Study and enhancement of the heritage value of a fortified settlement along the Limes Arabicus. Umm ar‐Rasas (Amman, Jordan) between remote sensing and photogrammetry

The Limes Arabicus is an excellent laboratory for experimenting with the huge potential of historical remote sensing data for identifying and mapping fortified centres along this sector of the eastern frontier of the Roman Empire and then the Byzantine Empire. Remote sensing, combined with modern surveying techniques and tools such as photogrammetry and laser scanners, now makes it possible to identify, document and study ancient settlements in the area, as well as to develop site valorisation programmes, including the design of real and virtual routes for better use of archaeological areas. We offer here a preliminary contribution to the site of Umm ar-Rasas (Amman, Jordan), a fortified town on the Via Traiana Nova.

Since the early 1800s, certain explorers have recorded this location, which is marked by the presence of a tetrarchic castrum and a Byzantine settlement to the north. It has been the subject of archaeological study since the second half of the 1980s. The excavations, carried out by the Studium Biblicum Franciscanum of Jerusalem (Piccirillo and Alliata 1994) and the Swiss Max van Berchem Foundation (Bujard 2008), involved a portion of the castrum and, in particular, the settlement north of this fortified site, bringing to light precious mosaic floors that have made Umm ar-Rasas famous. From 2013 to 2019, the Italian National Research Council's Institute of Heritage Science (CNR-ISPC) conducted topographic and 3D surveys of the Saint Stephen, Bishop Sergius, and Saint Paul Churches in the inhabited area north of the castrum, in collaboration with the Jordanian Department of Antiquities (DOA) and co-financed by the Italian Ministry of Foreign Affairs and International Cooperation (MAECI), in order to both document the status of conservation of the mosaic floors and prepare for better tourist access to the area (Gabrielli, Portarena, and Franceschinis 2017). Beginning in 2021, the CNR-ISPC investigations concentrated on the castrum and the region to the north and east of the Byzantine village.

Topographical and architectural surveys were carried out to confirm on the ground the crop marks discovered through the study of historical and modern remote sensing images, as well as to document the fortification's construction phases. An examination of multitemporal remote sensing documents obtained from aerial and satellite platforms preceded the field studies. To begin, historical aerial photographs taken by Sir Marc Aurel Stein in 1939 and space photos acquired by the Corona KH-4B and Hexagon KH-9 satellites throughout the 1960s and 1970s were georeferenced, processed, and interpreted. The analysis of aerial photos taken from 600 to 1200 feet in altitude and space photos with spatial resolutions ranging from 1.8 to 0.60 m allowed for the documentation of preserved structures as well as the identification of archaeological crop, dump, and shadow marks linked to buried ancient features. This research endeavour enabled the design of ground checks for the investigation and reconstruction of the site's ancient topography and historical landscape. Furthermore, the panchromatic and multispectral data from two very high-resolution satellite images, a Pléiades 1B from 2020 (max. spatial resolution 0.5 m) and a Pléiades Neo from 2022 (max. spatial resolution 0.3 m), were processed to identify other marks associated with buried ancient remains and to generate orthorectified images used as base maps during field surveys. Finally, the archaeological elements acquired from multitemporal recording improved the site's archaeological map when combined with comprehensive plans of the castrum and excavated sectors of the inhabited area made during recent topographic surveys or previous studies. The castrum walls were recently topographically surveyed and 3D architecturally surveyed using the following techniques: terrestrial photogrammetry with a Canon 5D Mark II (24 MPixel) and laser scanner survey with two laser scanners, Faro 120 and Faro 330 x. Ad hoc systems were created with the goal of finishing the high-resolution documentation of the masonry; specifically, photogrammetry techniques were applied with multiple pictures to acquire a higher chromatic definition of the surfaces and better photographic detail. Instead, a correct description of the distorted walls was achieved through the capture of 97 laser scanner scans, which showed substantial irregularities. Many relevant and credible facts relating to the building phases and subsequent alterations of the castrum's walls, as well as its state of conservation, have been collected and archived over the course of two years. An initial mapping of the locations of stone material extraction, water reserves, canalization systems, and the organisation of cultivated land around the community was completed. The data are being combined into an archaeological map that will be put into a GIS platform to document the ancient topography of Umm ar-Rasas and will serve as a knowledge foundation for the site's valorization initiatives. With the collapses that hide much of the interior of the castrum, it is impossible to identify a construction phase of the Severian age, which has been hypothesised based on items found in earlier excavations. It is hoped that future missions will be able to complete the aerial laser scanner surveys and conduct geophysical prospecting, allowing for the acquisition of elements on the topography of the area inside the walls of the tetrarch fortification and the highlighting of any pre-existing structures, with the goal of creating a more updated castrum plan.