Drones and Stones - Using UAV SfM photogrammetry to investigate petroglyph degradation of Neolithic megaliths at Cromeleque dos Almendres, Portugal.

Abstract

The Almendres Cromlech is the largest and most important of the Neolithic Megalithic enclosures in Portugal. The scale and significance is reflected in the presence of a series of petroglyphs on the site, which provide insight into the lives of the Neolithic people. The stones sit in shallow sockets in thin lithosoil, which has been eroded increasingly quickly with the loss of stabilising vegetation due to the rise in footfall at the fully accessible site. The petroglyphs protrude just a few centimetres at the most and are at risk from both natural weathering and damage by human contact. This makes the cromlech particularly vulnerable to erosion, putting the archeological record at risk.
To both preserve this site for education and future study, and examine the effects of erosion on the site, UAV SfM photogrammetry was used to create millimetre accurate 3D models of each stone. These were then analysed in Blender using a virtual grazing light technique in order to identify the petroglyphs that were present. The results were then compared with previous surveys. Overall, this study found fewer petroglyphs than earlier studies, but does show two megaliths to have previously undiscovered decorations. It is concluded that the discrepancies between the results of this and previous studies is mostly due to erosion and differences in interpretation, although human error, bias and methodological differences likely played a part as well.