Plot-GEM Example #1

Do we have enough plots to adequately address our issues?

In 2000, Region 1 (R1) conducted a pilot study in the Idaho Panhandle National Forest. The study area contained 59 existing FIA primary sampling units (PSUs). One of the questions R1 wished to answer via the pilot study was "How much of an intensification of the existing FIA grid (if any) will achieve ±20% estimation accuracy?"

Plot-GEM was used to produce the graphs in Figure 1. The percent error graph on the left shows that the goal of estimating the mean of the attribute percent tall shrub cover to within ±20% (represented by the horizontal, red line) is not met with the existing FIA grid. The ±20% accuracy goal is met when 85 PSUs are used. Thus R1 was able to conclude that if they wanted to estimate mean percent tall shrub cover with an accuracy of ±20% the existing FIA grid was insufficient. They would need at least an additional 26 PSUs.