Geometric calibration for geomorphological photogrammetric surveys

Abstract

Remotely Piloted Aircraft System (RPAS)-based Structure-from-Motion (SfM) photogram metry is a recognised tool for mapping topography in geomorphological applications that can be applied with consumer grade sensors. Previous research, often conducted in favourable survey environments, has indicated that the use of sensor pre-calibration instead of self-calibration can be effective to avoid systematic errors and digital elevation model (DEM) deformation. However, there is a lack of applicable workflows and best-practice examples to address the often more challenging prerequisites of surveys determined by study site, such as footprint, topography and surface type. This thesis presents a time-efficient, in-situ pre-calibration workflow for thermal and opti cal multi-sensor systems, that employs a terrestrial laser scanner (TLS)-scan of a 3D-object present on-site as reference; is optimized to meet both the needs of practitioners and pho togrammetric rigour. Performing the calibration on-site allows retaining its validity until the survey flights by avoiding shocks and temperature changes during transport and en abling constant sensor-object distance. We transferred pre-calibrated sensor parameters to a survey dataset acquired on a reach of the River Gairn, Scotland and assessed the influence of calibration approach, survey geometry and software choice on DEM accuracy. We found the overall best results (root mean square error (RMSE): 0.02 m) using a 30 m nadir-only dataset, pre-calibrated with the photogrammetric software vision measurement system (VMS), whereas when self-calibrated, the maximum image network complexity produced the best results. We applied the approach in a multi-temporal dataset to mon itor geomorphic changes following a river restoration with engineered log jams (ELJs) aiming to improve habitat quality. We used a change detection workflow with spatially discrete uncertainty analysis, water surface modelling and refraction correction to monitor geomorphic change on a centimetre scale. The results show several areas of real change in the first two years after the restoration, but only a small part of these can be attributed to the ELJ placement. The developed approach for multi-sensor calibration can be efficiently integrated into multi-temporal geomorphological surveys to improve SfM photogrammetric accuracy.