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Communication Dans Un Congrès Année : 2021

On the calibration of least cost path models: a large-scale simulation of boat and wagon transport in late Iron Age Gaul.

Résumé

The explosion of medium and long-distance contacts at the end of the Iron Age reflects major economic changes and the emergence of European societies increasingly dependent on trade. However, beyond the numerous discoveries of products of distant origin and a few textual indications (by Strabo, Diodorus of Sicily or Caesar), we know very little about the concrete organisation of these exchanges, the routes taken and the flows of people and goods that travelled through them. In order to bring new elements to the understanding of the modalities of these ancient exchanges, we seek to estimate realistic costs of transporting goods between the Gallic agglomerations on the basis of their location in the landscape. To do so, the classic solution consists in approximating the transport network by the set of least cost paths (LCP) between the sites based on the constraints of geographical space (relief, wetlands, etc.).Such routes must, however, take into account the multimodal nature of old transport, linking land transport and inland waterway transport on navigable rivers. The aim of this communication is to present the methodological contributions of this work in the framework of least-cost path analysis. The study area is located in the centre-east of Gaul (260,000 km²), and the paths that run through it are reconstructed using a digital terrain model with a resolution of 90 m. Different time slices are used to reconstruct the connections between the 309 grouped habitats identified in central-eastern Gaul between the 4th and 1st centuries. BCE. The land portions of the least cost paths are calculated from the cost function proposed by I. Herzog (2013) for wheeled vehicles. Inland waterway transport is permitted on the navigable sections of the main rivers of Gaul, differentiating between upstream and downstream navigation. The articulation of costs between these three transport possibilities is addressed through the transport cost ratios proposed in the literature (De soto 2019, Scheidel 2014). However, the literature does not provide any indication for estimating the costs of changing modes: (dis-)embarkation and river crossings. Uncertainties related to parameters (e.g. costs of changing modes) and methods (e.g. method of calculating directional or smoothed slopes) are investigated using complementary procedures : - By comparing the calculated paths with a set of control sites (newer, smaller or badly dated sites). The hypothesis is that some of these sites may have developed close to some of the paths that we are trying to reconstruct. - Through the systematic use of sensitivity analyses, which make it possible to specify the behaviour of each parameter and the influence of their modification. We introduce several aggregated comparison metrics to highlight differences between the sets of paths computed for each configuration of the parameters. These analyses allow us to identify ranges of insensitivity of the model to the parameters, to exclude ratios which induce unrealistic behaviours and to identify plausible ratios. An added difficulty is the inherent coupling between those parameters. For instance, the river crossing cost is not independent from (dis-)embarkation costs: if the crossing cost is too high, LCP will simply embark and then disembark to cross a river. Another typical coupling is between (dis-)embarkation costs and terrain to river cost ratios. Indeed an increased interest of fluvial paths should be compensated by a comparable increase of (dis-)embarkation costs to avoid LCP with multiple short river segments or LCP with unreasonably long detour to catch a river. This work brings new elements to some of the still very topical challenges of least cost path calculations, such as the consideration of several modes of transport and the development of calibration procedures for certain parameters. The costs of transportation on these reconstructed paths are then used for the restitution of the extent of economic interactions between the conurbations at the end of the Gallic Independence. Taken together, these models provide new insights to assess the level of economic integration of these changing societies. De Soto, Pau. 2019. “Network analysis to model and analyse Roman transport and mobility”. In Finding the limits of the limes: modelling demography, economy and transport on the edge of the Roman empire, Philip Verhagen, Jamie Joyce, and Mark R. Groenhuijzen, editors, 271‑89. New York: Springer. Herzog, Irmela. 2013. “The potential and limits of optimal path analysis”. In Computational Approaches to Archaeological Spaces, Andrew Bevan and Mark Lake, editors. Left Coast Press, 179‑211. Walnut Creek. Scheidel, Walter. 2014. “The Shape of the Roman World: Modelling Imperial Connectivity”. Journal of Roman Archaeology 27: 7‑32. https://doi.org/10.1017/S1047759414001147.
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hal-03911362 , version 1 (22-12-2022)

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Clara Filet, Fabrice Rossi. On the calibration of least cost path models: a large-scale simulation of boat and wagon transport in late Iron Age Gaul.. CAA 2021 “Digital Crossroads”, Computer Applications and Quantitative Methods in Archaeology, Jun 2021, Limassol, Cyprus. ⟨hal-03911362⟩
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