Efficiently measuring complex sessile epibenthic organisms using a novel photogrammetric technique

This paper describes a stereo photogrammetry method that allows accurate measurements of volume for sessile epibenthic organisms. It represents a novel approach based on multiple views (five stereo image pairs) from a purpose built stereo digital still camera system combined with a three-dimensional reconstruction software program (CAM). The bias, accuracy, precision and efficiency of the method were assessed in the laboratory using models with three levels of morphological complexity (simple, moderate and complex morphologies) and two sizes (large and small). The technique did not show any biases to observer experience, with no significant difference among observers (p > 0.05). Volume measurements made with CAM were very accurate when compared with the water displacement volume of each model, with an overall mean error of about -3% (S.E. +/- 1%). The CAM volume measurements were more accurate on complex models and moderately complex models than on models with simple morphologies. Also, large models had a higher accuracy than small models. Volume measurements made with CAM were also highly precise with the lowest precision observed being 2% of the volume estimate. The time required for a volume estimation using the CAM method was also highly efficient, and the longest time taken for a volume estimation was on average 1 h and 36 min, making it the fastest reported three-dimensional reconstruction method. In field applications, volume estimations of four sponges were all within the observed accuracy, precision and efficiency established during laboratory trials. The accuracy, precision and efficiency demonstrated by the CAM method make this technique highly suitable for routine measurements of the volume of sessile epibenthic organisms.