Photogrammetry

Measuring Marine Mammals without Touching Them

Unless one measures a beached specimen, it is almost impossible to take measurements of a whale manually. However, biologists need this information – length, width, height, circumference, mass, etc. – to get an idea of ​​the animal’s state of health.

Taking accurate morphological measurements of cetaceans presents several challenges: the animals are immersed in water, seldom sit still and their bodies, far from being flat like a pancake, are full of curves.  Researchers have therefore developed a number of methods to achieve this: some, such as photogrammetry, are used in the presence of the animal, while others can be applied remotely, such as measuring the size of elephant dung to determine the animal’s mass.

Converting photos to measurements

Aimé Laussedat © Wikimedia Commons

In the 19th century, Aimé Laussedat, a French colonel and geometry professor, discovered that it is possible to measure distances using photographs of landscapes. The measurements of an object or an animal can be calculated from photographs when the ratio of the focal length of the lens and the distance between the camera and the object are known. This method is still used today, for example to measure sperm whales at the surface. Researchers must combine Laussedat’s method with a rangefinder to determine the distance between the camera and the object.

Biologists also use parallel lasers to directly obtain a measurement scale. Thanks to the lasers, the measurement is projected onto the animal. This way, one does not need a rangefinder or complex analyses. This method is used on cetaceans when they come to the surface.

In recent years, photogrammetry has been combined with a high-resolution camera installed on a drone to capture images of whales without intruding into their daily lives. Drones are relatively quiet and stable in flight, which allows them to fly close to the water without disturbing the whales. This non-intrusive approach eliminates the need for sedatives to immobilize the animals, handle them, or to lug bulky equipment into the field.

The crew of the Bleuvet, GREMM’s research vessel, uses images taken using a drone, combined with photogrammetry, to measure the health of St. Lawrence belugas. © GREMM

Are photos reliable?

To describe marine mammals, biologists combine photogrammetric data with morphological models. Mathematically, they transform the animal into a series of truncated cones to estimate the dimensions of the animal.

The next step is to make sure the models are reliable: are the results representative of reality? This is done by calculating the discrepancy between the mathematical model data and flesh-and-bone data (either from captive animals or stranded carcasses). If the difference is minor, the measurements obtained by photogrammetry can be considered to be as effective as a measuring tape or a scale!

Biologists use truncated cones to calculate the volume of blubber on a Weddell seal (Leptonychotes weddellii). This layer of fat protects marine mammals from the cold, icy waters. Blubber provides an indication of the animal’s energy balance. © GREMM

For more on this subject

How Do We Measure the Length of a Whale? (Baleines en direct, 01/11/2017)

With the belugas… And Their Waist Measurements! (Baleines en direct, 27/07/2018)

Sources

(2007) Waite, J.N., W.J. Schrader, J.A.E. Mellish and M. Horning. Three-dimensional photogrammetry as a tool for estimating morphometrics and body mass of Steller sea lions (Eumetopias jubatus). Can. J. Fish. Aquat. Sci. 64, 293-303

(2011) Rohner, C.A., A.J. Richardson, A.D. Marshall, S.J. Weeks and S.J. Pierce. How large is the world’s largest fish? Measuring whale sharks Rhincodon typus with laser photogrammetry. Journal of Fish Biology, 78, 378–385

(2014) Polidori, L., E. Simonetto and E. Labergerie. L’enseignement de la photogrammétrie au conservatoire national des arts et métiers: De Laussedat à la formation des géomètres en photogrammétrie numérique. Revue Française de Photogrammétrie et de Télédétection, 206 :55-61.

(2014) Shero, M. R.,  L.E., Pearson, D.P., Costa and J.M. Burns. Improving the Precision of Our Ecosystem Calipers: A Modified Morphometric Technique for Estimating Marine Mammal Mass and Body Composition. PLoS ONE 9(3): e91233

(2015) Durban J.W., H. Fearnbach, L.G. Barrett-Lennard, W.L. Perryman, and D.J. Leroi. Photogrammetry of killer whales using a small hexacopter launched at sea. J. Unmanned Veh. Syst. 3: 131–135

(2016) Chapman, S.N., H.S. Mumby , J.A. H. Crawley, K. U. Mar, W. Htut, A. T. Soe, H. H. Aung and V. Lummaa. How Big Is It Really? Assessing the Efficacy of Indirect Estimates of Body Size in Asian Elephants. PLoS ONE 11(3)

Last update: August 2018