Sampling was carried out with a fixed-radius plot with plot radius varying from 3 to 11 m.
NFI systems differ in terms of inventory cycles, sampling system, plot radius, diameter threshold etc. (Table 1).
Fig. 13 RMSE of stems per ha and mean volume for test areas with different point patterns as a function of plot radius (m) for fixed-radius plot.
The optimal plot radius was a little bit larger (7 m), but trees with d1.3 less than 15 cm were only measured within 5 m plot.
It also turned out that the optimal plot radius in the tested area was somewhat smaller than the one used in current Finnish NFI, 9 m.
The plot radius 15 m used in this study was mainly because we used the same field instructions as in the NAFORMA inventory.
For circular plots, no side of the outer rectangle should be, anywhere, closer to the forest edge than the plot radius and for square or rectangular plots, no closer than half the length of the plot diagonal.
The use of "flexible radius" plots that is varying the plot radius according to the estimated stocking density, was employed to maintain an approximately constant workload throughout the survey (Melville et al. 2015).
Fig. 9 The weighted mean of relative standard errors for mean volume, basal area and stems per ha as a function of plot radius (maximum radius for relascope plots).
Fig. 6 Number of subsample trees as a function of plot radius (m) with the two subsample tree selection strategies (S1 = fixed, S2 = relascope) for fixed-radius (a), relascope (b) and concentric (c) plots.
Circular plots are more convenient for remote sensing studies compared to square or rectangular plots because only a single coordinate together with a plot radius are needed to match the two data sources geographically [19,28,29].
