AUTOMATING THE MEASUREMENT OF RED CORAL IN SITU USING UNDERWATER PHOTOGRAMMETRY AND CODED TARGETS
Keywords: Underwater photogrammetry, coded target, red coral
Abstract. A photogrammetry tool dedicated to the monitoring of red coral populations in situ has been developed by LSIS in Marseille (France). This tool is used to collect in an efficient and precise manner key data for the study of the population dynamics of red coral. In selected red coral populations, scuba-divers obtain a series of photographs from the permanent plots (about 2 m2) on an annual basis. To facilitate the photographic sampling and measurements, the scuba-divers use a 20 x 20 cm quadrat to cover the permanent plots. The analysis of the photographs provides reliable measurements on colony sizes (basal diameter and maximum height), occurrence of breakage of colonies and the occurrence of necrosis.
To minimize the divers' tasks during the acquisition phase, we opted for stereoscopic acquisition using a single device to easily adapt the measurement procedure to the scene configuration. The material is quite light, one camera and two electronic strobes and a simple procedure with two photographs taken for each site.
To facilitate the measurement phase of colony sizes; the exploitation of photographs consists of four key steps: orientation, scaling, measurement of the characteristic points of coral colonies and result validation (checking measurement consistency to detect possible errors in measurement or interpretation).
Since the context of the shooting can vary widely, dominant colors, contrast, etc. may often change. In order to have a stable and common reference in all photographs independently of the site, we decided to always include a quadrat in the scene which then will be used for the orientation and scaling phases.
The automation of orientation and the lack of constraints to adapt the analytical technique to the features of each site offer the possibility to multiply field surveys and to measure hundreds of quadrats from several different populations in a very efficient manner.
The measurement results are exported into a spreadsheet application and integrated into the biologists' workflow. The results obtained from different red coral populations displaying contrasting characteristics (small versus large colony sizes) are presented and discussed at the end of this article.