Using multiple scales to estimate the projected frontal surface area of complex three-dimensional shapes such as flexible freshwater macrophytes at different flow conditions

Pierre Sagnes

Limnol. Oceanogr. Methods 8:474-483 (2010) | DOI: 10.4319/lom.2010.8.474

ABSTRACT: For different purposes, the projected frontal surface area (Af) of various organisms is often measured through image analysis. Such Af estimates are particularly used to assess the drag coefficient (Cd) in hydro- or aerodynamic studies of organisms. So far, estimates of Af from image analysis were generally biased (as subsequent Cd assessments) for flexible organisms at a given flow, as these estimates were obtained using the same scale for parts of the body being close to or far from the camera. To assess this problem, I used image analyses on 3 flexible freshwater macrophytes, each exposed to 3 different flows. For these, using a single image scale resulted in ~20% errors of Af estimates if compared with a multiple scale (16 subscales, providing the "reference area") assessment. Using 4 subscales and a human-controlled plant contour definition for Af estimates was a good trade-off between accuracy of Af estimates (error < 3% if compared with the reference area) and analysis effort (6 to 10 min for 1 plant in 1 flow condition). In comparison, using an automated definition of the plant contour (through computer software) and a composite scale (i.e., the mean of the 4 previous subscales) provided slightly worse Af estimates (error < 5% if compared with the reference area). Using the automatic procedure reduced the time to estimate Af for 1 plant in 1 flow condition by ~30%. Therefore, future studies requiring accurate Af assessments of flexible, complex, and elongated bodies should use an appropriate number of subscales as reference.