Vol 4 - Issue 1

Digital Archaeology

List of Articles

The LiDAR (Light Detection and Ranging) technique allows a very fine restitution of the topography from a laser scanner on board an aircraft. For several years now, this technology has been making archaeological remains visible over square kilometers of forest areas that were previously invisible from the sky. The archaeological sites are reconstructed with their environment (roads, plots of land, agrarian structures, etc.) at scales that were previously difficult to analyze. This special issue provides us with an overview of French approaches to the archaeological exploitation of LiDAR without claiming, far from it, to be exhaustive, as there is so much work to be done, both in France and abroad. However, the range of articles gathered here reflects the diversity of current research, in terms of scales of work (site, forest massifs, region, etc.), LiDAR acquisition procedures (aircraft, drone), the fields of investigation (from the Aisne to the Allier, from the Vendee to the Vosges and as far as Mexico) or the periods and chronological ranges covered (from the long period of agrarian history to the short period of the Battle of Verdun). A number of methodological points are also covered in the course of the articles, concerning the processing of LiDAR data, the manual or semi-automatic interpretation of relief, field verification protocols, comparison with environmental data, etc. We hope that these insights will bear witness to this research which is full of relief (in every sense of the word) and will provide food for thought on the contributions of LiDAR to archaeological investigation in the forest, on the approaches to be developed to make good use of it, by working with digital data, but also, and always, with historical information and the reality of the field.

The vestiges of past landscapes have come down to us and their study can inform us about the ancient occupation of the territories. LiDAR, coupled with traditional data sources, GIS and modelling, opens up new perspectives to better understand past landscapes and their transformations. In particular, these data show the frequency of patchy remains in present-day forests in France, occupying up to several hundred square kilometres in a single block. These vestiges materialize spaces formerly dedicated to agro-sylvo-pastoral activities. 17 areas (about 2200 km2 ), including parcel vestiges attributed to Antiquity, have been studied within the framework of the ERC Rurland programme devoted to the spatial and historical dynamics of rural territories between the Seine basin and the Limes de Germanie, from the Final Tène to Late Antiquity. Half of these areas, mainly under forest, incorporate LiDAR. Documented by archaeological data, they have been put into perspective with environmental characteristics: slopes, soil textures, soil water reserve capacities, degrees of fertility or silvicultural productivity, limits to agricultural use, and other topographical, geological and pedological information. In addition, a detailed study of plot morphologies was carried out in six areas to better understand how the plots fit into their environment. Examination at different scales of plot systems identified some factors that appear to favour or hinder past agro-pastoral use. Insufficient soil water reserve capacity and low natural fertility could be factors limiting this use in antiquity; the presence of slopes may have favoured the ancient abandonment of an agro-pastoral use after the Roman period.

Frequently highlighted by remote sensing airborne laser scanning, the agrarian features and field systems are preserved in the forest, in the form of microrelief, covering large areas. The range of forms in which they are shown raises the question as to their origin and their function. With the increasing number of lidarLiDAR surveys since the 2000’s the need to set up a framework for interpretation quickly became apparent. We propose here to present the typology of agrarian and field system remains preserved in the Lorraine forests based on a study consisting of four lidarLiDAR surveys over an area of about 300 km². A three-step approach to these remains has been developed: 1) identification of elementary forms, particularly those which are revealed by the visualization of lidarLiDAR data; 2) study of the morphogenesis of the remains, based on observations on the ground and some excavations enabling us to gain more insight into their features and their taphonomy; 3) morphological analysis of the field systems which contributes to the interpretation of some of these morphological features. This analysis is completed by a chronological approach.

LidarLiDAR mapping have multiplied in recent years, revealing everywhere fossilized ancient landscapes in the topography. This truly consists of an archaeological revolution, comparable to the development of aerial surveys in the 1970s or preventive archaeology twenty years later. LidarLiDAR fever has taken over the world of archaeologists, whose main question is how to date ancient plot networks, as it is rare to find archaeological artefacts during forest surveys. The various methodological approaches undertaken recently tend to provide solutions to this problem, but without extensive forest excavations, can one be sure to have a large majority of ancient parcel boundaries? The analysis of the LidarLiDAR survey of the Rouvray Forest in Normandy reveals several ancient parcel networks. After having analysed them, a 12 000m² excavation was conducted in the heart of the forest on the Gallo-Roman farm of “Le Grésil”, in order to understand the topographical anomalies identified on the LidarLiDAR. This has revealed varied cases. Some anomalies are linked to archaeological structures, others were geographical anomalies, even fantasized structures. In the case of “Le Grésil”, all the ditches defining spaces other than the heart of the settlement were only seen during the excavation. The LidarLiDAR did not lead to their identification. While the LidarLiDAR remains a wonderful source of information, it is therefore necessary to only take into account the structures whose anthropic origin is unambiguous and to keep in mind that other parcel boundaries might not have left any traces in current topography. The rare spaces which present an absence of anomalies should not be considered as immutable forests without complementary approaches (palynology…).

