3D Mapping of Glacially-Sculpted Bedrock in Central Park

Abstract:

The movement of glaciers and ice sheets through sliding over bedrock depends on the configuration of the subglacial hydrological system. Over time, the glacier erodes the bedrock, which in turn changes water drainage pathways, the overall interaction with the ice, and potentially sliding rates. Drainage can take many forms. At the largest scale, subglacial lakes tens of kilometers in length store water, but the individual pathways are often on the order of meters or smaller. Studies at such a fine scale are only possible by looking at deglaciated beds to infer water drainage. 3D mapping can resolve centimeter scale features and inform studies of the processes that created them. In this survey, Agisoft Photoscan’s structure from motion algorithm is used to create a map of Umpire Rock in New York’s Central Park from digital photographs. Over 3300 photographs are taken at a separation of roughly half a meter to cover the 1000 square meter survey area. The surface is imaged in separate sections and the resulting point clouds are each aligned with a central section using Photoscan’s Align Chunks tool. This process allows additional areas to easily be added to the 3D map. The scale of the final model is accurate to 1mm across the survey area and 3D meshes with a surface resolution of up to 5mm can be created. The distribution of striation directions and sizes on surfaces across the outcrop gives the overall flow direction of the ice and, more locally, illustrates how ice deforms around bedrock features. In addition to striations, we identify cavities and subtle drainage features that are oblique to ice flow. This study demonstrates the relative ease of 3D mapping bedrock outcrops from digital photographs, and indicates the utility of applying this process to more recently deglaciated areas.