| Remote measurements of size are helpful for studying unhabituated primate populations. Photogrammetry uses photographs as scaled representations from which to accurately measure subjects. Typical photogrammetry methods require costly hand-made parallel laser mounts to establish scale. Here, we test an unpublished method that uses only focal length and subject distance to establish scale. The equation for this relationship is: Sa=((D*F)/Sp)+Ci , where Sa is actual subject size, D is distance from the lens, F is lens focal length, Sp is subject size in the photo, and Ci is a correction factor unique to each camera/lens. To calculate Ci, we photographed a ruler at several distances and focal lengths using a Nikon D700. We measured D to within ± 1.6 mm using a Leica DISTO D2 Rangefinder. Sp was measured using Gimp, and subtracted from Sa for measurement error. We regressed error against log(D*F) for all photos to calculate Ci. To validate, we estimated measurement error in photos of a new subject: a field assistant. In contrast to previous findings, mean error was quite large (3000 mm; compared to previous reports of 0 mm), suggesting his method does not reliably give accurate measurements of primate body lengths (in our vervet monkey subject, Chlorocebus pygerythrus, sizes range from 25 to 50 cm). |
