Morphometry of Middle Bronze Age palstaves by Discrete Cosine Transform

Abstract

a b s t r a c tThe Discrete Cosine Transform (DCT) is a Fourier-related transform widely used in signal processing and well suited to the analysis of open outlines. This method was applied here to evaluate the discrimination power of the inner lateral rib for two palstave populations dating from the Middle Bronze Age, excavated in northwest France. A corpus of almost 400 palstaves (bronze axes) of the Breton and Norman types was processed, and compared to specimens found at Sermizelles in Burgundy. The procedure is robust and produces a discrimination in good agreement with the traditional typology. Besides the definition of a 'standard' shape for each population, the morphometrical approach allows shape disparity, which is generally inaccessible to the naked eye, to be visualised and quantified. Shape disparity indicates that, contrary to previous assumptions, the bronze axes from the Sermizelles hoards cannot be explained as an assortment of Breton and Norman palstaves alone. We believe that this approach is quick, reproducible, and generalisable enough to be applied to a wide variety of artefacts from different periods, in order to clarify their typology and even their origin.

Unsupervised model-based clustering for typological classification of Middle Bronze Age flanged axes

“…In fact, the presence of two distinct areas: (i) zone A, with flanged axes belonging to G1 and G2, and (ii) zone D, with axes from G5 and G6, tends to confirm a finer structuration of geographical space, already evoked by Butler (1963), and supported by the study of all metallic Middle Bronze Age artefacts discovered in north-western France (Gabillot, 2003). Note that zone D also corresponds to an area presenting a high density of contemporary Norman palstaves (Forel et al, 2009;Monna et al, 2013), suggesting the existence of a specific cultural area (Gabillot, 2006;Monna et al, 2013). Comparison with existing typologies reveals that most of the artefacts from G5 and G6 are members of concave-blade flanged axes ( Table 2).…”

“…Data acquisition, drawings, and outlines Acquisition follows the procedure described in Forel et al (2009), Monna et al (2013) and Wilczek et al (2014). Briefly, all published images obtained from the available documentation (Supplementary materials S1, Fig.…”

“…They provide a continuous morphospace allowing more complex statistical analyses, including the reconstruction of the mean shape and shape diversity within the group of interest (Adams et al, 2004;Navarro, 2003;Zelditch et al, 2004;Slice, 2005;Wilczek et al, 2014). Two recent studies undertaken on Bronze Age palstaves (Forel et al, 2009;Monna et al, 2013) have already demonstrated that combining geometric morphometrics with spatial analyses can be very effective for the better understanding of artefact production and use.…”

Le dépôt du Bronze moyen II de Mouilleron-en-Pareds (Vendée) et les haches à rebords de type vendéen

“…De nouveaux travaux sont ainsi menés pour dépasser ces biais (Forel et al, 2009). Pour ce qui est de la largeur du tranchant cependant, pour les exemplaires étudiés de Mouilleron-en-Pareds, les plus larges ne correspondent pas nécessairement à des tranchants plusieurs fois réafûtés.…”

Morphometrics for Nonmorphometricians

“…Fourier decomposition has been applied to archaeological data by , Cardillo (2005), Cardillo and Charlin (2007), to investigate the outlines of flake tools; by Karasik et al (2005) and Goel et al (2005) in the analysis of asymmetrical deformations of pottery vases; by Forel et al (2009) to study the morphometry of European Bronze Age (2300-800 BC) bronze tools; and by Peterson (1992) to estimate shape differences among the limits of particular fields during Roman Times. The problem is that corners are difficult to approximate with the waveform represented by sine and cosine functions; hence a large number of terms are needed to force the curve represented by the Fourier series to represent a corner.…”