calc_co: Calculate canopy openness
In GastonMauroDiaz/rcaiman: CAnopy IMage ANalysis

calc_co

R Documentation

Calculate canopy openness

Description

Calculate canopy openness

Usage

calc_co(bin, z, a, m = NULL, angle_width = 10)

Arguments

`bin`	SpatRaster. Binarized hemispherical canopy image.
`z`	SpatRaster built with `zenith_image()`.
`a`	SpatRaster built with `azimuth_image()`.
`m`	SpatRaster. A mask. For hemispherical photographs, check `select_sky_vault_region()`.
`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 \insertCiteGonsamo2011;textualrcaiman:

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 applied using the m argument, the equation is adjusted to account for the reduced portion of the image that contributes to the calculation:

\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}.

The denominator ensures that the canopy openness remains correctly scaled even when part of the image is masked. Without this normalization, the total openness could be underestimated since the summation in the numerator will be one only when the full hemisphere of an totally unobstructed image is included. Therefore, masking requires the normalization provided by the denominator.

Value

Numeric vector of length one.

References

\insertAllCited

Examples

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

GastonMauroDiaz/rcaiman documentation built on April 14, 2025, 9:39 a.m.