title

Method for estimating animal size by camera trap images using reference objects

author(s)

Sergii Gashchak (orcid: 0000-0002-7582-6742)

affiliation

National Museum of Natural History, NAS of Ukraine (Kyiv, Ukraine)

bibliography

Theriologia Ukrainica. 2025. Vol. 30: 97–108.

DOI

language

Ukrainian, with English summary, titles of tables, captures to figs

abstract

Camera traps (CT) are increasingly used in zoological studies, primarily to identify species, estimate their abundance, distribution, behaviour, etc. However, the full potential of CTs has not been widely adopted, for instance, the opportunity to estimate animal size. This information is vital for analysing population structure and status, and for individual identification, especially in species lacking distinct external markers. This study presents an author-developed method for estimating animal size based on CT images. It is based on comparing the relative size of the animal in the image with that of reference objects with known linear dimensions, which were photographed by the CT at the same location. As reference objects, the author used a set of one-metre poles with brightly coloured 20 cm markings, installed in front of the CT, along a predefined radial grid within a sector of c. 30° and a radius up to 8 m. The key conditions for precise size estimation are: strict adherence to the grid design, visibility of the point where the pole meets the ground surface, and the presence of unchanging elements within the CT’s field of view. It is also important to select images where the animal’s body is positioned parallel to the frame, and the legs are vertically aligned. The grass in front of the CT is recommended to be kept at the lowest level to ensure visibility of where the legs meet the ground. All procedures with the images of the animals and the poles were carried out in Adobe Photoshop (overlapping, aligning, measuring). The precise alignment of the pole and animal positions favours an accuracy of size estimation up to a few centimetres. An error of ±1 m in determining the pole and animal positions results in an error of size estimation of ±25% at a distance of 3 m from the CT, decreasing to ±10% at 7 m. A function of the change in linear dimensions with distance from the CT helps to estimate size more precisely. All examples and calculations were made for conditions using the Browning BTC-7 camera trap in Ultra Video mode. For other CT models and modes the parameters of the grid, the dimension-distance function, and the evaluation error will vary, and they can be recalculated, though the main regularities will remain consistent.

keywords

Remote studies, camera trap, mammals, animal sizes, indirect methods of measurements, setup of the field equipment, graphics editor

references

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