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

Abstract

The application of structure from motion (SfM) photogrammetry for digital elevation model (DEM) and orthophoto generation from visible imagery enjoys ever-growing popularity in geomorphological research. Photogrammetry experts, however, urge that a rigorous approach is a prerequisite for reliable results—a requirement that may conflict 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 aircraft system (RPAS), the method facilitates time-efficient topography mapping and the integration of other wavelengths to create orthophotos providing additional surface information. The approach utilizes an on-site 3D structure—for example, a building, as calibration object, by extracting coordinates of natural features from lidar scans and sensor imagery. We assess the workflow with specialized calibration software (VMS) and widely applied commercial SfM photogrammetric software (AM), using a DJI Phantom optical and a Workswell thermal sensor. We achieved calibration accuracies below one-third (optical) and one-quarter (thermal) of a pixel. Subsequently, we transfer the sensor parameters to pre-calibrate the SfM application and compare the results to a self-calibrated workflow. In a systematic experiment using the 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 complexity (i.e., flying heights/view angles) improves pre-calibrated but compromises self-calibrated scenarios. Pre-calibrating (VMS) imagery from a single height (30 m nadir) yielded the best results. This finding could have implications for geomorphological surveys, in which single-scale datasets are widespread practice, despite the literature's urge towards more complex imaging networks. The self-calibrated results legitimise this insistence: The same dataset resulted in pronounced dome-shaped DEM distortion, indicating systematic errors, whereas additional flying heights and angles significantly improved the results.