Recent LiDAR-based surveys in Mesoamerica (Mexico and Guatemala) slowly reveal the underestimated magnitude of prehispanic landscape management. Known residential sites prove to be integrated within deeply modified landscapes, primarily to be cultivated, thus highlighting once more the importance of agriculture in these societies deprived of beasts of burden or cattle. Consistent with this research trend, this contribution presents the compelling case study of the Malpaís de Zacapu LiDAR (90 km²), in West Mexico, focusing on agrarian features. The study combines field work and digital imagery (LiDAR, satellite imagery) to address landscape modification in this volcanic environment characterized by chaotic lava flows displaying little or no soil cover. We used methods from archaeogeography and applied geopedology to update the archaeological and soil maps of the area. The numerous remains brought to light dramatically change our understanding of local settlement history. In the meantime, the study underlines agrarian features adapted to specific landforms and soil types. It finally points out that the selection of cultivated areas is also governed by cultural choices that have changed through time.

LiDAR covers under forest allow to identify rural settlements and roads. Since 1989, the association Escles-Archéologie has been organizing surveys and listing the ancient ways of the Vôge. One of them attracts particular attention because its substructures make it easy to identify on the ground. It is thought to have been a portage route between the Saône and the Moselle; the Gallo-Roman village of Escles may have been a mid-way stopover between the two rivers. In 2016, the examination of a LiDAR survey revealed a strip of land 40 meters wide, bordered by straight ditches, which follows the road to the north of the Saône. This strip of land is part of a 27 kilometers straight line from Corre to Escles with an azimuth of 30 ° E. When crossing the valleys, while the road deviates to follow easier slopes, this strip of land retains its rectilinear orientation until the change in the gradient at the edge of the plateau, then crosses the obstacle by very steep ramps and resumes the same width and direction on the other side of the river. Surveys carried out on the Corre-Escles section now make it possible to know quite precisely the orientation and the layout of these two contiguous paths and to confirm the existence of a large rectilinear portage way probably intended for pedestrians, riders and pack animals. On the other hand, the paved way was to be used to carry heavy loads. This study, which reveals the topographical and chronological complexity of the road connections between the Saone and the Moselle, requires new archaeological investigations and LiDAR surveys to clarify the routes, in particular for the Escles-Portieux section which is so far poorly documented.

In 2013, an airborne LiDAR mission conducted over the Verdun battlefield has brought to light World War IOne landforms. Concealed by a large forest cover of 10,000 ha, these reliefs, called polemoforms in reference to J.-P. Amat’s research (1987, 2001, 2015), are direct remnants from the 1916 battle and constitute a prime archeological reserve for the future generations. Despite the high density of features, they should be exhaustively inventoried in the framework of the national label “forêt d’exception du champ de bataille de Verdun”. However, at the scale of the entire site, this work is only feasible if an automated mapping method is developed. This paper especially focuses on the trench network inventory. The methodology used is threefold: (i) Landforms are extracted from the digital terrain model by means of a semi-automatic algorithm; (ii) Trench geometry is studied using a sinuosity index; (iii) Map interpretation is carried out on the field site and supplemented by historical documents collected in French and German archives centers. The resulting maps reveal approximately 420 km of fire and communication trenches. Their morphological signature is rich and due to the different ways of building facilities. Although a large part of the initial network disappeared during the last century, the crossing of LiDAR data with historical archives allows to estimate the length of the erased features and to locate the affected areas. Beyond the reproducible nature of the method, which may contribute to the research development on other battlefields, this research provides operational tools for management and conservation of the historical, cultural and natural heritage of the Verdun forest. As a result, the produced iconographic and cartographic corpus will be directly used in the next forest management plan, in order to optimize the safeguarding of polemoforms and associated remnants.

