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Aerial photography as a methodology to investigate the siege of Groenlo. The archaeology of droughts
The summer months of 2018 and 2019 were exceptionally dry and many crop marks appeared in the landscape. Various factors, with differing timeframes, cause appearance and disappearance of crop marks. The French Encampment near Groenlo shows this very well: the southern part was visible only in August 2018, while the northern part was visible only in September 2018. Taking infrared imagery into account, the crop mark season can be extended. Vegetation shows a difference in growth rather than a difference in reaching maturity, as shown by the Frisian Rampart. The problem of the right place, right time, and suitable sensors can be mitigated by using drones (uavs). Furthermore, it is relatively easy for the public to engage in this kind of aerial research with the expected increase in drone use by the general public.