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#' @title Diurnal Anisotropic Heat Index
#' @description
#' Simple approximation of the anisotropic diurnal heat (Ha) distribution
#'
#' @param x An elevation raster of class terra SpatRaster
#' @param amax The Alpha Max (amax) parameter in degrees defined
#' as: minimum = 0, maximum = 360 with the default = 202.500
#'
#' @details
#' The Diurnal Anisotropic Heat Index is based on this equation.
#' Ha = cos(amax - a) * arctan(b)
#' Where; amax defines the aspect with the maximum total heat
#' surplus, a is the aspect and b is the slope angle.
#'
#' @return terra SpatRaster class object Diurnal Anisotropic Heat Index
#'
#' @author Jeffrey S. Evans <jeffrey_evans@@tnc.org>
#'
#' @references
#' Boehner, J., and Antonic, O. (2009) Land-surface parameters specific to
#' topo-climatology. In: Hengl, T., & Reuter, H. (Eds.), Geomorphometry -
#' Concepts, Software, Applications. Developments in Soil Science,
#' 33:195-226
#' @examples
#' library(terra)
#' elev <- rast(system.file("extdata/elev.tif", package="spatialEco"))
#' Ha <- dahi(elev)
#' plot(Ha)
#'
#' @export dahi
dahi <- function(x, amax = 202.500) {
if(!inherits(x, "SpatRaster"))
stop(deparse(substitute(x)), " must be a terra SpatRaster object")
tr <- terra::terrain(x, v=c("slope", "aspect"), unit="degrees")
return( cos(amax - tr[[2]]) * atan(tr[[1]]) )
}
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