TY - JOUR

T1 - Structure-from-Motion on shallow reefs and beaches

T2 - potential and limitations of consumer-grade drones to reconstruct topography and bathymetry

AU - David, C. Gabriel

AU - Kohl, Nina

AU - Casella, Elisa

AU - Rovere, Alessio

AU - Ballesteros, Pablo

AU - Schlurmann, Torsten

N1 - Funding Information: The authors would like to thank Ali Ahmed, Tatiana Ivanova, René Klein, Ibrahim Shiyan (Panda), Zahid as well as Marion and Uwe Zander for their help in the field campaigns. On the Maldives, the authors were supported by the Maldives Meteorological Service (MMS), Fuvahmulah Island Council and Fuvahmulah DIVE School. The authors would also like to thank James T. Dietrich for feedback on his refraction correction routine. Funding Information: This study took place in the project “Dealing with change in SIDS: societal action and political reaction in sea level change adaptation in Small Island Developing States (DICES),” Grant No. SCHL 503/17-1 (CGD, NK, PB, TS) and “Holocene sea-level changes in Southeast Asia (SEASchange),” Grant No. RO-5245/1-1 (AR). The projects are framed within the priority program (SPP-1889) regional sea level change and society of the German Research Foundation (Deutsche Forschungsgemeinschaft, DFG). Elisa Casella acknowledges the Leibniz Center for Tropical Marine Research for economical support. Open Access funding enabled and organized by Projekt DEAL.

PY - 2021/6

Y1 - 2021/6

N2 - Reconstructing the topography of shallow underwater environments using Structure-from-Motion—Multi View Stereo (SfM-MVS) techniques applied to aerial imagery from Unmanned Aerial Vehicles (UAVs) is challenging, as it involves nonlinear distortions caused by water refraction. This study presents an experiment with aerial photographs collected with a consumer-grade UAV on the shallow-water reef of Fuvahmulah, the Maldives. Under conditions of rising tide, we surveyed the same portion of the reef in ten successive flights. For each flight, we used SfM-MVS to reconstruct the Digital Elevation Model (DEM) of the reef and used the flight at low tide (where the reef is almost entirely dry) to compare the performance of DEM reconstruction under increasing water levels. Our results show that differences with the reference DEM increase with increasing depth, but are substantially larger if no underwater ground control points are taken into account in the processing. Correcting our imagery with algorithms that account for refraction did not improve the overall accuracy of reconstruction. We conclude that reconstructing shallow-water reefs (less than 1 m depth) with consumer-grade UAVs and SfM-MVS is possible, but its precision is limited and strongly correlated with water depth. In our case, the best results are achieved when ground control points were placed underwater and no refraction correction is used.

AB - Reconstructing the topography of shallow underwater environments using Structure-from-Motion—Multi View Stereo (SfM-MVS) techniques applied to aerial imagery from Unmanned Aerial Vehicles (UAVs) is challenging, as it involves nonlinear distortions caused by water refraction. This study presents an experiment with aerial photographs collected with a consumer-grade UAV on the shallow-water reef of Fuvahmulah, the Maldives. Under conditions of rising tide, we surveyed the same portion of the reef in ten successive flights. For each flight, we used SfM-MVS to reconstruct the Digital Elevation Model (DEM) of the reef and used the flight at low tide (where the reef is almost entirely dry) to compare the performance of DEM reconstruction under increasing water levels. Our results show that differences with the reference DEM increase with increasing depth, but are substantially larger if no underwater ground control points are taken into account in the processing. Correcting our imagery with algorithms that account for refraction did not improve the overall accuracy of reconstruction. We conclude that reconstructing shallow-water reefs (less than 1 m depth) with consumer-grade UAVs and SfM-MVS is possible, but its precision is limited and strongly correlated with water depth. In our case, the best results are achieved when ground control points were placed underwater and no refraction correction is used.

KW - Bathymetry from drones

KW - Bathymetry from photogrammetry

KW - Coral reefs

KW - Drone mapping

KW - Structure from motion underwater

UR - http://www.scopus.com/inward/record.url?scp=85105378178&partnerID=8YFLogxK

U2 - 10.1007/s00338-021-02088-9

DO - 10.1007/s00338-021-02088-9

M3 - Article

AN - SCOPUS:85105378178

VL - 40

SP - 835

EP - 851

JO - CORAL REEFS

JF - CORAL REEFS

SN - 0722-4028

IS - 3

ER -