Fig. 5

Formation of groups of four sample locations in Matérn’s estimator of variance under systematic sampling from a regular grid of sample locations (black dots). The domain of interest (forest) is the grey polygon. A group of four must have at least one location in the domain of interest, and be (spatially) nearest neighbours (NNs). Groups satisfying this condition are indicated by two dashed diagonals. The formation of groups was initiated with the four NNs in the upper left corner