Geomorphic analysis of digital elevation model generated from vintage aerial photographs: A glance at the pre-urbanization morphology of the active Campi Flegrei caldera

Accurate digital elevation models represent the basic tool for a large spectrum of applications, including geological, architectonic, archaeological, and urbanistic studies. However, aggressive urbanization may significantly alter the morphology of areas of interest. Such is the case of the active Campi Flegrei caldera in southern Italy, where all the buildings, facilities, infrastructures, quarries, and landfills altered the original morphology. To avoid these limitations, we analyzed a set of vintage aerial photographs acquired in 1943 by Italy's Military Geographic Institute (IGM). We reconstructed the study area's topography before the deep anthropic modification from the 1960s onwards by applying Structure-from-Motion photogrammetric processing. On the up to 1-m resolution orthomosaic and a historical-Digital Surface Model (hDMS), we conducted geomorphic analyses along 18 longitudinal stream profiles outside, across the border, and inside the caldera, underlining that the rectilinear sections with preferred orientations and increased incision values that suggest a strong tectonic underpinning on valley incision and stream paths. The analysis of the spatial distribution of sinuosity, SL index, Chi value and knickpoint maps suggests the presence of tectonic lineaments that influence the stream network from the pre-caldera (>40 ka) to recent times, producing an articulated caldera geometry that affects the localization of volcanic vents. We also compared the 1943 hDSM and 2013 LiDAR (Light Detection and Ranging) DSM, spotlighting the areas with the most significant differences due to anthropic intervention obliterating critical features, supporting our motivation to use this base. Overall, the results indicate that long-lived volcano-tectonic and tectonic structures control the orientation of drainage patterns and their re-arrangements during volcano-tectonic deformation phases. Finally, a comprehensive structural map, based on the merged 1943 hDSM and the bathymetric DEM, is presented together with a conceptual evolutionary model of the stream network is proposed. Under the proper acquisition conditions, vintage aerial photographs can provide a useful tool for morphological analysis applicable to several topics in the geosciences.