Effect of Errors in Visual Assessment of Distance in Line Transect Sampling

The effect of errors in visual assessment of distance to objects, on density estimate obtained through line transect sampling was investigated. An ex situ trial was conducted to assess the agreement between actual distance and visual estimates made by the observers. Wooden poles were fixed at known distances in random sequence on a flat ground and volunteers were asked to estimate the distance by visual observation. Linear regression equation fitted through the origin showed that there was underestimation by 2m for every 100 m of actual distance, which is negligible. The mean bias in the visual estimation of actual distance was not significantly different from zero. However, the coefficient of variation of visual estimates of distance varied from 54 per cent in 0-20 m class to 34 per cent in 80-100 m class. An examination of the theory of line transect sampling showed that for a given set of detections, overestimation of distances in the field would lead to underestimation of density and vice versa. Increasing disruption of random nature in distance measurements was found to bring down the density estimate on an average for a fixed set of detections and transect length.