Abstract
Abstract. Images captured by unmanned aerial vehicles (UAVs) and processed by structure-from-motion (SfM) photogrammetry are increasingly used in geomorphology to obtain high-resolution topography data. Conventional georeferencing using ground control points (GCPs) provides reliable positioning, but the geometrical accuracy critically depends on the number and spatial layout of the GCPs. This limits the time and cost effectiveness. Direct georeferencing of the UAV images with differential GNSS, such as PPK (post-processing kinematic), may overcome these limitations by providing accurate and directly georeferenced surveys. To investigate the positional accuracy, repeatability and reproducibility of digital surface models (DSMs) generated by a UAV–PPK–SfM workflow, we carried out multiple flight missions with two different camera–UAV systems: a small-form low-cost micro-UAV equipped with a high field of view (FOV) action camera and a professional UAV equipped with a digital single lens reflex (DSLR) camera. Our analysis showed that the PPK solution provides the same accuracy (MAE: ca. 0.02 m, RMSE: ca. 0.03 m) as the GCP method for both UAV systems. Our study demonstrated that a UAV–PPK–SfM workflow can provide consistent, repeatable 4-D data with an accuracy of a few centimeters. However, a few flights showed vertical bias and this could be corrected using one single GCP. We further evaluated different methods to estimate DSM uncertainty and show that this has a large impact on centimeter-level topographical change detection. The DSM reconstruction and surface change detection based on a DSLR and action camera were reproducible: the main difference lies in the level of detail of the surface representations. The PPK–SfM workflow in the context of 4-D Earth surface monitoring should be considered an efficient tool to monitor geomorphic processes accurately and quickly at a very high spatial and temporal resolution.
Highlights
During the past decade, unmanned aerial vehicles (UAVs) or unmanned aerial systems (UASs) have emerged as a very valuable tool for aerial surveying (Passalacqua et al, 2015; Tarolli, 2014)
We aim to (i) assess the accuracy and repeatability of post-processing kinematic (PPK) and non-PPK solutions in georeferencing to examine the capability of using PPK without the need for ground control points (GCPs), (ii) assess the reproducibility of surface topography change detection using PPK solutions for two different UAV–camera setups, and (iii) evaluate different approaches to estimate uncertainties using PPK solutions and their implications for surface change detection
The check points (CPs) root mean square error (RMSE) for the GCP solution are better than 1 pixel for the X, Y and Z coordinates
Summary
During the past decade, unmanned aerial vehicles (UAVs) or unmanned aerial systems (UASs) have emerged as a very valuable tool for aerial surveying (Passalacqua et al, 2015; Tarolli, 2014). An important application in geoscience is the generation of high-resolution topography (HRT) data (i.e., point clouds, digital surface models – DSMs – or digital elevation models – DEMs) from 2-D imagery using structure-from-motion (SfM) and multi-view stereo (MVS) photogrammetry (Eltner et al, 2016; James and Robson, 2012). With the capability of detecting topographical change at a very high resolution and accuracy, the UAV–SfM framework has become an increasingly used tool for the monitoring of landslides (e.g., Clapuyt et al, 2017; Turner et al, 2015), overland flow erosion (e.g., Eltner et al, 2017; Pineux et al, 2017), river dynamics (e.g., Hemmelder et al, 2018) and vegetation dynamics (e.g., Candiago et al, 2015). The use of ground control points (GCPs) surveyed with precise GPS systems or total stations is generally employed for accurate positioning. A major limitation arises from the fact that GCPs cannot be placed in poorly accessible terrain due to practical or safety reasons (e.g., swamps, landslides or glaciated areas)
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