Fig. 3
From: Take five: about the beat and the bar of annual and 5-year periodic national forest inventories
Spatial design of the US FIA annual forest inventory survey as adopted after the Farm Bill of 1998. The basic design is based on the interpenetrating panel approach applied to a basic hexagonal-grid tessellation of the territory. Each number assigns one grid cell to one yearly panel, the spatial design of which being a parallelogram-shaped grid (represented for panels 1 and 2). Within each cell, the location of the sampling units—a permanent cluster of 4 field plots as illustrated in the gray-filled cell—is drawn at random to permit unaligned systematic sampling (redrawn and adapted from Bechtold and Patterson 2005)