Anza palaeoichnological site, Late Cretaceous, Morocco. Part III: Comparison between traditional and photogrammetric records
Résumé
The present study evaluates a methodological workflow that could identify dinosaur tracks and trackways more comprehensively at outcrop scale. The approach described here is based both on 3D modelling by photogrammetry at different resolutions, and on suitably processed digital elevation models (DEMs). The ichnosite of Anza, Morocco, was chosen to demonstrate the efficiency of the proposed pipeline, because 323 dinosaur and pterosaur tracks discovered there have already been published. One subsector containing 89 tracks, identified in the two companion works that followed a traditional approach, was selected and divided into four subzones. By combining different DEM processes (hill-shade, slope, sky-view factor, and positive openness), almost twice as many tracks (175 vs 89) are now identified in these subzones. However, the improvement is not homogeneous. In the first subzone, the previous works reported 25 tracks vs. 22 with the 3D modelling techniques used here, whereas results for the second and third subzones show considerable improvement with 3D (21 vs 38 tracks and 42 vs 81 tracks, respectively). The enhancement is even more dramatic for the fourth subzone, where 34 new tracks are now identified, whereas with the traditional approach, only one track was previously reported. It is likely that such improvements depend on several factors, i.e. the surface conditions of the rocks (e.g. irregularities, cracking, etc.), and on the preservation state and depth of the tracks. Morphometric measurements of tracks and trackways obtained from 3D models are very similar to those derived from traditional fieldwork methods. The digital approach can be applied rapidly at different resolutions, but the models acquired with the pole-mounted camera provide a good compromise, with a resolution high enough (~2 mm/pix) to spot tracks, while respecting computational constraints. Once treated, DEMs greatly facilitate the reproduction of track outlines, drawn according to criteria defined by the operator.
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