From Archived Historical Aerial Imagery to Informative Orthophotos: A Framework for Retrieving the Past in Long-Term Socioecological Research

Aerial photographs have been systematically collected from as early as the 1930s, providing a unique resource to describe changes in vegetation and land cover over extended periods of time. However, their use is often limited by technical constraints, such as the lack of ground control information and precise camera parameters, which hamper an accurate orthorectification of the raw imagery. Here, we describe the historical aerial photographs orthorectification (HAPO) workflow, based on a conventional photogrammetric procedure (the direct linear transformation (DLT) Method), integrated as a geographic information systems (GIS) procedure, in order to perform the image orientation and orthorectification, thereby converting historical aerial imagery into high-definition historical orthoimages. HAPO implementation is illustrated with an application to a rugged landscape in Portugal, where we aimed to produce land-cover maps using an aerial photograph coverage from 1947, as part of a study on long-term socioecological dynamics. We show that HAPO produces highly accurate orthoimages and discuss the wider usefulness of our framework in long-term socioecological research.