Shadows analysis and viewshed georeferencing for dating of historical photographs

New possibilities for chronology studies of lost built heritage

Abstract

To facilitate the use of historical photography in studies of lost heritage and its modifications over time, it is crucial to have dated images, in order to chronologically organize the visual information they contain. Although there are dating methods based on optical metadata for digital images, little attention has been paid to historical photographs, where no optical information is available, and there is insufficient data redundancy to infer them by automatic means. To address this problem, the present work proposes a method to date historical photographs based on the analysis of the shadows contained in the image, as they are known to evidence a measurable position of the sun. The key observation is that, having recovered the volumetry of the scene and its georeferencing by photogrammetric means, it is possible to project the visual information of the shadows on this geometry and thus obtain magnitudes and angles that, with the help of an astronomical formula, allows determining the time of day and the day of the year associated with them. Based on this principle, a processing workflow is proposed and validated in a case study. The results show that the method generates consistent and replicable results in different scenarios and photographic material; the margin of error has been calculated, and open issues for future studies have been identified. This confirms the validity of the approach proposed, and its feasibility to complement existing dating methods, thus improving the use of historical photography in studies of lost heritage with interest in the evolution of its interventions.