Location uncertainty and the tri-areal design
The U.S. Department of Agriculture Forest Service Forest Inventory and Analysis Program (FTA) uses a field plot design that incorporates multiple sample selection mechanisms. Not all of the five FIA units currently use the entire suite of available sample selection mechanisms. These sampling selection mechanisms could be described in a number of ways with respect to the optional mechanism known as the annular plot. The annular plot is an auxiliary sampling mechanism intended for sampling rare attributes of interest. One explanation is that the subplot, which samples all trees greater than or equal to 5 in diameter at breast height (d.b.h.), is surrounded by an annular plot, concentric with the subplot for the estimation of rare but regionally important events. To date this selection mechanism has only been used to increase the sample of larger trees above a predetined d.b.h., known as a breakpoint diameter. Alternatively, the selection mechanisms could be viewed as disjoint concentric circles. The SUbplot in this latter view would sample all trees that are greater than or equal to 5 in and less than the breakpoint diameter. The larger circle can be referred to as a macroplot and it serves as the sole sampling mechanism for trees greater than or equal to the breakpoint diameter. This article focuses on the importance of clarity between these two descriptions and the estimation bias that can result from a misunderstanding of the distinctions between them, especially with respect to change estimates.