UAV photogrammetry for archaeological site survey. 3D models at the Hierapolis in Phrygia (Turkey)

Abstract

Highlights:• The paper aims to validate UAV photogrammetry as a very flexible tool for archaeological areas; a fix wing eBee device by Sensefly is tested.• Derived DSM and aerial orthoimages in complex areas with different formal traits are discussed, targeting high mansory ruins and collapsed parts.• Up to 2 cm accuracy and high resolution 3D models are convenient to extract morphological data. Abstract:Unmanned aerial vehicle (UAV) photogrammetry has shown a very rapid development in many fields, especially in archaeological excavation areas and architectural complexes, where it offers a detailed generation of three-dimensional (3D) data including the possibility of updating over time. It also proves to be a very flexible tool applicable to many types of complex areas with a variety of different features. The use of aerial acquisition provides highly effective results, adding to both rapid capture and lower costs. In fact, today in the field of archaeological research, great efforts are invested in the generation of very large-scale models and orthophotos, and the technology seems to promise further future developments, not only from the terrestrial (orthogonal) point of view, but also from the nadiral direction from a low altitude, as a preferential and often optimal point of view. Here an effective workflow for photogrammetric product generation is presented for selected case studies in some monumental areas of ancient Hierapolis in Phrygia (Turkey), in which the Italian Archaeological Mission of Hierapolis (MAIER) has been working since the 1960s. The recent experiences achieved by UAV photogrammetry are quite innovative. The variety and complexity of the buildings, as well as the height of their ruins, offer numerous challenges, which are interesting to deal with. The 3D aerial survey was performed for multiple purposes with the eBee system by Sensefly. Specific attention was paid to the digital surface model (DSM) and aerial orthoimages of three test areas: the Plutonium area; the Thermal Bath-Church; and the Necropolis. Starting from the same technical approach, a comparative assesment among the three sites was carried out, taking into account the specific goals, the type of the structure and the terrain conformation.Keywords: unmanned aerial vehicle (UAV); digital surface model (DSM); aerial photogrammetry; archaeological heritage; 3D modelling; data integration Resumen:La fotogrametría con vehículos aéresos no tripulados (Unmanned Aerial vehicle, UAV) ha mostrado un desarrollo muy rápido en muchos campos, especialmente en áreas de excavación arqueológica y complejos arquitectónicos, donde ofrece una detallada generación de datos tridimensionales (3D), junto con su actualización en el tiempo. También demuestra ser una herramienta muy flexible aplicable en muchos tipos de áreas complejas, con diferentes características formales. El uso de la toma aérea proporciona hoy resultados altamente efectivos, lo que aumenta la rapidez de adquisición y menores costes. De hecho, hoy en día en el campo de la investig...

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“…Several published papers report to what extent UAV photogrammetry has been affirmed in the field of rapid mapping systems, for a numerous series of reasons: because you can bring objects closer and document difficult areas as well (Xiao et. al 2012;Chiabrando et al 2017a). Moreover, the recent use of oblique images, already known in airplane systems (Höhle, 2008) allow to improve the knowledge of the surveyed object with different non-conventional point of view.…”

“…This last point was a planned decision precisely to increase the accuracy of the photogrammetric processes for areas shooted from very low UAV flights. In 2015, the Hierapolis site had already been affected by UAV flights and consequent generations of DSM and orthophotos of some main areas of major transformation of the city, for both massive excavations and restoration works (Chiabrando et al 2017a). We cannot make direct comparisons with the 2015 flights, since at the time the images were acquired with a fixed wing drone at a higher altitude.…”