METHOD FOR RECORDING STRATIGRAPHIC INFORMATION ACROSS COMPLEX 3D SURFACES FOR FLUVIAL ARCHITECTURAL ELEMENT ANALYSIS

Architectural element (AE) analysis of fluvial strata necessitates the collection of detailed stratigraphic information at a scale of centimeters to 100s of meters. Consequently, most AE analyses utilize photomosaics of near 100% exposed rock faces that present near-planar surfaces. Here we report a method to assess the AE of outcrops with complex 3D topography that allows seamless collection of fluvial architecture at several scales. Initially, a high-resolution camera mounted to an unmanned aircraft system (UAS), which flies in a grid pattern, captures highly overlapped geotagged digital images of an outcrop. Subsequently, Pix4D software is used to assemble a 3D digital outcrop model (DOM). The DOM, and initial field observations, help locate suitable outcrop faces for AE analysis. In the field, a tablet computer with a high-resolution camera is used to photograph and annotate small sections of the outcrop. Defining surfaces and determining surface hierarchy on the outcrop improves interpretation when compared to using only remote imagery. Using a stylus enables tracing of bounding surfaces directly on images, with a different color assigned to each surface order. Once surfaces are defined, measurements made on the outcrop are cataloged directly on the image where they were measured. Notes taken in this way permit annotation of overhead images with accuracy, as UAS images alone do not have the spatial or vertical resolution to facilitate AE analysis. Importantly, a record of the orientation of the primary face in each image allows diagrams to be positioned in space. After collecting all the information for the area comprising a given panel, examination of images containing notes form the basis to begin annotating the DOM. The resolution of the DOM is not high enough to properly document many low-order features, but each surface can be placed accurately using field information. Completing the tablet-based portion of the method before UAS coverage is also possible, as photographs can be stored until surveillance becomes available. In this case, determining the locations of each photograph requires GPS geotagging. Iteration between UAS and tablet imagery, collecting successively higher-resolution DOMs and more detailed field measurements, increases the accuracy of finished stratigraphic panels.