3D MODELLING OF ARCHAEOLOGICAL STRUCTURES AND DEPOSITS AS A METHOD OF DOCUMENTATION AND DISSEMINATION: THE CASE OF SAN ESTEBAN ARCHAEOLOGICAL SITE (MURCIA, SPAIN)

Abstract

The research project carried out between 2018 and 2020 at San Esteban archaeological site (Murcia, Spain), under the agreement signed between the University of Murcia and the Murcia Townhall, was developed by a series of archaeological excavation campaigns in different parts of this enclave. Of the sectors involved, we have chosen the four most relevant ones: the buildings known as “Recinto I” and “Recinto II, the "Oratory" and the cemetery or Maqbara (Fig. 1). The research project objectives were the review and diagnosis of the state of conservation, the interdisciplinary study, the consolidation measures implementation and the temporary exposure of the sectors involved (Eiroa et al., 2019; 2021). San Esteban archaeological site is located within the current urban centre of the city of Murcia. It is a good example of medieval Islamic urban design in a part of the old suburb of Arrixaca, a neighbourhood outside the walls of the city that seems to have been formed during the 11th century. The graphic record in archaeology has been an intrinsic part of the discipline since its inception (Caballero, 2006). Photography and archaeological drawing have been the main ways of documenting an archaeological excavation with the double objective of achieving a topographical reconstruction and ordering the material remains by means of a stratigraphic sequence. Photogrammetry has become one of the main techniques in archaeology and is a great support in the topographic and planimetric survey of high precision and graphic representation. Drones or unmanned aerial vehicles (UAV) (Korumaz & Yıldız, 2021), high-resolution digital cameras and the appropriate software have enabled their application in numerous cases, generating extensive quality records; this can be seen in the field of medieval archaeology (Enríquez et al., 2020; García-Molina, González-Merino, Rodero-Pérez & Carrasco-Hurtado, 2021; Ruiz, Gallego, Peña, Molero & Gómez, 2015) or in the study of medieval buildings. In the fieldwork at San Esteban archaeological site, a series of topographic instruments and photogrammetric software in a 4D sequence have been used; this has allowed both the documentation in the plan of the archaeological structures, as well as the location of the deposits in a stratified sequence. For the generation of these diachronic three-dimensional (3D) models, the Agisoft Metashape software was used, based on the SfM (Structure from Motion) technique of geometric analysis of clusters of pixels from photographic perspectives with different parallaxes. This technique has become an important methodological dynamic in the archaeological record (Maldonado, 2020: 196-203), improving the work of topography in the geomatics field (Marín, 2020). The authors created also a vector plan of the documented structures; the registration through the SfM photogrammetry was also carried out (Fig. 6), together with the depositional process of the sectors involved (Charquero, 2016; Semeraro, 2008). As an example of this methodology, a practical case within the building known as “Recinto I” is highlighted (Fig. 7). This exercise, developed in the Espacio 4, has made it possible to measure the excavation process by continuously recording the activity at a specific point of the site. Also, a detailed delimitation of the extension and volumetric shape from the contact surfaces of a stratigraphic unit has been made, reflecting the physical aspects of the deposit itself at a visual level (Fig. 9). This method emphasises the documentation of the depositional process from a 3D and orthophotographic perspective of the strata (Montalvo, Dyrdahl, Cantisani, Fabririis, & Vinci, 2020; Orengo, 2013), allowing greater detail in the delimitation and shape of the excavated stratigraphic units and assemblages, either synchronously or diachronically. All this graphic evaluation work allows for new possibilities in relation to the processes of data management of an archaeological intervention. Their implementation in a geographic information system (GIS), being the graphic base in a vectorised digital format, opens the possibility of creating environments where interrelated information of their virtual recreation is available, from the 3D models obtained during the archaeological excavation. All this makes 3D models versatile products for use in archaeological heritage research and dissemination. From the latter approach, it is the geometric basis for the subsequent development of virtual surveys and 3D recreations or their reproduction in models at variable scales of the whole or a part of them, in greater detail, using 3D printers.