Deriving observation distances for camera trap distance sampling

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Abstract
Camera trap distance sampling (CTDS)o is a recently developed survey method to estimate animal abundance from camera trap data for unmarked populations. It requires the estimation of camera‐animal observation distances, which previously was done by comparing animal positions to reference labels at predefined intervals. Here, we test a photogrammetry approach to derive camera‐animal observation distances. We applied both, the reference label and photogrammetry approaches to five ungulate species varying widely in body size (Giraffa camelopardalis,Equus grevyi,Oryx dammah,Kobus megacerosandEudorcas thomsonii) and one ground‐dwelling bird species (Numida meleagris) inhabiting a large enclosure and estimated their density with CTDS. Both procedures provided highly correlated observation distances (ρ = 0.99,p < 0.001). A paired t test revealed a minor but significantly higher mean of the photogrammetry approach (MD = 0.28 m,p < 0.001). This, however, seems negligible as for analyses, distances were grouped in intervals of 2 to 5 metres. In general, estimated animal abundance was close to the true number of individuals in the enclosure for both approaches, with the exception of zebra, whose density was underestimated. The photogrammetry approach offers an alternative approach for deriving camera‐animal observation distances and is particularly useful for application in open habitats, with little occlusion of animals.

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