calc_co {rcaiman}

Calculate canopy openness

Description

Calculate canopy openness

Usage

calc_co(bin, m = NULL, z, a, angle_width)

Arguments

bin

SpatRaster. Binarized hemispherical canopy image.

m

SpatRaster. A mask. For hemispherical photographs, check mask_hs().

z

SpatRaster built with zenith_image().

a

SpatRaster built with azimuth_image().

angle_width

Numeric vector of length one. It should be ⁠30, 15, 10, 7.5, 6, 5, 3.75, 3, 2.5, 1.875, 1⁠ or 0.5 degrees. This constrain is rooted in the requirement of a value able to divide both the 0 to 360 and 0 to 90 ranges into a whole number of segments.

Details

Canopy openness calculated as in the equation from Gonsamo et al. (2011):

CO = \sum_{i = 1}^{N} GF(\phi_i, \theta_i) \cdot [(cos(\theta_1) - cos(\theta_2))/n],

where GF(\phi_i, \theta_i) is the gap fraction of the cell i, \theta_1 and \theta_2 are the minimum and maximum zenith angle of the cell i, n is the number of cells on the ring delimited by \theta_1 and \theta_2, and N is the total number of cells.

When a mask is provided through the m argument, the equation is modified as follow:

\frac{ CO = \sum_{i = 1}^{N} GF(\phi_i, \theta_i) \cdot [(cos(\theta_1) - cos(\theta_2))/n] }{ \sum_{i = 1}^{N} (cos(\theta_1) - cos(\theta_2))/n} .

This allows the masking of any individual cell.

Value

Numeric vector of length one.

References

Gonsamo A, Walter JN, Pellikka P (2011). “CIMES: A package of programs for determining canopy geometry and solar radiation regimes through hemispherical photographs.” Computers and Electronics in Agriculture, 79(2), 207–215. doi:10.1016/j.compag.2011.10.001.

Examples

caim <- read_caim()
z <- zenith_image(ncol(caim), lens())
a <- azimuth_image(z)
m <- mask_hs(z, 0, 70)
bin <- apply_thr(caim$Blue, thr_isodata(caim$Blue[m]))
plot(bin)
calc_co(bin, m, z, a, 10)