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11 pages; International audience; The present study proposes a workflow to extract from orthomosaics the enormous amount of dry stones used by past societies to construct funeral complexes in the Mongolian steppes. Several different machine learning algorithms for binary pixel classification (i.e. stone vs non-stone) were evaluated. Input features were extracted from high-resolution orthomosaics and digital elevation models (both derived from aerial imaging). Comparative analysis used two colour spaces (RGB and HSV), texture features (contrast, homogeneity and entropy raster maps), and the topographic position index, combined with nine supervised learning algorithms (nearest centroid, naive Bayes, k-nearest neighbours, logistic regression, linear and quadratic discriminant analyses, support vector machine, random forest, and artificial neural network). When features are processed together, excellent output maps, very close to or outperforming current standards in archaeology, are observed for almost all classifiers. The size of the training set can be drastically reduced (to ca. 300 samples) by majority voting, while maintaining performance at the highest level (about 99.5% for all performance scores). Note, however, that if the training set is inadequate or not fully representative, the classification results are poor. That said, the methods applied and tested here are extremely rapid. Extensive mapping, which would have been difficult with traditional, manual, or semi-automatic delineation of stones using a vector graphics editor, now becomes possible. This workflow generally surpasses pedestrian surveys using differential GPS or a total station.

This study focuses on traces of paint on stone stelae of Mongolia, dating from the late second to the mid-first millennia BC. The painting was made in various shades of red. In all cases, remains of paint were found on facets which had been protected from weathering because the stelae had collapsed or been reused. Additional protection might have been provided by a calcite crust formed where the stelae were in contact with the ground. On the basis of the role of painting in visual imagery, two groups of paintings are described: supportive (filling in the engraved figures or outlining them) and independent. The first group is the largest. In terms of composition, stelae fall into two types:1) those whose front is on the narrow vertical facet, carrying images of deer in the Mongolian-Transbaikal style;2) and those whose front is on the wide vertical facet, where the upper tier is emphasized, and neither deer nor belts with weapons are represented. The relative chronology of the two types is established by the fact that in certain instances, stelae of the first type were reshaped into those of the second type. Also, the first type is related to funerary and memorial sites of the Khereksur and Deer Stone culture of Central Mongolia ; the second, to burials of the Slab Grave culture. The tradition of decorating engraved figures with red mineral paint originated among eastern Eurasian steppe pastoralists in the Early Bronze Age., Cette étude porte plus particulièrement sur les traces de peinture présentes sur certaines stèles dites « pierres à cerfs » de Mongolie, datant de la fin du deuxième au milieu du premier millénaire BC. La peinture apparaît selon diverses nuances de rouge. Dans tous les cas, les traces de peinture ont été trouvées sur les faces protégées des intempéries notamment sur les stèles qui furent en partie ensevelies ou qui avaient été réutilisées dans des tombes. Une protection supplémentaire pourrait avoir été apportée par une croûte de calcite formée au contact des stèles avec le sol. De façon générale, l’utilisation de la peinture dans la constitution des iconographies se répartit en deux catégories. La première, la plus rencontrée, est le remplissage des figures gravées ou le rehaussement de leurs bords. La seconde correspond aux motifs peints distincts des figures gravées. En termes de composition, les stèles se répartissent en deux types :l) celles dont la face avant est sur le côté étroit, portant des images de cerfs dans le style mongol-Transbaïkal ;2) celles dont la face avant est sur un côté large, où la partie supérieure est mise en valeur par rapport à une base sans figurations de cerf ni de ceinture portant des armes. La chronologie relative entre ces deux types est établie par le fait que dans certains cas, les stèles du premier ont été transformées pour devenir celles du deuxième type. Aussi, la première catégorie appartient aux sites funéraires et commémoratifs de la culture des Khereksur et « pierre à cerfs » de Mongolie centrale ; la seconde, aux sépultures de la culture des tombes à dalles. La tradition de colorer des gravures à l’aide de peinture minérale rouge a commencé chez les pasteurs des steppes de l’est de l’Eurasie au début de l’âge du Bronze.

Rock art studies are facing major technical challenges for extensive documentation. Nowadays, recording is essentially obtained from time-consuming tracing and rubbing, techniques that also require a high level of expertise. Recent advances in 3D modelling of natural objects and computational treatment of the modelled surfaces may provide an alternative, and reduce the current documentation bottleneck. The aim of this study is to examine the extent to which such treatments can be applied. The case study presented here concerns the famous deer stones erected by ancient Mongolian nomad populations. The 3D acquisition workflow is based on structure-from-motion, a versatile photogrammetric technique, well adapted to various field conditions. From the 3D geometry of objects of interest, elevation raster maps are produced by projection on four sides of the stela. These digital elevation models are then tested using algorithms based on differential geometry, sky visibility and local morphology, the general principles of which are briefly exposed. All these approaches may be appropriate with essentially planar surfaces. However, in the case of irregular carved surfaces, such as those of deer stones, the most efficient algorithm appears to be positive openness. In favourable cases, the incisions can be automatically delineated, facilitating the final drawing. Results obtained at the end of the process are comparable to the best drawings available in the literature, and can also include archaeological information about rock surface conditions. The procedure considerably accelerates the workflow in comparison with traditional techniques, reduces the level of expertise required, and provides 3D models, which can easily be shared, or further analysed by morphometric methods, for instance. (C) 2018 Elsevier Masson SAS. All rights reserved.

During his archaeological missions the Museum of Prehistoric Anthropology tested the 3D modelling of several typesof objects. His team chose the ‘Structure-from-Motion’ (SfM) method which works according the basic principle of stereoscopicphotogrammetry, namely that 3D structure can be resolved from two or more overlapping, offset images. Most of other methodscapable of surveying, at high resolution, often complex objects and landforms are very expensive and have a difficult portability.Using only a consumer-grade digital camera and a scale on the ground, the user moves through the environment, acquiringphotographs of the area of interest from as many locations and perspectives as possible. Software is then used to reconstruct scenegeometry. Results from example applications of the technique to engraving art, archaeological zone and architecture are presented.Such data are invaluable for the archaeological heritage inventory on difficult and remote field. To conclude, SfM output is directlycompared with that obtained from a classic archaeological documentation., Au cours de ses missions archéologiques, le Musée d’Anthropologie préhistorique de Monaco a testé la modélisation 3D de plusieurstypes d’objets. Son équipe a choisi la méthode ‘Structure-from-Motion’ (SfM) qui fonctionne selon le principe de base de la photogrammétriestéréoscopique, à savoir que la structure 3D peut être résolue à partir de deux images superposées et décalées. La plupart desautres méthodes capables d’acquérir, à haute résolution, des objets et des paysages souvent complexes, sont très coûteuses et d’une portabilitédifficile. En utilisant uniquement un appareil photo numérique de qualité grand public et une échelle sur le sol, l’utilisateur se déplacedans l’environnement, en prenant des photographies de la zone d’intérêt depuis le plus possible d’endroits et de points de vue. Un logicielest ensuite utilisé pour reconstituer la géométrie des scènes. Les résultats des exemples d’applications de la technique à l’art rupestre, auxchantiers archéologiques et à l’architecture sont présentés. De telles données sont précieuses pour l’inventaire du patrimoine archéologiquesur des terrains difficiles et éloignés. Pour conclure, la production de SfM est directement comparée à celle obtenue à partir de la documentationarchéologique classique.