On-site geometric calibration of RPAS mounted sensors for SfM photogrammetric geomorphological surveys

The application of structure from motion (SfM) photogrammetry for digital elevationmodel (DEM) and orthophoto generation from visible imagery enjoys ever-growingpopularity in geomorphological research. Photogrammetry experts, however, urgethat a rigorous approach is a prerequisite for reliable results—a requirement that mayconflict with real-world survey. We present a method that unites the two disciplines,using the example of a challenging SfM photogrammetric survey at a Scottish river.Using simultaneous geometric pre-calibration of a multi-sensor remotely piloted air-craft system (RPAS), the method facilitates time-efficient topography mapping andthe integration of other wavelengths to create orthophotos providing additional sur-face information. The approach utilizes an on-site 3D structure—for example, a build-ing, as calibration object, by extracting coordinates of natural features from lidarscans and sensor imagery. We assess the workflow with specialized calibration soft-ware (VMS) and widely applied commercial SfM photogrammetric software (AM),using a DJI Phantom optical and a Workswell thermal sensor. We achieved calibra-tion accuracies below one-third (optical) and one-quarter (thermal) of a pixel. Subse-quently, we transfer the sensor parameters to pre-calibrate the SfM application andcompare the results to a self-calibrated workflow. In a systematic experiment usingthe optical river survey dataset, we assess the effectiveness of pre-calibration,oblique imagery, scale variation and masking to mitigate systematic DEM errors.Opposing trends show between the calibration strategies. Decreasing network com-plexity (i.e., flying heights/view angles) improves pre-calibrated but compromisesself-calibrated scenarios. Pre-calibrating (VMS) imagery from a single height (30 mnadir) yielded the best results. This finding could have implications for geomorpho-logical surveys, in which single-scale datasets are widespread practice, despite theliterature’s urge towards more complex imaging networks. The ...