The Photogrammetric Survey of an Historical Map

Abstract. This study investigated the effectiveness of photogrammetric surveying for the digital reproduction of historical documents unsuited to traditional scanning techniques due to their fragility or large dimensions. The experiment focused 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 objective was to obtain high-resolution, metrically accurate digital outputs by designing a specific acquisition protocol, including a tailored flight plan and the use of a metric camera. Image acquisition was conducted indoors through oblique photography from external positions. To address surface irregularities caused by the semi-rigid support, a high-precision digital surface model (DSM) was generated to enable accurate orthorectification. Geometric reliability was ensured by establishing a topographic control network, defining the coordinates of six ground control points with sub-millimetric precision. The resulting orthophoto validated both the methodological approach and its implementation, providing a reliable and detailed representation suitable for territorial analysis. The outcomes contribute to the objectives of the PRIN project and offer a replicable methodological reference for the digital reproduction of large-format historical documents, supporting the safeguarding of documentary heritage and the dissemination of its informational content.