Underwater close-range photogrammetry

Recent advances in computer vision and specifically in the field of automated digital image recognition and the extraction of geometric information from images, supported by the constant growth of the processing power of modern computers, have given a boost in the development and refinement of image-based modelling techniques. Structure from Motion (SfM) is one of the most widely used such technique , and refers to the process of finding the 3D structure of objects by analysing local motion signals over time, thus using monocular vision and motion cues instead of stereoscopic vision for geometry inference from 2D images. Assuming that all objects in a given scene remain static, their relative displacement in subsequent photographic frames captured from a moving imaging platform depends only on their depth, and by analysing this displacement, a spatial model of the scene can be created.

During the last years I have been trying to apply a multi-view 3D surface reconstruction technique based on the SfM algorithm for the production of high-resolution spatial data in the context of seafloor mapping for marine geomorphology, underwater archaeology and marine ecology investigations. The results of this effort is being showcased below. Raw spatial data in the form of 3D point clouds or polygons can be extracted for other spatial-related quantitative analyses in Computer Generated Imagery (CGI) modelling, Computer Added Design (CAD), and Geographical Information System (GIS) software environments. 

 

Shallow coastal scene

A shallow (~5 m depth) coastal scene comprising a rocky formation of about 7 m in length found near HCMR premises at Anavyssos, Greece.

 

Submerged coastline

A 3D photogrammetric reconstruction of a large (~30 m long, ~12 m wide) underwater scene depicting the coastal geomorphology of an island in the Ionian sea. This is a fully georeferenced model whose study and interpretation will help scientists form the greater picture of the submerged prehistoric landscapes of the Inner Ionian Archipelago. Project funded by the Honor Frost Foundation

 
Combined georeferenced orthophoto and bathymetic map of the survey area.

Combined georeferenced orthophoto and bathymetic map of the survey area.

 

17th - 18th century old deep water shipwreck

A 17th - 18th century old shipwreck was found by HCMR and the Greek Ephorate of Underwater Antiquites at the North Ionian Sea, in depth of around 1260m. Extensive vertical video documentation was conducted using HD camera and HID lights on HCMR’s MAX Rover medium work class ROV operated from the R/V Aegeo. 3D reconstuction was based on static images that were extracted from the continuous video with a frequency of 1 Hz.  

 
Combined photomosaic and shaded-relief map of the shipwreck. 

Combined photomosaic and shaded-relief map of the shipwreck. 

 

Coralligenous wall

A 3D model of coralligenous wall surface at 25 m depth at Lambiri, Korinthiakos Gulf, Greece. Constructed for the CIGESMED project, as a baseline reference for biological monitoring of the coralligenous communities of the specific region and as a high-resolution reference map of sampling localities and sensor placements. Scene dimensions: 20 m length, 2.4 m maximum height, 104 sqm total area surface, 1 cm spatial accuracy. 

 

Picarel spawning grounds

A honeycomb-like formation on the sandy bottom at 46 m depth created by the picarel (Spicara smaris) for its mating and egg-deposition. Each hexagon is about 30 cm in diametre.