APPLICATIONS OF GEOREFERENCED PRECISION PHOTOGRAPHY TO DIGITAL OUTCROP SURFACE MAPPING

Low-altitude aerial georeferenced photography using a unique assemblage of equipment, techniques, and software creates new opportunities for geologic mapping. Georeferenced photo mosaics in GIS help to form a virtual world in which it is possible to view an outcrop surface at scales ranging from a low altitude “birds-eye view” to close-ups of features too small for the most precise survey equipment. This technique can be enhanced by the addition of on-screen digitizing where features may be mapped as shape files directly from georeferenced photographs revealing a greater density and complexity of structures than is possible to map in the field. This process allows mapping at a complete and previously unattainable range of scales incorporating both digital survey (RTK GPS and total station) and digital photomosaic techniques. Photographs were taken with at least two reference points per frame whose positions were established using a Trimble 5700 RTK GPS (+/- 1cm). Three methods of photography were employed: fine structures were photographed using a Hasselblad 500c/m 6x6cm camera with a 50mm planar lens, mounted on a 1 meter tripod. Larger structures were shot by hand using an Olympus 4.1 megapixel digital camera. Entire outcrops were shot using the same digital camera mounted on a 6m pole extended above the outcrop surface. Photographs were loaded into Adobe Photoshop in order to build seamless photomosaics using the Photomerge function. This function assists in combining frames by using calculated histograms to match features within each frame. The photomosaic and georeference point files were loaded into ArcMap where the mosaic was positioned and oriented using the georeference tool. The visual georeference point markers within the photo were aligned with their corresponding measured geographic reference points. Different photographic methods resulted in varying degrees of success (ie. minimal distortion and maximum detail) for the photo-georeferencing process. Scanned film showed more detail than the digital frames, although both clearly delineate all major features. Mosaics shot using the camera pole resulted in less distortion than mosaics created from frames taken at lower heights. Increasing the camera height diminishes the distortion resulting from varying elevation across the outcrop surface.