Casa del Samarín, una estación de grabados rupestres en deterioro. Documentación, análisis y diagnóstico en Los Llanos de Ifara, Granadilla, Tenerife

Abstract

[EN] In the archaeology of the Canary Islands (Spain), there are many studies based on the usage of new technologies to contribute to the identification and description of rock art engravings through high-resolution digital models (Martín, 2005; Martín, Velasco, González & Ramírez, 2007; Senén & Cuenca, 2016; Navarro & Cancel, 2019). This paper is supported by these documentation techniques and digital analysis in order to deepen into the characterization of the damaged rock art station Casa del Samarín (House of Samarín), or Tagoro del Rey, in Los Llanos de Ifara, south of the island of Tenerife (Figs. 1). Twenty-one panels conserved in situ were documented (Fig. 6). Geometric-linear, geometric with an oval and rectangular trend and figurative ones can be distinguished. The blocks [1] that compose the engravings station belong to a rocky basalt outcrop, to which other free-standing blocks are attached, forming a circle. The shape that describes this set of blocks is defined as a "cabin" or circular-shaped structure.

This set of engravings, made on a basalt rocky outcrop with a planar factory, show a tendency to suffer from exfoliation and are affected by internal stresses. The intrinsic characteristics of this stone support, together with their exposure to anthropic actions and strong insolation, condition its fragility, with the risk of losing part of the representations that it houses. Given the threat posed by its gradual deterioration, we seek to ensure its digital preservation through precision three-dimensional (3D) records, the engravings inventory, the record of their conservation state and the understanding of the degradation processes that are affecting the outcrop. What has been explained will be addressed quarterly, to observe the evolution of any material changes every three months.

The registration work consisted of taking four photogrammetric surveys in eight months; the surveys were georeferenced by means of a centimetric Global Navigation Satellite System (GNSS) and a total station. Structure from Motion (SfM) technology enabled the researchers to generate high-precision 3D models in an affordable way, not only in terms of cost but also ease of use. Digital copies with Geographic Information System GIS technology were extracted from them, being exportable in shapefile format (Fig. 7).

As regards the documentation of existing pathologies, assuming standardized lexicon and classification criteria (IPCE, 2013), together with a rigorous information systematization, was key for achieving agile handling of the data collected and for facilitating monitoring tasks (Fig. 8). Damage maps were created for collecting the location and scope of the alterations. The complex volumetry of the outcrop and the varied orientation of the panels marked the need to resort to 3D editing so that all their faces could be properly registered (Fig. 10). This project was performed with a 3D design program, Blender®.

Thanks to an imaging analysis process, internal textures of 3D models also provided relevant graphic support for the pictographic content and the conservation state (Figs. 11 & 14). DStretch® (Harman, 2008), a plugin implemented in the scientific image processing software ImageJTM, was used for this purpose. To conclude, researchers relied on CloudCompare (Girardeau-Montaut, 2015), an advanced 3D data processing software, to tackle a morphometric analysis that allowed us to detect the appearance of formal changes along with the recorded sequences (Figs. 12 & 15). In this process, the distances between two records, taken after six months, were computed with the Cloud to Mesh (C2M) tool, based on the Chamfer distance algorithm (Ruiz et al., 2016: 120).

Registration file cards and damage maps clearly highlighted the main conditions: material losses (shown in orange) and breaks (in green) have affected the outcrop in a generalized way. Furthermore, sedimentary deposits (blue) are concentrated in interstices; while lichen colonies (idem) do so in the least sun-exposed areas. The use of DStretch® highlighted modern excoriations of anthropogenic origin and contributed to distinguishing recent material losses from the older ones, already affected by an incipient patina. Finally, thanks to morphological analysis, a new detachment (Fig. 15b) and a generalized displacement of exempt elements (Figs. 12 & 15) were detected. These displacements indicate outstanding manipulation, which could lead to decontextualizations or new fragmentations.

Regarding the archaeological interpretation, macroscopic observation of exempt blocks located in the vicinity of the station and the zenith representation of the immediate environment from photogrammetry, have shown that they are forming a set of attached structures (Fig. 13). The site redefinition and the diagnosis of its very weakened defensive system show the need to intensify the archaeological study of this area, so emblematic for the archaeology of the south of Tenerife, in addition to establishing preventive conservation measures that can contribute to its stabilization.

[ES] En el presente artículo nos respaldamos en técnicas de documentación y análisis digital para profundizar en la caracterización de la estación de grabados rupestres denominada como Casa del Samarín o Tagoro del Rey, situada en Los Llanos de Ifara, Granadilla de Abona (Tenerife). En el enclave arqueológico, de adscripción guanche, se analizan 21 paneles con diversas tipologías de grabados. Se pueden distinguir los clasificados como geométricos-lineales, geométricos de tendencia oval y rectangular, así como los figurativos. Los bloques que conforman la estación de grabados pertenecen a un afloramiento rocoso de basalto, desde el cual se adosan otros bloques que se disponen formando un círculo. La planta que describe el conjunto de bloques se define como "cabaña" o estructura de superficie. Las acciones antrópicas y una fuerte insolación, unido a la problemática intrínseca de las rocas basálticas, condicionan la conservación de este conjunto. Ante la amenaza que supone su deterioro paulatino, perseguimos asegurar su representación digital, inventariar sus grabados, monitorizar su estado de conservación y comprender los procesos de degradación que le afectan. Para ello nos valemos de la georreferenciación y el levantamiento fotogramétrico del sitio, así como de calcos digitales, para generar registros tridimensionales (3D) de precisión. Esta documentación geométrica se realiza con una secuenciación trimestral para observar la evolución de los cambios materiales. El registro patológico se realiza por medio de fichas de registro, encargadas de enumerar, describir y clasificar las lesiones observadas por escrito; también de mapas de daños 3D, que recogen su alcance y ubicación. Además, las propias texturas de los modelos 3D se someten a un procesamiento de imagen para destacar las características cromáticas; por otra parte, el análisis computacional de sus geometrías permite detectar la aparición de cambios formales a lo largo de las secuencias registradas. El protocolo llevado a cabo en cuanto a registro y documentación incidieron no sólo en el diagnóstico y análisis de esta estación de grabados, sino también en la redefinición arqueológica del propio yacimiento.Lo más destacado:Se aborda el registro fotogramétrico y calco digital de una estación rupestre, en avanzado estado de deterioro, con el fin de registrar su estado actual y contribuir a su conservación digital.La observación en detalle de los elementos analizados y la representación cenital a partir de la fotogrametría, han contribuido a la redefinición arqueológica del propio yacimiento.El análisis computacional de la información geométrica, unido a un registro del estado de conservación sistemático, nos ha permitido subrayar los problemas que afectan a su preservación.