CONSTRUCTING PHOTOGRAMMETRY-BASED THREE-DIMENSIONAL MODELS FOR USE IN GEOLOGIC MAPPING

The use of orthoimage and topographic base maps for traditional geologic mapping is often inadequate for constraining the areal distribution of structure and stratigraphy in areas that include vertical cliffs and other rugged terrain. We present a workflow for developing georeferenced, collocated, color-textured models through the use of modern photogrammetry augmented with conventional surveying to provide a more detailed and accurate 3-D base for mapping of surface exposures. This case study focuses on modeling the Rocas de Benet structure in the Catalan Coastal Range of Spain. Advances in the computer vision Structure-from-Motion (SfM) technique were used to develop a model over a region with several km² areal extent and close to 500m vertical relief. Several thousand geocoded field photographs of Rocas de Benet were taken in similar lighting from a variety of angles and distances from the outcrops and processed using Agisoft Photoscan. This SfM software package is capable of robustly inverting for the 3-D color and positioning information of the large field area. The resulting model offers high levels of detail that allow for the accurate interpretation of specific features such as unconformities, faults, bedding contacts and internal bed forms. Final reconstructions were georeferenced using six distinct control points defined by differential GPS coordinates taken in the field. Model accuracy was compared to independently obtained 2m gridded airborne lidar and ground-based lidar data sets with 10-20 cm-scale precision. Preliminary analysis suggests the SfM models easily have sub-meter accuracy with collocated color and texture information crucial for geologic mapping. Furthermore, the models image complex terrain such as vertical cliffs and even overhanging terrain in ways that are difficult to obtain with other methods. The ability to have such a dataset in a 3-D environment provides views that are otherwise impossible to obtain in the field and allows for better insight into the geology of the area.