Structure-from-Motion on shallow reefs and beaches: potential and limitations of consumer-grade drones to reconstruct topography and bathymetry

Abstract

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.

Methodology for Combining Data Acquired by Unmanned Surface and Aerial Vehicles to Create Digital Bathymetric Models in Shallow and Ultra-Shallow Waters

“…The method developed also has operational limitations. The study further confirmed the importance of the underwater photogrammetric control network included in the study [70]. However, in some cases, the assumption of such a photogrammetric control network may prove problematic.…”

“…An additional constraint is the noticeable decrease in accuracy with increasing depth, water turbidity, and diverse bottom material [61]. The research undertaken presents a photogrammetric approach that confirms the possibility of obtaining data using an underwater photogrammetric control network [70] and fills the gap in coastal water mapping where the opportunities for obtaining bathymetric data with passive remote sensing sensors are significantly limited. The need to create hybrid methods using hydrographic and optical methods was pointed out in [13].…”

“…Other approaches are also reported, e.g., [69] land-based GCPs are used and two buoys are placed on water for the study. Another approach is to use both land and underwater GCPs [70]. The authors of [70], while conducting surveys in the coastal waters of one of the Maldives islands, pointed out that the best results can be obtained using an underwater network without refraction corrections.…”

“…Another approach is to use both land and underwater GCPs [70]. The authors of [70], while conducting surveys in the coastal waters of one of the Maldives islands, pointed out that the best results can be obtained using an underwater network without refraction corrections. In the present study, we used this option, taking into account the underwater photogrammetric network.…”

Concept of an Innovative Autonomous Unmanned System for Bathymetric Monitoring of Shallow Waterbodies (INNOBAT System)

“…It has become the basis for recognizing that an autonomous unmanned system for bathymetric monitoring of shallow waterbodies can be developed using the currently available research tools. However, bathymetry estimation methods are subject to errors (non-linear distortions) due to the wave refraction phenomenon [34] that occurs in the water environment, so it is necessary to eliminate the refraction effect at the water-air interface in the photogrammetric processing [35]. The decisive solutions enabling a positive conclusion of the proposed study include:…”