Geomorphic Change Detection Using Cost-Effective Structure-from-Motion Photogrammetry: Evaluation of Direct Georeferencing from Consumer-Grade UAS at Orewa Beach (New Zealand)

Abstract

Unmanned aerial systems (UAS) and structure-from-motion photogrammetry are transforming the way we produce topo- graphic data, with applications covering many disciplines in the geosciences, including coastal studies. To overcome limitations of ground control points (GCPs), we evaluate direct georeferencing (DG) of consumer UAS imagery for the cost-effective measurement of beach topography. Using DG, camera positions determined with on-board instruments provide air control points for photogrammetry, obviating the need for presurveyed GCPs. We validate the approach at Orewa Beach, New Zealand, achieving vertical accuracies similar to light detection and ranging (< 0.2 m) at a higher resolution (< 0.1 m). A low-quality global navigation satellite system onboard a consumer UAS remains the main constraint on measurement quality. We show how independent topo- graphic data sets, which are increasingly available world- wide, can improve measurement quality, and hence change detection capacity. Our understanding of measurement quality achieved in this study is applied to the assessment of morphological and volumetric change at Orewa Beach.