The photogrammetric survey of an historical map

This study aimed to test the effectiveness of photogrammetric surveying for the digital reproduction of historical documents for which traditional direct scanning techniques are unsuitable due to their fragility or large dimensions. The experiment was conducted on a mid-18th-century territorial map, known as the “Calcato”, that in Italian means ‘trodden’, in the sense of a landscape that has been explored and described through walking it on foot. The work focused on the possibility of obtaining high-resolution and metrically accurate digital outputs, starting from the design of a proper flight plan that defined the specific parameters for the photographic acquisition phase, based on the use of a metric camera. Image acquisition was performed indoors, through oblique shots taken from external points. To compensate for the non-planarity of the surface, due to the semi-rigid nature of its support, a high-precision digital surface model (DSM) was produced to enable accurate orthorectification. To ensure the geometric reliability of the digital copy, a high-precision topographic support was established, defining the coordinates of six control points, with sub-millimetric precision. The characteristics of the resulting orthophoto validated both the approach and the execution, also providing a valuable tool for territorial study and analysis. The results, in addition to contributing significantly to the PRIN project’s objectives, offer a reliable methodological reference for the reproduction of large-format historical documents. This outcome is of considerable value both in terms of safeguarding documentary heritage and promoting and disseminating its informational content.