The national forest of Tronçais is one of the most beautiful oak forests in Europe, covering 11,600 hectares in the center of France, in Allier. Known for a long time to shelter remains, especially Gallo-Roman, this forest and its surroundings have benefited from a LidarLiDAR acquisition in 2016. Very many anthropic reliefs were then observed, which largely complement our vision of ancient occupations in this area. A diachronic surveying program was set up in 2017-2019, in order to check on the field these clues, to characterize better the structures and to date them. In this paper, the procedures of these ground interventions, combining visual survey, collecting of artifacts on the ground and use of a metal detector, are detailed. It is also an opportunity to make an initial assessment of the assets and limits of the archaeological exploitation of LidarLiDAR data in this forest area. Thus, we can distinguish the remains that we see much better thanks to ALS (buildings, enclosures, mounds, quarries, ponds, coal platforms), those that have appeared entirely (field systems), but also those that we do not see yet (prehistoric occupations, structures in perishable materials) and which are still largely lacking to our knowledge.

Since 2015, the Regional Natural Reserve of Val Suzon (Côte-d’Or) has been the subject of archaeological researches based particularly on the analysis of LiDAR datas acquired in 2013. Thanks to these datas, many traces of activities preserved under the forest can be identified and studied. These archaeological structures, characterized by a great diversity of ages, origins and natures, make it possible to apprehend the evolution of this territory over the long term. In addition to LiDAR, various sources are also exploited, such as textual archives and old plans. These documents provide additional information to those from LiDAR and also provide information, for example, about the exploitation of Val Suzon’s natural resources or the transformation of the landscape over time. This diachronic and multi-source approach is illustrated presently through the study of some structures identified on the plateau of Messigny. From the Bronze Age tumulus to the trenches of the war of 1870, this area of approximately 200 hectares presents a wide range of archaeological structures testifying to the composite history of this territory which is occupied nowadays by a forest. In addition, the geomorphological conditions of this area have allowed the preservation of exceptional traces left by the repeated passage of animals. It appears however that these traces, like other micro-reliefs, are only visible in the areas where the forest has been maintained for several centuries, despite the episodes of deforestation that occurred in Messigny. Finally, the research carried out on the Val Suzon addresses issues specific to forest archaeology as well as forest history and historical ecology, in particular by focusing on the question of the impact and legacies of past dynamics on the current environment.

In the panel of non-invasive techniques, LiDAR airborne surveys by airplanes, helicopters or even satellites have, for several decades, proved their utility in the exploration of large wooded, forest or inaccessible sectors. The miniaturization of the sensors made it possible to carry powerful LiDAR sensors under light civil UAVs, multirotors or fixed-wing drones. These microdrones can overcome the limitations and disadvantages of traditional aviation. Archaeology has, of course, benefited from this technology, which is complementary to the LiDAR plane survey, the photogrammetry and even the terrestrial scanner survey. Points clouds, shaded MNTs and 3D models reveal microtographic artifacts that are not always identifiable in the field, which feeds archaeologists research work and helps to establish excavations. New digital DTM treatment tracks are being drafted.

The paper presents research carried out as part of a Collective Research Project on the Mervent-Vouvant forest in Vendée since 2018 following LiDAR acquisition. The latter made possible to substantially renew and complete the archaeological knowledge of this forest and to outline some ways of research. From research initially focused on the problem of fortified protohistoric habitats – because of the excavation of a fortified enclosure from Hallstatt D3 in the village of Mervent – the project has evolved into a diachronic study of the whole of the forest due to the mass of data attributable to other periods, in particular medieval and modern. LiDAR data turns out to be an extremely rich source of information, which remains to be analyzed further by combining the contributions of archeology, geoarchaeology, archaeogeography and history.

The ten articles in this issue have given us a broad overview of the use of LiDAR technologies for the study of archaeological remains under dense forest or vegetation cover. Coming from a variety of contexts with studies of which a good part is unpublished, the numerous data presented in this special issue cover a large part of France, from the Pays de la Loire to the Great East, from the Hauts-de-France to the Auvergne-Rhône-Alpes, with an excursus in Mexico. We have therefore taken the opportunity to draw up, from these contributions on 19 different zones, a repertoire of the anthropic reliefs observed on LiDAR images, which are evoked in the 10 contributions. 59 types of relief, divided into nine areas of activity have been identified. This document is incomplete and still very perfectible, but we hope that it will be useful to have an overall view of the traces studied in this special issue, and will make it possible to begin the implementation of a more exhaustive reference system, at least on the scale of the French territory.