In situ Underwater Measurements of Red Coral: Non-Intrusive Approach Based on Coded Targets and Photogrammetry

Abstract

Since a decade, we develop a photogrammetry tool dedicated to monitoring red coral populations. This tool is used to collect, effectively, accurate data for the study of population dynamics of red coral. In this article, we present new developments that automate the steps of calculating scale and orientation of images. This automation is made possible through the use of coded targets. In measuring red coral by random quadrat sampling, scuba-divers use a set of photographs on which are visible colonies surrounded by a 20×20 cm quadrat. 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 multi-view photogrammetric acquisition using a single device to easily adapt the measurement procedure to the scene configuration. The material is quite light: one digital camera, two electronic strobes and a simple procedure with a few 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 use the quadrat in the scene for 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.