Testing the use of Unmanned Aerial Vehicle and structure from motion technique for acquisition of ultra-shallow water bathymetric data.

Bathymetric surveying in shallow water can be performed by multiple ways: traditionally using a vessel but also techniques like LIDAR, satellite bathymetry or hovercrafts (and smaller) are used. For ultra-shallow bathymetry, data acquisition, using either singlebeam or multibeam echosounder systems, often present difficulties in obtaining reliable data where submerged obstacles are present; whereas the use of LIDAR technology or satellites data implies relatively high-costs. Multiple studies on land environment have validated the accuracy of Structure from Motion (SfM) techniques for high-resolution 3-dimensional (3D) topographic reconstruction and analysis, and in some cases found SfM to be highly comparable to substantially more expensive LIDAR techniques. In the last 5 years this methodology start to be applied in ultra-shallow marine environment, using the collection of videos or still images by underwater cameras along snorkelling transect, especially on coral reefs, where ship-based survey are unfeasible. The small spatial extent of the resulting 3D models, in this peculiar marine setting, is the main limit of this technique. Our methodology attempt to increase the coverage of SfM techniques in ultra-shallow water environment coupling the snorkelling video transects with an Unmanned Aerial Vehicle (UAV) survey in order to obtain: (1) a complete submetrical scale DEM (Digital Elevation Model) on reef-flat areas, and (2) provide unique opportunities to better quantify topography, rugosity and other structural characteristics of this ultra-shallow marine environment. This study utilized SfM 3D reconstruction software tools to create textured mesh models of a coral reef flat area of a small Maldivian island (Magoodhoo Island, Faafu Atoll) and used a proper GIS-based tool to further extract geomorphometric parameters from the surveyed area, providing evidence of the adeptness of the survey design for a number of applications