Application of automated photogrammetry and lighting techniques with GIS tools for visualisation and analysis of a slab with anthropomorphous reliefs

Abstract

We present a methodological approach for the representation, visualisation and analysis of three-dimensional (3D) models of meaningful details in stone reliefs provided by digital documentation tools and subsequent processing. For this aim, anthropomorphous shapes engraved on a flat stone slab found in Sierra de Fontcalent (Alicante) are studied. The object under consideration was located near two archaeological sites, Cova del Fum–a cave with presence of the Chalcolithic material (López, 2010)–and the archaeological site of Fontcalent, with remains from different phases of occupation spanning from 7th-6thBC to the 20thcentury (Ximénez, 2012).

In the last few years, the use of digital tools provided by new technologies and software development has left traditional work methodology behind (De Reu et al., 2014)while enabling the development of new approaches to both minimise heritage alteration and provide objective and accurate information (Lopez-Menchero, Marchante, Vincent, Cárdenas, & Onrubia, 2017). 3D documentation allows recording of cultural heritage at a reasonable cost with precision and quality through digital photography and SfM (Structure from Motion) photogrammetry with specialised software (De Reu et al., 2013).

In this project, recording and documentation with digital photography and automated photogrammetric techniques are applied to the Fontcalent stone slab for its digitisation and subsequent 3D representation. From the resulting model, a two-folded line of study is obtained. On the one hand, a Digital Elevation Model (DEM) is generated to study the microtopographies of the stone with geographic analysis techniques provided by Geographic Information Systems (GIS) from different lighting conditions and surface reflections, which are calculated by hillshading or LRM (Local Relief Model) for the interpretation of the object (Carrero-Pazos, Vilas, Romaní, & Rodríguez, 2014;Gawior, Rutkiewicz, Malik & Wistuba, 2017).On the other hand, from both the 3D model and the point cloud, the study is completed with the application of the methods of analysis and visualisation based on the Morphological Residue Model (MRM) which stands out every single detail of the surface morphology of the object (Caninas, Pires, Henriques, & Chambino, 2016;Correia, Pires, & Sousa, 2014). Further visualisations are based on Reflectance Transformation Imaging (RTI) which provides different shadows and reflections over the object from the application of a multidirectional illumination (Happa et al., 2010; Malzbender, Gelb, Wolters, & Zuckerman, 2000; Mudge et al., 2010).

The  results thus obtained of the  Fontcalent  stone  slab  allow  us  to  visualise  several  characteristic  elements. The anthropomorphous figure awaking interest is also combined with the figure resulting from different visualisations applied with GIS techniques which may resemble a zoomorph. The use of visualisation techniques shown in this study has been fundamental in order to recognise the latter element. The composition reveals a zigzag line already appreciated before the study so that it is interesting to check if visualisations based on GIS techniques are able to highlight it though being shallow incisions. In our experience regarding this study, visualisation by using the hillshading technique shows a greater level of 3D detail than that provided by the application of the sky-view factor technique which offers a flattering view. However, the former  technique  may occasionally show  shadows  which  hide  other  details, unlike the  latter  technique  which  plots  the entire slab surface illuminated while differentiating the associated microtopography on the basis of its marks. The use of shaders  in  combination  with  hillshading  and  particularly  combined  with  high  pass filtering, contributes  to improving the visualisation and accuracy of shadowed areas. As a result, we conclude that the results obtained in this work by lighting techniques with GIS add a greater level of detail in comparison to those provided by the mesh or the point cloud.

The study of the Fontcalent stone slab paves the way for two working hypotheses to be developed: on the one hand, its anthropological origin possibly related to the Chalcolithic, and on the other hand, its study as natural geological formations with ichnofossils.

The digitisation of cultural heritage with available 3D technologies should be a mandatory requirement when facing any study, analysis or intervention.  With  the  current  development  of  such techniques, we  have  verified  their  contribution  to fundamental characteristics in the corresponding stages of visualisation and study. Thus, the proposed methodology is presented as an accurate and complete alternative for the study and analysis of the existing cultural heritage, and opens new  ways  for  the  revision,  reinterpretation  and  revaluation  of  the  previously  evaluated  heritage  through  traditional techniques.