Three-dimensional photogrammetry as a tool for estimating morphometrics and body mass of Steller sea lions (Eumetopias jubatus)

Abstract

A technique was developed to estimate morphometrics and body mass of Steller sea lions (Eumetopias jubatus) using three-dimensional (3D) photogrammetry. 3D photogrammetry reduces many of the problems associated with camera and body position encountered with two-dimensional photogrammetric techniques, allowing body mass estimation of free-ranging, active sea lions, without sedation, heavy weighing equipment, and disturbance. 3D computer wireframes of 53 Steller sea lions of various age classes were generated from multiple time-synchronous digital photos and used to estimate length, girth, and volume. Average estimates of standard length and axillary girth were within ±2.5% and ±4.0% of physically measured dimensions, respectively. Average estimates of standard length and axillary girth using only wireframes based on ideal body postures were within ±1.7% and ±3.1% of physically measured dimensions, respectively. Regressions of physically measured mass on photogrammetrically estimated body volume yielded a predictive model. Body mass estimates using this model were on average within 9.0% (95% confidence interval = ±1.7%) of the physically measured mass. This technique was also successfully applied to reptiles and fish.

How to weigh an elephant seal with one finger: a simple three-dimensional photogrammetric application

“…Calibration for each camera/ lens combination occurs only once before its first use. We initially attempted to create a 3D model of each seal based on the technique used by Waite et al (2007) for Steller sea lions, whereby orientation points on the seal are cross-referenced between photographs to create a 3D space. Our attempts at this method failed because, firstly, natural marks on seals are scarce and/or difficult to identify for cross-referencing between photographs, and, secondly, seals (even when immobilised) move when breathing or otherwise, resulting in slight shifts of orientation points between photographs.…”

“…The Photomodeler Pro measuring tool was used to assign a scaling measure to the project based on the measuring stick in the photographs. To test if morphometric measures of the seal should be used as a scaling measure to improve ultimate estimates (see Waite et al 2007), we marked the standard length of the seal on the photographs, assigned this as the scaling measure for the model, and compared the derived volumetric estimate with that gained from using the measuring stick in the image. Standard-length-scaled and measuring-stick-scaled volume estimates were compared for all 53 projects.…”

“…The photographer is required to know the exact distance between the camera and the seal and scaling measure. More recently, Ireland et al (2006) and Waite et al (2007) made significant advances using new technology to estimate the masses of Weddell seals Leptonychotes weddellii and Steller sea lions Eumetopias jubatus, respectively. These methods have increased the accuracy of mass estimation for the particular species, but introduced (or maintained) various constraining field procedures, restricting their use in the field.…”

“…However, the study did not simplify the field photographic component. The Waite et al (2007) technique required sophisticated targeting on the seal and synchronized images from different angles to allow the 3-dimensional modelling of the subjects and required best estimates to remain morphologically correlated. Thus, all these methods require physical contact with the animal to acquire a morphometric measure or to manipulate posture.…”

A practical guide to the use of consumer-level digital still cameras for precise stereogrammetric <I>in situ</I> assessments in aquatic environments

“…Boyce, 1963;Christie and Green, 1982;Leatherdale and Turner, 1983;Bythell et al, 2001;Brandou et al, 2007;Waite et al, 2007). As in terrestrial ecology, stereophotography enables reference-free and non-invasive measurements of object coordinates in the 3D realm, thus eliminating the need for a parallel estimation of quantitative parameters such as distance or volume (e.g.…”