Low-altitude remote sensing and GIS-based analysis of cropmarks: classification of past thermal-contraction-crack polygons in central western Poland

The examination of low-altitude aerial photographs reveals the presence of more than 400 polygonal nets in central western Poland. Polygons range from 5 to almost 70m in diameter. Based on the polygons' diameter and intersection angles, we identified seven main types of nets geometry. Based on ground verification, we interpreted them as past thermal-contraction-cracks, filled mostly with sand (i.e. sand-wedge casts). As favourable weather conditions and a proper land cover (i.e. cultivated land) are necessary for identifying polygonal nets, the observed number of polygons is probably much underestimated.