.