Compute fractional tree canopy cover of a subplot/microplot by crown overlay

crown_overlay_pct() computes the proportion of a circular polygon covered by a given set of tree crowns modeled as discs and having spatially explicit stem locations. The sampled area is generally an FIA subplot with radius 24 ft (7.315 m) for trees with diameter >= 5 in. (12.7 cm), or an FIA microplot with radius 6.8 ft (2.073 m) for trees >= 1 in. (2.54 cm) but < 5 in. (12.7 cm) diameter (denoted as "saplings"). Stem locations are specified as distance and azimuth from subplot/microplot center.

Usage

crown_overlay_pct(tree_list, sample_radius, digits = 1)

Arguments

tree_list: A data frame containing tree records for a subplot/microplot. Must have columns DIST (stem distance from subplot/microplot center in the same units as sample_radius), AZIMUTH (horizontal angle from subplot/microplot center to the stem location, in the range 0:359) and CRWIDTH (tree crown width in the same units as sample_radius and DIST).
sample_radius: A numeric value giving the radius of the circular subplot/microplot.
digits: Optional integer indicating the number of digits to keep in the return value (defaults to 1, will be passed to round()).

Value

Estimated tree canopy cover as percent of the area specified by sample_radius that is covered by a vertical projection of circular crowns.

Examples

# subplot 1 of the `plantation` plot
subp1_trees <- plantation[plantation$SUBP == 1 &
                          plantation$DIA >= 5, ]
crown_overlay_pct(subp1_trees, 24)
#> [1] 87