3D Reconstruction of Coastal Cliffs from Fixed-Wing and Multi-Rotor UAS: Impact of SfM-MVS Processing Parameters, Image Redundancy and Acquisition Geometry

Gil Gonçalves,Umberto Andriolo,Diogo Gonçalves,Juan Antonio Pérez-Alvárez,Álvaro Gómez-Gutiérrez

doi:10.3390/rs13061222

Abstract

Monitoring the dynamics of coastal cliffs is fundamental for the safety of communities, buildings, utilities, and infrastructures located near the coastline. Structure-from-Motion and Multi View Stereo (SfM-MVS) photogrammetry based on Unmanned Aerial Systems (UAS) is a flexible and cost-effective surveying technique for generating a dense 3D point cloud of the whole cliff face (from bottom to top), with high spatial and temporal resolution. In this paper, in order to generate a reproducible, reliable, precise, accurate, and dense point cloud of the cliff face, a comprehensive analysis of the SfM-MVS processing parameters, image redundancy and acquisition geometry was performed. Using two different UAS, a fixed-wing and a multi-rotor, two flight missions were executed with the aim of reconstructing the geometry of an almost vertical cliff located at the central Portuguese coast. The results indicated that optimizing the processing parameters of Agisoft Metashape can improve the 3D accuracy of the point cloud up to 2 cm. Regarding the image acquisition geometry, the high off-nadir (90°) dataset taken by the multi-rotor generated a denser and more accurate point cloud, with lesser data gaps, than that generated by the low off-nadir dataset (3°) taken by the fixed wing. Yet, it was found that reducing properly the high overlap of the image dataset acquired by the multi-rotor drone permits to get an optimal image dataset, allowing to speed up the processing time without compromising the accuracy and density of the generated point cloud. The analysis and results presented in this paper improve the knowledge required for the 3D reconstruction of coastal cliffs by UAS, providing new insights into the technical aspects needed for optimizing the monitoring surveys.

Highlights

IntroductionCoastal cliffs are specific coastal landforms characterized by steep rocky walls, which are present on about 52% of the global shoreline [1,2]
To test the impact of the acquisition geometry on the quality of the generated Structure-from-Motion and Multi View Stereo (SfM-MVS) point clouds, two Unmanned Aerial Systems (UAS) platforms were used in the topographic survey of the cliff face: a fixed-wing (Ebee) and a multi-rotor (Phantom)
Due to the differences in flight path design and the view angle of each payload camera, the number of images acquired by the multi-rotor were approximately eight times the number of images acquired by the fixed-wing

Summary

Introduction

Coastal cliffs are specific coastal landforms characterized by steep rocky walls, which are present on about 52% of the global shoreline [1,2]. Their complex face topography, with ledge crevices and overhangs, is the result of geological, physical and environmental processes interplaying at the coast. The erosional processes of cliffs are mainly caused by the notching at their base due to wave forcing, and/or by the collapse of the cliff face due to the combination of atmospheric and marine processes [2]. The current sea level rise in the actual climatic change scenario is further intensifying the erosional processes [1]

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