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. 2017 Nov 29;12(11):e0187465.
doi: 10.1371/journal.pone.0187465. eCollection 2017.

An accurate and adaptable photogrammetric approach for estimating the mass and body condition of pinnipeds using an unmanned aerial system

Douglas J Krause  1 , Jefferson T Hinke  1 , Wayne L Perryman  2 , Michael E Goebel  1 , Donald J LeRoi  3
Affiliations

An accurate and adaptable photogrammetric approach for estimating the mass and body condition of pinnipeds using an unmanned aerial system

Douglas J Krause et al. PLoS One. .

Abstract

Measurements of body size and mass are fundamental to pinniped population management and research. Manual measurements tend to be accurate but are invasive and logistically challenging to obtain. Ground-based photogrammetric techniques are less invasive, but inherent limitations make them impractical for many field applications. The recent proliferation of unmanned aerial systems (UAS) in wildlife monitoring has provided a promising new platform for the photogrammetry of free-ranging pinnipeds. Leopard seals (Hydrurga leptonyx) are an apex predator in coastal Antarctica whose body condition could be a valuable indicator of ecosystem health. We aerially surveyed leopard seals of known body size and mass to test the precision and accuracy of photogrammetry from a small UAS. Flights were conducted in January and February of 2013 and 2014 and 50 photogrammetric samples were obtained from 15 unrestrained seals. UAS-derived measurements of standard length were accurate to within 2.01 ± 1.06%, and paired comparisons with ground measurements were statistically indistinguishable. An allometric linear mixed effects model predicted leopard seal mass within 19.40 kg (4.4% error for a 440 kg seal). Photogrammetric measurements from a single, vertical image obtained using UAS provide a noninvasive approach for estimating the mass and body condition of pinnipeds that may be widely applicable.

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Conflict of interest statement

Competing Interests: I have read the journal's policy and the authors of this manuscript have the following competing interests:Co-author Donald J. LeRoi is the owner of Aerial Imaging Solutions LLC, which produces the APH-22 drone used in the experiments. His technical knowledge was invaluable to the design of the study, therefore he deserves co-authorship credit; however, he was not involved in the execution of the study, data analysis or interpretation and writing. As lead author of the study I certify to the editorial staff that he did not attempt to influence the results of the study in any way. And, this does not alter our adherence to PLOS ONE policies on sharing data and materials.

Figures

Fig 1
Fig 1. The APH-22 VTOL UAS system.
Photo Credit: D. Krause/NOAA.
Fig 2
Fig 2. Example photos of leopard seal body positions and substrates.
A) A dorsal-straight body position on sand substrate. The ground reference scale is marked by two red crosses. B) A lateral-straight body position, C) a dorsal-curved body position, and D) a lateral-curved body position.
Fig 3
Fig 3. Photogrammetric measurements.
An example measured leopard seal with labeled photogrammetric measurements.
Fig 4
Fig 4. Observer precision.
Photogrammetrically-derived measurements of leopard seal overall length (n = 50) compared between three independent observers (ANOVA F2,147 = 2.009, p = 0.138).
Fig 5
Fig 5. Aerial versus ground precision.
Observer-derived variances compared between ground-based SL (n = 9) and aerial PSL (n = 9) measurements. Residuals were calculated by summing the absolute difference between each measurement and the mean value for a given animal. Leven’s Test F1,9 = 2.439, p = 0.14.
Fig 6
Fig 6. Linear mass estimation models per observer.
Linear regressions of overall length (POL) times umbilicus width (PUW) to leopard seal mass with 95% confidence intervals (gray ribbon) for each observer. For all models n = 15, R2 > 0.85, and P << 0.0001.
See this image and copyright information in PMC

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