Edge effects in line intersect sampling with segmented transects

Transects consisting of multiple, connected segments with a prescribed configuration are commonly used in ecological applications of line intersect sampling. The transect configuration has implications for the probability with which population elements are selected and for how the selection probabilities can be modified by the boundary of the tract being sampled. As such, the transect configuration also affects the performance of methods designed to eliminate edge-effect bias. We show that the reflection method solves the edge-problem for designs that use symmetric radial transects (e.g., straight-line and X-shaped transects centered at the sample point). This method also applies to designs that use asymmetric radial transects, provided the orientation is selected uniformly at random. Asymmetric radial transects include straight lines emanating from the sample point, and L- and Y-shaped transects. The reflection method does not apply to designs where polygonal transects (e.g., triangular, square, and hexagonal transects) are used, or where the orientations of asymmetric radial transects are fixed. We provide a new method that eliminates edge-effect bias for designs that use asymmetric radial transects with fixed orientation, but a workable solution for polygonal transects remains elusive.